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eHam Forums => HomeBrew => Topic started by: KB1WSY on May 05, 2015, 05:06:10 AM



Title: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 05, 2015, 05:06:10 AM
This thread is split out from my separate "Tube Portable Transceiver" thread. My current challenge: designing and building a subminiature-tube regenerative receiver with good sensitivity and stability, plus the best selectivity that can be achieved with a regenerative design.

In his article "High Performance Regenerative Receiver Design," Charles Kitchin, N1TEV, argues that a properly designed regenerative receiver is "quite capable of direct-conversion or superheterodyne-level performance, although it does require greater operator skill." The article was published in the Nov/Dec 1998 issue of QEX
and is available on the ARRL website: http://www.arrl.org/files/file/Technology/tis/info/pdf/9811qex026.pdf (http://www.arrl.org/files/file/Technology/tis/info/pdf/9811qex026.pdf). (I thank eham member JAHAM2BE for bringing it to my attention a couple of years ago.)

Kitchin reminds us that the regen was the standard design among hams of the 1920s and early 1930s. He argues that today's viewpoint of regens as "only suitable for beginner experimentation" is erroneous. This attitude, he says, arose in part because regen circuits designed for novices in the 1940s and '50s had poor performance. Much of the regen-design knowledge possessed by the "old-timers" of 100 years ago had been forgotten.

Here are some design elements of a high-performance regen (these points are a mixture of Kitchin's, and mine). Note that Kitchin's published designs are solid-state; so part of the challenge is to "translate" them to a tube context.

--A broadband (untuned) RF stage preceding the detector; this prevents radiation by isolating the oscillator from the antenna; reduces loading on the detector; provides a constant load for the detector; and reduces instability resulting from an antenna that swings in the wind. The gain of such a stage does not have to be substantial and can even be negative, because of the high sensitivity of the following detector stage. Kitchin argues against using a tuned, selective RF stage although some other authors (Lindsay Publications, "The Impoverished Radio Experimenter") do advocate a conventional tuned stage (with its own separate tuned circuit, either ganged or un-ganged with the detector tank circuit).

--Some kind of variable input attenuation, to control overload, which is a common problem with regens. This could be a resistive or capacitive control in the antenna circuit; or some kind of control on the gain of the RF stage.

--A "throttle" variable capacitor to control regeneration, eliminating the "hysteresis effect" ("overshooting") and the beat-note drift of the nowadays more-common potentiometer or the detuning effect of a variometer.

--Voltage regulation to hold the detector voltage constant; Kitchin's solid-state designs use a Zener diode for this.

--Something that is confusing me quite a lot. Both Kitchin and some other sources argue that the gain of the detector should be kept low (with a JFET, by keeping high negative bias) because this permits "very smooth regeneration control." Kitchin says that "the old-timers of the 1920s knew this and operated their tube detectors from low supply voltages" to achieve this. However it is not clear to me whether this is necessary only for good reception of AM signals; or whether it is also a good idea if (as is my case) you are only interested in CW. In "The Impoverished Radio Experimenter" it is recommended to run the detector at such low gain that it is operating in the non-linear part of its curve; does this mean that detection is, in effect, a form of harmonic distortion??

--Use of a non-metallic cabinet, apart from an (optional) grounded metal front panel to reduce hand-capacitance effects. "A metal chassis, shield cans etc. all absorb energy from the main tuning coil and add to its losses, which directly affect the overall circuit Q and the selectivity of the receiver" (Kitchin).

My challenge is to replicate the above design elements using small, low-current subminiature tubes. My "baseline" is the 1AD4, a sharp-cutoff RF pentode with a directly heated 1.25V cathode/filament and a nominal B+ of 45 volts. I've already successfully built (with a lot of help from eham Elmers!) a conventional "beginner's regen" using two such tubes. It has none of the above-mentioned "high performance" design elements and furthermore I took little care in construction, wanting instead to do a rapid "proof of concept." The receiver does work. It is very sensitive; however it is also very unstable, easily overloaded and extremely sensitive to hand capacity effects or indeed the presence of any objects in its vicinity. It is hard to get optimal loading; and grounding the set shuts off regeneration altogether. These are the typical "issues" that cause today's builders to shun regenerative sets.

My first question is: With the tube constraints that I have (directly heated filament), what sort of circuit would function well as a broadband RF amplifier in front of the detector? Kitchin uses a grounded-base transistor. What would be the hollow-state equivalent: Can I use a conventional RF amplifier circuit and if so what does the input circuit in front of the grid look like? Or do I need to figure out a grounded-grid or a (degenerative) cathode-follower design in order to get wide bandwidth? If so, how are those designs possible with a directly heated tube? Alternatively, should I use a tuned RF amplifier, as advocated by "The Impoverished Radio Experimenter" even though it's a PITA to introduce tracking between the RF stage and the detector?

73 de Martin, KB1WSY (with apologies for the typo in the word "Performance" in the topic title; once posted, it cannot be fixed....)


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on May 05, 2015, 07:35:09 AM
The best regenerative detector I had used a 6AU6. The control of regeneration was by varying the screen voltage and it was SMOOTH! Something like an RF choke in the plate, then a 1000pf to ground and a 47k plate load to an 8 mfD electrolytic and then via 1k to about 250 volts of B+. From the plate, a 100pF fed the tickler coil. I used a 6BA6 RF amplifier with a tuned grid and a pot - probably about 5 k - in the cathode for an RF gain.

Now if we look at the US Navy RAL design from 1936, it has two RF stages using 6D6s, a 6D6 detector, a 41 output and a 41 AGC detector: the regeneration was by controlling the screen and suppressor of the detector and the tickler was in the cathode of the detector. It covered 300kHz to 23 MHz in 9 bands, using 3 coils per stage, each one of which had a tuned winding with two taps giving three possible inductances.

The 2 RF stages was probably to give good isolation between detector and antenna: the receiver is not so much built like a battleship as needing a battleship to carry it! It is 13-5/16 inches high, 18 inches wide and 16-3/32 deep and weighs 69 pounds. The power supply is 12-1/4 by 14 by 8 and weighs 41lbs...But they have a reputation as probably the best TRF made. Its sister RAK covered 15kHz to 300kHz in 6 bands

Now as you have been talking of only covering 7 MHz, there's no need to worry about tracking an RF amp - a single tuned circuit in the RF amp grid will suffice. I would still go for a metal box from the viewpoint of hand capacity, but space the coil at least two diameters away from the metal. By varying the bias on the tube (you will need a C battery), you can get variable RF gain to avoid overload. Bear in mind that with the very low voltage supply, overload will happen much more easily than with high voltage tubes.

Try looking in the pre WW2 QSTs for TRF designs.



Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 05, 2015, 08:27:21 AM
The 2 RF stages was probably to give good isolation between detector and antenna: the receiver is not so much built like a battleship as needing a battleship to carry it!

I see what you mean! The RAL manual is here: http://bama.edebris.com/manuals/rca/ral/ (http://bama.edebris.com/manuals/rca/ral/).

I will trawl through the pre-war QSTs.

Now as you have been talking of only covering 7 MHz, there's no need to worry about tracking an RF amp - a single tuned circuit in the RF amp grid will suffice.

So presumably, at the front of the RF amp, an antenna coil with its secondary forming the L part of a fixed LC tuned circuit, peaked for whatever favorite part of the band? Maybe I'll try building one experimentally with a 1AD4 and putting it in front of my existing, sub-optimal "breadboard" set as yet another "proof of concept." Then I'll come up with a full schematic for the "high performance" set and we can look over it -- this time, "design before building" (until now, it's been the other way round!!).

So now we're working on the RF amp, I have two questions on other stuff:

--Concerning the AF stage. Am I right that if I put in a two-tube AF stage (triode, then pentode) with relatively low gain on the triode, it will load the detector output less and help with stability? (Perhaps just a resistive load on the triode rather than a choke.)

--Does it make sense to add voltage regulation, and if so how? With the relatively low B+ none of the usual VR tubes is appropriate.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on May 05, 2015, 11:11:46 AM
Quote
--Concerning the AF stage. Am I right that if I put in a two-tube AF stage (triode, then pentode) with relatively low gain on the triode, it will load the detector output less and help with stability? (Perhaps just a resistive load on the triode rather than a choke.)

The choke presents a high impedance load for the detector with a low resistance. Going to resistance coupling means losing plate volts and load resistance, Getting the right value of resistor is then a compromise. With two AF stages, you get around the loss of signal by going to a resistor: if you used a pentode stage, you could have a lower plate load on the detector and thus more volts. But you might get the noise of the AF stage becoming a problem. Probably not, but worth noting the possibility.


Quote
--Does it make sense to add voltage regulation, and if so how? With the relatively low B+ none of the usual VR tubes is appropriate.

No, but at  least 2mFd and preferably more across the B+ supply. If you use an electrolytic, make sure it is on the rx side of the B+ ON/OFF switch.

I would suggest you go on both RF and detector for about 10 microhenries and around 50pF to tune it. The higher L circuit gives a higher impedance.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 05, 2015, 11:59:54 AM
No, but at  least 2mFd and preferably more across the B+ supply. If you use an electrolytic, make sure it is on the rx side of the B+ ON/OFF switch.

OK. I have just added a 10µF/100V electrolytic across the 48V B+ of my ramshackle breadboard regen test-bed. There is no audible difference in stability, but the causes of instability in this crude set probably don't have much to do with the purity of the B+ supply. However, the new capacitor does seem to have slightly improved the smoothness of the regeneration control -- but it was already rather smooth anyway. With the new capacitor, the onset of regeneration appears slightly farther round the dial and with a slightly more gradual transition.

I would suggest you go on both RF and detector for about 10 microhenries and around 50pF to tune it. The higher L circuit gives a higher impedance.

I will start by building a "breadboard" RF amplifier and connect it to the existing breadboard set, with the LC setup as you suggest. When I build the "high-performance" set I will probably play around with LC ratios to see what works best.

I would still go for a metal box from the viewpoint of hand capacity, but space the coil at least two diameters away from the metal.

I know from my experience with building the ARRL "3-Transistor Receiver for the Beginner" that you can build a good regen, with high sensitivity, even if it is completely enclosed in a metal cabinet. In fact my initial build of that project was just an "open" metal chassis/panel combination and I ended up building a metal cabinet to deal (successfully) with hum issues. Adding the cabinet didn't appreciably damage sensitivity. That set has hand-capacity issues, but they are very mild compared to my "open breadboard" tube experiment.

With the current "high-performance" project I will first attempt to do things the "N1TEV way" i.e. with a largely wooden chassis, with metal only on the front panel; but will do it in such a way that metal shielding/groundplane can be added experimentally if there are problems with hand capacity or hum. Kitchin's point is that the knowledge of The Ancient Ones concerning building a good regen is worth paying attention to, including using a breadboard-style design rather than metal; so I will give it a go, and change course if it doesn't work.

Edited to add: one possibility would be to have a metal groundplane for most of the set, except for the detector stage and the area around the detector coil. I do want this set to "play nicely" with the rest of my station, including being connected to the ground system and not drifting all over the place when other equipment is nearby/active.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on May 05, 2015, 05:11:29 PM
If you look back to the TRF sets in the 1938 RSGB Radio Amateur's Handbook, you can see a move to metal cabinets and chassis....Same as the admittedly,  pretty crappy, RAF aircraft TRF sets of the pre WW2 period.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 05, 2015, 05:52:51 PM
Now as you have been talking of only covering 7 MHz, there's no need to worry about tracking an RF amp - a single tuned circuit in the RF amp grid will suffice. I would still go for a metal box from the viewpoint of hand capacity, but space the coil at least two diameters away from the metal. By varying the bias on the tube (you will need a C battery), you can get variable RF gain to avoid overload. Bear in mind that with the very low voltage supply, overload will happen much more easily than with high voltage tubes.

Why the C battery? No way to get self-bias? How come the audio stage works fine without battery bias? Hmmph, all those batteries....

On another tack, how about a cascode RF amp? They are all the rage in the 1968 RSGB manual, even at HF....

Edited to add: This evening I did a head-to-head comparison between my two regenerative receivers, both on 40m, both with the same antenna, swapping betwen the two. The two-tube 1AD4 "messy breadboard" receiver is hands-down better in sensitivity and is bringing in noticeably more signals, however the ARRL "3-Transistor Receiver for the Beginner" (1968 design) is dramatically more stable and usable at this point. A fun comparison.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on May 06, 2015, 01:54:24 AM
The cascode needs more volts if you use the simpler series circuit: other wise, it gets more complex. You don't NEED an RF stage at 7 MHz from a noise factor viewpoint, but having an extra tuned circuit helps reject those nasty strong broadcast signals above 7.2 MHz. I would go for about 10 microhenries for both detector and RF stage: the higher L/C ratio gives a higher impedance and more step up.

No bias means very low signal handling capability: getting gain control by varying the screen volts also gives signal handling problems. So get the bias by connecting the negative side of the B battery through a resistor to ground: choose it for about 2 to 3 volts drop across it, and now you have a negative bias supply. As done in the BC348 and R1155 receivers.

You may well find less AF distortion with some bias....depends on how big the AF signal is.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 06, 2015, 03:12:48 AM
No bias means very low signal handling capability: getting gain control by varying the screen volts also gives signal handling problems. So get the bias by connecting the negative side of the B battery through a resistor to ground: choose it for about 2 to 3 volts drop across it, and now you have a negative bias supply. As done in the BC348 and R1155 receivers.

You may well find less AF distortion with some bias....depends on how big the AF signal is.

Reading my post yesterday, it looks a bit dyspeptic! I can either use your suggested biasing method, or your original suggestion of a C battery. I'm already using a rechargeable 1.2V AA NiMh battery for the filaments, and can easily add a new, separate "C supply" battery holder using two 1.2V batteries in series -- I recharge those NiMh cells four at a time anyway.

"Leafing through" the digital QST archive from the late 1920s, what strikes me is the sophistication of the regenerative designs: complicated coil combinations, plethoras of batteries, and great attention paid to filtering and bypassing in the power supply lines.

I also found a quote from the November 1929 edition of the Radio Amateur's Handbook:

And now, when the receiver has been built, adjusted and placed in satisfactory working condition it will be permissible to sit back and take a long breath. For the receiver is one of the two essential parts of an amateur station. If the receiver has been correctly built and if the location of the station is satisfactory it will receive as far as any transmitter can send. If it has open tuning scales; if it has lots of sensitivity and amplification; and if it smooth and quiet in operation, it will be a very great comfort and a source of splendid pleasure.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 06, 2015, 05:05:25 AM
Here's my first try at a circuit for the RF amplifier. This time, I'm trying the novel idea of designing first, then building....

(https://lh3.googleusercontent.com/-ID2tFy6LvCA/VUoCPV_CTUI/AAAAAAAAE1k/ShQkIwIJ0Go/s576/Submini_regen_large_layout_schematic_06.jpg?gl=US)

I'll wait for comments, then build a rough breadboard version as a first test.

One question: instead of putting the "C" battery in series with the gain pot, would it make more sense to connect the bottom end of the pot directly to ground and put the negative battery connection on the wiper? Edited to add: in which case I'd need to add a fixed resistor in series, to avoid shorting the battery at one end of the pot travel....

Edited again to add: today I played with the B+ voltage, in an effort to test the assertion (made by Kitchin and others) that The Ancient Ones deliberately dialed back the gain on the detector stage to permit smooth regeneration control. It didn't work: lowering the B+ from 48V to 36V only resulted in an almost dead receiver. It was possible to achieve oscillation at the extreme end of the pot range, but sensitivity was way, way down and I did not notice any greater smoothness. This doesn't mean the idea of operating at low gain is necessarily wrong, however it doesn't seem to be working in the case of my current tube/circuit combination. (It should also be noted that regeneration control with my current circuit is already quite smooth anyway.)

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: JS6TMW on May 06, 2015, 05:23:15 AM
I don't think the directly-heated cathode is a drawback, maybe a necessity in a small volume. Wow, but those subminiature tubes were really magic back in the day - not very popular among hams but they were a "missing link" between VTs and transistors and widely used in hearing aids.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 06, 2015, 05:34:08 AM
I don't think the directly-heated cathode is a drawback, maybe a necessity in a small volume. Wow, but those subminiature tubes were really magic back in the day - not very popular among hams but they were a "missing link" between VTs and transistors and widely used in hearing aids.

Yes indeed. For those who may be unfamiliar with these cute tubes, here's a picture of the 1AD4s that I'm using in this set (I bought 30 of them for $29):

(https://lh3.googleusercontent.com/-q5Fwos9jteo/Ux4HVf53wLI/AAAAAAAAB4k/_z3Xh9lMN9c/s640/Miniature_Tube.JPG?gl=US)

Here's my Rat's Nest Experimental Regenerative Radio. I used 1AD4s that haven't had their leads cut for sockets, and the other components also have long leads to make them easier to re-use later. This kind of wild construction is a recipe for instability and leakage ... I'm looking forward to settling on a future "high performance" design and building it properly!

(https://lh3.googleusercontent.com/--nCrd8sZ0Ug/VUoM2nVRWNI/AAAAAAAAE10/zjG2Q1riinw/s640/Submini_regen_7.JPG?gl=US)

I'm not sure why hams didn't seem to use subminiature tubes much -- even the numerous QST designs for portable gear passed them over and used larger standard battery-type tubes. I have two three theories: (1) I suspect that they were quite expensive, on a "bang for your buck" basis. (2) Logically it probably makes more sense to use PCB construction for a finished project, given the tubes' small size, and those construction techniques were uncommon in the late '40s and early '50s. (3) Those were the days when more power was often regarded as an unalloyed "good thing" and the subminiature tubes seem more in harmony with today's QRP trend.

I like them a lot because they are so easy to experiment with -- similar to the ease with which one can throw together a solid-state circuit. There's no need for an AC power supply and the power consumption is really low. The main drawback is modest performance for the directly heated tubes, but presumably that applies to larger-sized directly heated "battery" tubes too.

Other advantages: (1) Ruggedness. (2) Minimal heat emission (even after they've been on for hours, the 1AD4s are barely warm to the touch) which should make for lower drift in receivers and VFOs. (3) Built-in shielding, in the form of a "sprayed-on" shield on top of the glass envelope (some full-size tubes used this technique too).

Disadvantage: you don't get that warm vacuum-tube glow!

General advantage of directly heated tubes (not just subminis): "instant-on" because you don't have to wait for the cathodes to warm up.

Later on, I'll be experimenting with the cathode-type versions of these tubes (6.3V filaments). These have good performance; some of them are similar in size to the one shown above; there are also subminiature tubes that are very slightly larger (8-pin, circular-body) that would be good in transmitter applications.

You can get an idea of the great variety of subminiature tubes in this Raytheon catalogue: http://www.tubebooks.org/tubedata/RaytheonTubes.pdf (http://www.tubebooks.org/tubedata/RaytheonTubes.pdf).

The tubes are still quite widely available. The sockets are much harder to find; but if you use the leaded types you don't need sockets at all (although that makes them harder to replace if they blow).

(Of course we also had Nuvistors later.)

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on May 06, 2015, 06:09:03 AM
The suggested circuit is a no-no. It won't put bias on the tube because of the DC path to ground through the coil. It also shunts the signal path.

Connect the grounded end of the grid coil to ground through a disc ceramic capacitor of  2 to 5000pF, with short leads, and connect the tuning capacitor grounded end to the same point. If the tuning cap is variable, you want the disc ceramic grounded end to be physically on the tuning capacitor. If it's a variable iron cored inductor, connect the tuning cap across the inductor. Connect the variable RF gain pot (I'd suggest something like 50 or 100kohm) across the C battery, and take the wiper through something like 10k to the 'groundy' end of the coil, the disc ceramic capacitor. You want about 4.5 volts in the C battery, and a 0.1 mFd from pot wiper to ground to minimise noise as you turn the pot.

The bypass from the end of the plate coupling winding is a bit on big side: with 1/2 inch leads, it's series resonant about 5 MHz. Use between 2 and 5000pF for that and the screen bypass.

It appears that most pre-war designs of successful TRFs used quite large coils (inch to inch and half diameter), well spaced from metal work, and with around a 1:1 length to diameter ratio for maximum Q.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 06, 2015, 06:56:54 AM
The suggested circuit is a no-no. It won't put bias on the tube because of the DC path to ground through the coil. It also shunts the signal path.

I was wondering about that already -- in particular the DC short through the coil to ground!!!

I know almost nothing about how to adapt cathode-type circuits for a directly heated tube. With your help, I am learning fast, it seems. Which is just as well because almost all the circuits in textbooks are for cathode-type tubes, after the first few pages of elementary tube theory with filament tubes.

Meanwhile, you can be grateful that I'm only playing with "low" voltages, huh? In this case the main result would have been a melted battery holder I suspect (yes, I've done that too, a few weeks ago when I shorted a battery).

Given the large number of changes you suggested, here's the revised schematic: is this what you meant?

(https://lh3.googleusercontent.com/-CJ5qeD3YulM/VUodOecoXaI/AAAAAAAAE2E/dr2qapIIdOo/s576/Submini_regen_large_layout_schematic_07.jpg?gl=US)

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on May 06, 2015, 09:35:12 AM
I'd suggest a capacitor from the pot wiper to ground. At least 0.1 microfarad: 1 mFd wouldn't hurt.

To avoid all doubt, "2-5000pF" means "2000 - 5000pF".

Deciding on how many turns for the plate coupling coil is another matter. One can go for a large coupling coil approximately self resonant  but loosely coupled to the detector coil, or for a smaller coil  more tightly coupled. Now you are currently running with a 5 microhenry coil: I would go for 10 microhenries and less capacity. This looks like around 27k with a Q of 60. Now have about a half as many turns on the coupling coil and the plate load for the RR stage is 27/4 kohm and the gain is about 6 to 9. The transformed plate resistance is thrown across the tuned circuit: that will have little effect. So there can be reasonably tight coupling between the coupling winding and the detector.  I wouldn't wind the coupling winding over the tuned winding but put it 1/8 inch away and close wind it.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 06, 2015, 12:09:27 PM
Deciding on how many turns for the plate coupling coil is another matter. One can go for a large coupling coil approximately self resonant  but loosely coupled to the detector coil, or for a smaller coil  more tightly coupled. Now you are currently running with a 5 microhenry coil: I would go for 10 microhenries and less capacity. This looks like around 27k with a Q of 60. Now have about a half as many turns on the coupling coil and the plate load for the RR stage is 27/4 kohm and the gain is about 6 to 9. The transformed plate resistance is thrown across the tuned circuit: that will have little effect. So there can be reasonably tight coupling between the coupling winding and the detector.  I wouldn't wind the coupling winding over the tuned winding but put it 1/8 inch away and close wind it.

Thanks, lots of stuff to ponder, as I re-work the coils.

It's interesting that there are two fundamental skills in radio design: the applied mathematics; and the practical knowledge derived from actually building radios and seeing how the theory works out in practice. You possess both in abundance, I am in awe!

Here's what it should look like after the re-work:

(https://lh3.googleusercontent.com/-j0f39yYBP7c/VUplesbYX6I/AAAAAAAAE2U/M3iBJ1FwCcw/s912/Submini_regen_large_layout_schematic_08.jpg?gl=US)

Edited to add a quick question: I dithered about the polarity for the 1µF electrolytic on the RF Gain wiper. I've now "fixed" it so that the positive plate goes to ground ... which I think must be correct since the bias is negative?

MDM

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 06, 2015, 12:48:52 PM
I wouldn't wind the coupling winding over the tuned winding but put it 1/8 inch away and close wind it.

Another question, something that's always confused me.

The tickler is currently wound on the bottom end of the coil form, underneath the "cold" end of the tank coil.

If I put the new primary winding (the plate coupling coil) above the detector tank coil, winding it (as usual) in the same direction as the tank coil, should the "cold" end of the coupling coil be immediately adjacent to the top of the tank coil, or should that be the "hot" end?

I suppose the same question also arises for the antenna coupling coil at the front of the RF stage....

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: WB6BYU on May 06, 2015, 02:05:37 PM
Generally I'd put the grounded ends of the coils together if I can - that reduces capacitive coupling
between the windings.

The direction of the winding doesn't matter for any of the coils except the tickler coil - that one
needs the proper phase to get the tube to oscillate.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 06, 2015, 09:10:22 PM
Spent much of day rewiring the detector: changing the capacitor values in the tuned circuit and rewinding the tank coil to try to get Peter/G3RZP's suggested 10µH/50pF LC ratio for the 40m band. (Haven't yet built the RF stage; decided to alter the detector stage first.)

Am rather discouraged. With antenna connected in the usual way, receiver is overloading drastically on almost every signal. Altering value of the series capacitor in the antenna connection doesn't help. Also, regeneration control is noticeably rougher and very "touchy" (I tried changing the tickler's turns and position, but it didn't help). Just much worse all around.

Tomorrow I will probably experiment with the tank-coil form factor. As of now, it is 15 turns that are 0.7" high, wound on a 1.25" pill bottle. That is nowhere near the 1:1 height/diameter ratio that Peter suggested for maximum Q. Looks like I should try a 1" or 3/4" form. On the other hand, the current symptoms (serious overloading) imply, don't they, that the Q has *increased* compared to the previous LC combination? Won't even higher Q make it even worse?

Another thing that's puzzling is that my calculations for number of turns was way off. To get 10µH it should have been 18 turns, not 15 (15 turns is supposed to be more like 7µH) and yet the only way I can bring in the desired band (6950-7150) is with the 15 turns (I don't have an inductance meter). I've done the calculations for the capacitance network over and over again, and believe them to be correct.

Edited to add: I've just measured the frequency coverage from one end of the dial to the other and it's only about 140kHz when it's supposed to be at least 200kHz. That tells me there's probably something screwy about the capacitor network (and also that the 15-turn coil is probably closer to 7µH than 10µH). It's time to go to bed -- I will check my wiring and calculations again tomorrow!!

The coil is also extremely fiddly: very small changes in the spacing of the windings yield very large changes in the resonant frequency of the tuned circuit.

As I understand it, Peter's rationale for the change in the LC parameters is to provide a better load/impedance match when the RF stage is added. My question therefore is: Should I worry about the sharp deterioration in the detector, as a stand-alone detector? Is everything going to be OK when the added load of the RF plate coupler winding and RF stage is added, will it just dampen all that overloading?

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on May 07, 2015, 03:26:11 AM
You could try the experiment of damping the Q with a shunt resistor to see if it's getting too high. Also the 2.2 megohm grid leak may well be a bit on the high side - I always used about 470k to 1 Meg. However, having said that, the 1927 ARRL handbook suggests values up to 7 Megohm with 201A or 199 tubes: the 199 data sheet suggests 2 - 4 Megohm. I found it easy to get squegging with such high values, but I was using tubes with an appreciably higher transconductance than they had. Typical transconductances for tubes back then was around 500 micromhos or so. One argument for the higher grid leak values is that they mean less damping of the tuned circuit, but if we assume a Q of 100 - which could be hopeful - that gives a dynamic resistance of 44kohm, so even a 470k will have little effect. The RSGB 1938 handbook suggests 1 - 2 Megohms. Sowerby's 1938 'Foundations of Wireless' likes higher values to avoid damping but he is concerned more there with the broadcast band where higher dynamic resistances were the norm anyway.

I have a feeling that some experimentation with tickler coupling and seeing where the oscillation starts as it is varied could be useful.

Interestingly, the Admiralty handbook of 1938 likes the idea of the detector not actually oscillating and a separate oscillator being used for the reception of CW. As I recall, this was done in the pre-WW2 Marconi CR200 receiver which covered something like 15kHz to 1MHz or so.

The 1927 ARRL handbook suggests an antenna coupling capacitor of two pieces of brass, 1/2 inch square, separated by 1/8 of an inch - I calculate that as 0.45pF, and too big a capacity there could affect where regeneration starts. Certainly, too big a capacity will affect the overload situation.

Terman (Radio Engineering, McGraw-Hill 1936) says that the feedback should be such that oscillation stops with a screen voltage between 20 and 40 volts for smooth regeneration, although it should be noted that he's talking of plate potentials of the order of 150 volts or so. If too low, the detection efficiency falls off because the tube gm is too low, while if too high, the oscillations start quite abruptly.
Some authorities like the idea of returning the grid leak to the positive side of the filament....

The idea of running with a higher L/C ratio is to get a better step up from the antenna.

Harmsworth's Wireless Encyclopedia (1924) doesn't go into optimum LC ratios, but likes the idea of a 500pF tuning capacitor for the medium wave (broadcast) band, which suggests the 50pF at 7 MHz is in the same ratio. The regenerative detectors in the ARRL handbooks use a wide range of LC ratios, tending to get smaller as the frequency goes up -presumably to take account of the effects of stray capacitance becoming more dominant.

Incidentally, what material is the coil former? Even at 7MHz, some plastics aren't very good, especially coloured ones. A quick and nasty check is to put a glass of water and the coil former in the microwave oven and zap it for 30 seconds or a minute or so to see if it gets warm. If it does, that tells you that it's lossy at 2.4GHz: if it stays cold, you can be pretty certain it's not lossy at 7MHz.

I think you need a capacitor to ground between the junction of the tickler coil and the RF choke. Something like 100pF.

So there has been an extensive trawl through my library for this. Interestingly, Scott-Taggart in 'Thermionic Tubes in Wireless Telegraphy and Telephony' (Iliffe, 1924) is very quiet on the subject.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 07, 2015, 04:48:55 AM
You could try the experiment of damping the Q with a shunt resistor to see if it's getting too high.

Across the coil, right?

Also the 2.2 megohm grid leak may well be a bit on the high side - I always used about 470k to 1 Meg. However, having said that, the 1927 ARRL handbook suggests values up to 7 Megohm with 201A or 199 tubes: the 199 data sheet suggests 2 - 4 Megohm. I found it easy to get squegging with such high values, but I was using tubes with an appreciably higher transconductance than they had. Typical transconductances for tubes back then was around 500 micromhos or so. One argument for the higher grid leak values is that they mean less damping of the tuned circuit, but if we assume a Q of 100 - which could be hopeful - that gives a dynamic resistance of 44kohm, so even a 470k will have little effect. The RSGB 1938 handbook suggests 1 - 2 Megohms. Sowerby's 1938 'Foundations of Wireless' likes higher values to avoid damping but he is concerned more there with the broadcast band where higher dynamic resistances were the norm anyway.

Good idea. I did some experiments with grid leak value early on in this saga, but haven't touched it since. For what it's worth, back then I tried various values between 2.2M and 4.7M. There was no audible difference, from what I remember. But I didn't try *lower* values. I will try a wider range: from 470K through 4.7M 6.8M.

I have a feeling that some experimentation with tickler coupling and seeing where the oscillation starts as it is varied could be useful.

The tickler coil was originally 2 turns, tightly coupled (close-wound directly below the tank, with no space in between). I tried 1 turn, which didn't work (hard to get any oscillation at all). I tried 3 turns, tightly coupled in the same manner, and this was slightly better than 2 turns although the difference is not great. In all cases (1, 2 or 3 turns) the tight coupling was necessary in order to get any oscillation. Moving the coupling even a few millimeters away from the tank killed the oscillation.

Incidentally, what material is the coil former? Even at 7MHz, some plastics aren't very good, especially coloured ones. A quick and nasty check is to put a glass of water and the coil former in the microwave oven and zap it for 30 seconds or a minute or so to see if it gets warm. If it does, that tells you that it's lossy at 2.4GHz: if it stays cold, you can be pretty certain it's not lossy at 7MHz.

It's an orange-colored pill bottle. I was wondering about that! I knew about the microwave trick but have never tried it -- will do so today (after removing the wire!!!!). By the way, the inductance-calculation errors also happened with the old 5µH coil: to get that value it should have been about 12 turns, 0.5" tall, but I couldn't get into the 40m tuning range without removing a couple of turns -- and theoretically 10 turns is only about 4µH.

(https://lh3.googleusercontent.com/-rhWUu6oFNzY/VUtm8BfbvLI/AAAAAAAAE28/IkbAulKK19o/s128/Submini_regen_coil_01.jpg?gl=US)

The next thing I'll try is an old, trusty Millen 45005 coil form: 5-prong, 1" OD. I'm usually reluctant to use them for experiments as they are hard to find, but I have a couple that are a bit messed up (drilled and scratched up by a previous owner, and with pins loose from excess soldering heat) and I'll play with those.

On a 1" form and with 20AWG wire (wire diameter 0.032"), 23 turns close-wound is about 0.92" long (allowing for the magnet-wire insulation thickness) and is supposed to yield 10.35µH. It comes close to your ideal 1:1 dia/length ratio.

Edited to add: however those Millen forms aren't very tall. They only have about 1.5" in useful height for winding. If the tickler and tank take up, say, 1.1" using 20AWG magnet wire, then there's hardly enough room at the top for the plate coupling coil from the RF stage, assuming that the coupling coil is half the height of the tank coil. At the very least the coils will be very close together. So perhaps I need to find another 1" form; or I could experiment with thinner magnet wire, to increase the number of turns available with a given coil height.

I think you need a capacitor to ground between the junction of the tickler coil and the RF choke. Something like 100pF.

Will try it.

So there has been an extensive trawl through my library for this. Interestingly, Scott-Taggart in 'Thermionic Tubes in Wireless Telegraphy and Telephony' (Iliffe, 1924) is very quiet on the subject.

It's clear that the old-timers managed to get excellent performance out of their regenerative sets, but it was hard work and very fiddly!

For what it's worth, monitoring the band this morning, the overload is much less. Some signals have no overload at all. So among other things, band conditions last night may have been a factor (in any case, nights are more propitious to strong-signal overload in my experience). I am hoping that the alterations that you suggest -- and changing the coil form factor -- will yield progressive improvements.

Edited to add: I also assume that you agree with me that it's useful to get the best possible performance out of the detector working in stand-alone mode, prior to adding the RF stage. One thing at a time....

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 07, 2015, 05:29:24 AM
I have a general question: how does a grid-leak detector work?

I've looked this up in several books but unfortunately they just tend to present you with a schematic and say, "this is a grid-leak detector" without further ado. This is often in the same chapter that explains diode detectors and triode/plate detectors -- which are easy to understand and usually explained in detail.

Why should a resistor, shunted with a capacitor, inserted into the grid circuit turn an amplifier into a rectifier/amplifier? The grid leak resistor is a standard technique for obtaining grid bias ... so it must have something to do with the addition of the shunt capacitor, huh?

Incidentally I love the way that the older British books describe it as the "leaky grid" detector. Sounds like a case of incontinence!

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 07, 2015, 06:10:31 AM
For the coil form, I just ordered a length of this stuff: 1" OD polycarbonate pipe (trade name Lexan). It will be useful if the Millen forms turn out to be too short to be useful; and will be a less destructive medium for experiments (I hate to use those Millen forms for anything other than a final product).

http://www.amazon.com/Polycarbonate-Tubing-Wall-Clear-Color/dp/B00193SSPG/ref=pd_sim_sbs_indust_2?ie=UTF8&refRID=04TKT98TWZRMBWZZZ12P (http://www.amazon.com/Polycarbonate-Tubing-Wall-Clear-Color/dp/B00193SSPG/ref=pd_sim_sbs_indust_2?ie=UTF8&refRID=04TKT98TWZRMBWZZZ12P)

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: JAHAM2BE on May 07, 2015, 06:37:50 AM
I have a general question: how does a grid-leak detector work?

A simple explanation can be seen starting on p.2 here:

http://www.inictel-uni.edu.pe/sites/default/files/archivos/2015/publicaciones/04/the_modern_armstrong_regenerative_receiver.pdf


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 07, 2015, 07:06:32 AM
A simple explanation can be seen starting on p.2 here:
http://www.inictel-uni.edu.pe/sites/default/files/archivos/2015/publicaciones/04/the_modern_armstrong_regenerative_receiver.pdf

Perfect, thanks; even I can understand it now! Also, thank you for stimulating my extended interest in regenerative radios, a couple of years ago -- when you gave me quite a lot of help with my then-balky ARRL regen build and its unusual Vackar oscillator.

Edited to add: incidentally, the article you link to has the alternative circuit in which the grid leak is connected between grid and ground, instead of being in series with the tuned circuit (while the capacitor remains in series). I guess I'll give that a try, too....

I will also be experimenting with a throttle capacitor (instead of the adjustable screen voltage) for regeneration control. But that comes later....

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on May 07, 2015, 07:14:18 AM
Martin

I'm having trouble with some of your numbers.

Quote
On a 1" form and with 20AWG wire (wire diameter 0.032"), 23 turns close-wound is about 0.92" long (allowing for the magnet-wire insulation thickness) and is supposed to yield 10.35µH.

0.004 inch thick insulation seems a lot.  Looking at my wire tables, I see ARRL quote 29.44 turns to the inch: Scroggie, for 21SWG (which is 0.032 inch) quotes 28.96 turns to the inch. That averages to 29.2 tpi or 0.03425 inches, giving a enamel thickness of 0.00123 inches, which sounds more likely. Thus 23 turns will be 0.79 inches long and have an inductance of 10.94 microhenries.

Not sure how you get 10.35 microhenries. I make 23 turns, 1 inch diameter and 0.92 inches long to have 9.3 microhenries.

I use Wheeler's formula  L = (a2 n2)/(9a + 10b), where a is the radius in inches, b is the length in inches and n is the number of turns.

How is the coil mounted? Is there a metal chassis or something that would pass as such? The coil needs to be at least one and preferably 2 diameters away. If you have a metal plate on which the coil is mounted, you could well get problems if either the tuning coil or the tickler get too close to the metal.

In the 6SN7 regen in the ARRL handbooks from about 1947 to 1957, for 40m, they use  4.3 microhenries from 13.5 turns and a tickler of 1.25 turns spaced 1/4 inch away: the plate and grid coil ends should be at opposite ends of the form. For 80m, they use 17.6 microhenries from 25 turns of #26, close wound, with a tickler of 4 turns, spaced 3/8 of an inch.

From which you can see quite a variation in L/C ratio, and surprisingly small tickler coils.

Shunt the detector coil with decreasing values of resistor, starting with 470k and see if it smooths the regeneration onset.

I also feel that you need a smaller coupling capacitor to the antenna.

Quote
For the coil form, I just ordered a length of this stuff: 1" OD polycarbonate pipe

It is about 1000 times more lossy than polystyrene, polyethylene or Teflon, and about twice as lossy as the mica loaded phenolic Millen forms. Slightly better than PVC, and about half as lossy as most woods. If it is black and carbon loaded, it is likely to be worse than that figure, while it's about 2/3 of the loss of FR4 printed circuit material.



Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 07, 2015, 07:31:31 AM
0.004 inch thick insulation seems a lot.

It was just a wild guess, so I'm not surprised I'm wrong.

Not sure how you get 10.35 microhenries. I make 23 turns, 1 inch diameter and 0.92 inches long to have 9.3 microhenries.

I use Wheeler's formula  L = (a2 n2)/(9a + 10b), where a is the radius in inches, b is the length in inches and n is the number of turns.

I used an online calculator which, itself, claims to use Wheeler's formula (the calculator is here: http://www.66pacific.com/calculators/coil_calc.aspx (http://www.66pacific.com/calculators/coil_calc.aspx)). Before building any more coils I'll do the calculations myself, which is a good exercise in its own right!

How is the coil mounted? Is there a metal chassis or something that would pass as such? The coil needs to be at least one and preferably 2 diameters away. If you have a metal plate on which the coil is mounted, you could well get problems if either the tuning coil or the tickler get too close to the metal.

It is mounted with a single brass screw in the middle of the base, onto a piece of wood. The shaft of the tuning capacitor is about 1.7 diameters away, but otherwise there is no metal.

...the plate and grid coil ends should be at opposite ends of the form.

Can you elaborate? You mean the "hot" ends right, with both of the "cold" ends in the middle? And the tickler underneath the grid coil?

Shunt the detector coil with decreasing values of resistor, starting with 470k and see if it smooths the regeneration onset.

I also feel that you need a smaller coupling capacitor to the antenna.

Will experiment with both.

It is about 1000 times more lossy than polystyrene, polyethylene or Teflon, and about twice as lossy as the mica loaded phenolic Millen forms. Slightly better than PVC, and about half as lossy as most woods. If it is black and carbon loaded, it is likely to be worse than that figure, while it's about 2/3 of the loss of FR4 printed circuit material.

Phooey. OK, will try to find something less lossy. BTW I have a large collection of Air Dux style coils, in all sorts of diameters and winding pitches. Presumably those would be good in this application, but "adjusting" them is a destructive process and I hate to do that until I have a better idea of the optimal inductance and configuration in this application. I also have a nice 1" grooved ceramic form but I was saving that for my future transmitter VFO.

Edited to add: Also have a sizable collection of original Amphenol 1.25-inch polystyrene forms, but those are too precious to waste on experiments....

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on May 07, 2015, 07:56:39 AM
Quote
Can you elaborate? You mean the "hot" ends right, with both of the "cold" ends in the middle? And the tickler underneath the grid coil?

Yes.

Quote
I used an online calculator which, itself, claims to use Wheeler's formula

Wheeler's formula has some constraints, but for coils with reasonable L/D ratios and not too small a diameter, it should be within 5%. Connecting leads, as a rule of thumb, are about 20 - 25nH/inch. We always reckoned on 1nH/mm in integrated circuit packages which is 25nH/inch.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 07, 2015, 08:23:20 AM
Thanks Peter. Must get back to my "day job" for at least a few hours but for the time being one last question.

While I cannot claim to have much to do with the design of the current experiment, I am taking pride in understanding exactly the purpose of every sub-circuit, and the function of every component.

So now let me make sure that I understand the RF amp biasing circuit that you designed. Here is my "take":

It's a directly heated cathode, therefore we cannot use the usual cathode biasing resistor, nor can we get enough bias with a grid-leak resistor. Therefore we need a source of negative potential, but the ground bus is already the negative side of the receiver's power supply. For that reason, we need to "float" the LC circuit. The 2000-5000pF capacitor at the bottom of the coil functions as a RF bypass (but why is it necessary, at that point in the circuit?). The 1µF on the pot wiper helps smooth the DC bias. The pot varies the grid bias to provide adjustable gain.

Correct?

(Concerning the -4.5V bias, this will either be a separate battery, or it will be tapped with a series resistor at the bottom of the B+ battery -- this is similar in general concept, I guess, to the taps that are sometimes used on HT transformers to provide bias voltages.)

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on May 07, 2015, 09:19:35 AM
Quote
The 2000-5000pF capacitor at the bottom of the coil functions as a RF bypass (but why is it necessary, at that point in the circuit?).


That point needs to be at RF ground potential. If you let it float, RF wise, you aren't providing an input to the RF stage between filament and grid.

 
Quote
The 1µF on the pot wiper helps smooth the DC bias. The pot varies the grid bias to provide adjustable gain
.

Pretty much. The electrolytic bypasses noise from the pot as it is rotated.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 07, 2015, 01:53:21 PM
I think you need a capacitor to ground between the junction of the tickler coil and the RF choke. Something like 100pF.

Come to think of it, I think that's where the variable "regeneration throttle" capacitor would go, in the older designs that use it. I was already planning to try this method of regeneration control.... I could wire the set with both the capacitor control and the potentiometer control and see which works better....

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 07, 2015, 06:19:06 PM
This evening I've spent a little while fiddling around with the current receiver, prior to attempting some of the coil and circuit changes mooted above. So, this is the version with (nominally) a 50pF/10µH tuned circuit, with the coil wound on a 1.25" diameter pill bottle.

In these evening hours plenty of signals are coming in on 40m, many of them heavily overloading the receiver -- this is not just the powerful SSB stations but lots of CW. The sensitivity is impressive so I have great hopes for this receiver once it's been improved!

Here's one new nugget. Until now I've been using a fixed 20pF series resistor on the antenna input (the schematic shows a variable capacitor, but actually I've been using fixed ones and playing around with the values). The capacitor is necessary because if you connect the antenna directly to the "hot" side of the tuned circuit, the damping is heavy enough to kill oscillation. It also helps control overload.

What I hadn't appreciated until now is the substantial "detuning" effect of this series capacitor. I had initially adjusted the value of the coil (adding or removing windings) with the 20pF capacitor on the antenna. This evening, when I changed that capacitor to 10pF, it seemed to be helpful in reducing overload (especially on SSB signals) but it also shifted the resonant frequency of the receiver upwards by at least 200kHz (out of the CW part of the band and into the top of the SSB area). Reducing the capacitor again, to 4.7pF, took the receiver out of the ham bands altogether and into SW broadcast.

So I suppose it's not surprising that, among other things, I've been having trouble using a "true" 10µH coil in that tuned circuit: the series capacitor is dramatically shifting the resonant frequency, so I've been having to adjust the inductance of the coil to compensate and bring the resonance back into the ham band.

The antenna is currently connected (through that capacitor) directly to the top of the tank coil. I have also tried winding an antenna coil above the tank coil, with the bottom connected to ground and the top connected to the antenna, but the set is almost dead when I try that.

If the antenna is connected directly to the detector, stability is very hard to achieve. Not only are there substantial "hand capacity" effects but almost any variable factor at the front of the detector messes things up in one way or another. So it will be very interesting to see what happens when that is replaced by the (relatively) stable load from the plate coil of a preceding RF stage.

Judging from the fact that when I wound an "antenna coil" above the detector coil, the signals were very weak, the coupling from the RF stage will have to be fairly tight and/or the gain from the RF stage will have to compensate. We'll see.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: W1BR on May 07, 2015, 08:51:15 PM
The antenna coupling capacitor can also end up limiting your tuning range, if it is set too tightly, or it can kill regeneration as you noted.

You might try playing around with a link coupled antenna on the coil...

Pete


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on May 08, 2015, 02:01:46 AM
The 1927 ARRL design used a fixed 0.45pF. For a variable coupling capacitor, you want something no bigger than 2pF, with a minimum  of around 0.2pF. That usually means making one....


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 08, 2015, 02:24:54 AM
You might try playing around with a link coupled antenna on the coil...

As I mentioned earlier, I have already tried an antenna coil wound above the tank coil, with its bottom grounded -- I didn't know the term "link coupling" until today but that's what it was. It worked, but sensitivity was extremely low.

The 1927 ARRL design used a fixed 0.45pF. For a variable coupling capacitor, you want something no bigger than 2pF, with a minimum  of around 0.2pF. That usually means making one....

A few minutes ago I tried a gimmick capacitor (I simply twisted the incoming long-wire antenna onto another piece of wire connected to the antenna input; only a couple of turns). This worked, with no overload. I don't yet know how sensitive the set is, in that configuration, because it was so "detuned" that all I could hear was a couple of broadcast shortwave stations. Now, let's rewind the coil and bring the ham band back into tuning range ... and see what happens. With the gimmick capacitor, regeneration occurs rather earlier in the dial rotation.

Edited to add: When I got into this hobby three years ago, and started building radios, I remember thinking that I would be spending much of the rest of my life winding coils....

An exciting election night in the UK. I'm from a multigenerational British Labour Party family. My grandparents helped run a constituency party office in deep-blue Hampshire and I was an adolescent Labour canvasser there (a hopeless task!). I see that in their constituency, the UKIP candidate came second to the winning Tory, this time around. On the one hand, given my quasi-tribal political proclivities, I find the national result depressing. On the other hand, I am always inspired by the spectacle of democracy in its full glory -- despite the unsavory aspects and low opinion of politicians that RZP sometimes evinces, and with which I sometimes agree! I was particularly impressed by Ed Balls' not only congratulating his opponent, but telling her that she will make a good MP. There really aren't that many countries where power changes hands (or, in this case, doesn't change hands nationally) so gracefully.

(For hams outside the UK: yes, there is a famous British Labour Party politician whose name actually is Ed Balls.)

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 08, 2015, 02:53:11 AM
Success! I added another turn to the coil, bringing 40m into range. With the "gimmick" antenna capacitor, sensitivity seems decent (a bit hard to tell for sure but at about 6 a.m. local, I am hearing plenty of CW stations).

Now here's the really interesting part: hand capacity effects are sharply reduced. I can now actually tune in a signal, then take my hand away from the tuning dial, and the change in CW beat note is relatively small. With the old antenna capacitors (20pF or 10 pF) a signal could drift completely out of range when I moved my hand.

I am not totally sure why [reduced antenna capacitor = reduction in hand-capacity effect] and would appreciate enlightenment on the theory side but this is extremely encouraging.

If this change is as beneficial as it initially seems, that's great news. Such a simple modication, eh?

Edited to add: Upon further testing, I notice that regeneration is extremely smooth. At one end of the 50K pot, nothing. Rotate about one quarter of dial, good for AM/SW-BC reception. Rotate another quarter, threshhold of "noisy" oscillation, good CW reception. Another quarter, reduced sensitivity but good for strong SSB signals. Fully rotated, a loud squeal!

Edited again to add: I am now able to ground the set and the set continues to work (in the past, with the "old" 20pF antenna capacitor installed, grounding the set killed the regeneration). I also grounded the small alumin(i)um front panel (confined to the area around the tuning capacitor). When the set is grounded, the regeneration control is somewhat rougher and the set is noisier -- there is somewhat better reception without grounding. We'll work on that; it is mainly an issue of S/N ratio rather than sensitivity per se.

Another edit: the audio volume is considerably lower than it was before (mind you, it was quite deafening before). This is probably partly because of reception conditions in the early daylight hours here, but I suspect it's also because of the change in value of the antenna capacitor. May need to add another stage of audio eventually, depending on what happens when the RF stage is added.

And another: I just checked the tuning range (for this, I use a Sony SW-1 miniature short wave receiver with digital readout, and spot the regen receiver's oscillator). The width of the tuning range is now 165 kHz. According to my calculations, with the current values of the capacitor network in the detector stage (see schematic http://tinyurl.com/lva95cm (http://tinyurl.com/lva95cm)), the width should be 230 kHz. Presumably the restricted tuning range is due in part to the presence of an antenna "capacitor" even if it's only a low-value "gimmick." A reminder: if you are looking at the schematic, the current build doesn't yet include the RF stage. The antenna is currently connected to the top of the detector tank coil, via the gimmick.

The 1927 ARRL design used a fixed 0.45pF. For a variable coupling capacitor, you want something no bigger than 2pF, with a minimum  of around 0.2pF. That usually means making one....

Couldn't you get the very low-C value by using a small-value variable in series with a fixed capacitor?

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 08, 2015, 06:07:43 AM
The 1927 ARRL design used a fixed 0.45pF. For a variable coupling capacitor, you want something no bigger than 2pF, with a minimum  of around 0.2pF. That usually means making one....

Well, if you confine yourself to fixed values, Mouser has plenty of choice:

(https://lh3.googleusercontent.com/-HJtl8JTliwc/VUy0dVmLI9I/AAAAAAAAE3Y/Ax5OZfNonKo/s512/Mouser%2520Capacitors.jpg?gl=US)

You could mount a selection on a rotary switch but I presume the capacitance of the switch would overwhelm the capacitors themselves!!

A strong argument for adding that RF stage is that varying the value of the antenna coupling cap at the front of the RF stage will then, presumably, have no major effect on the range of the detector tuning circuit. I have this extraordinary ambition to have a calibrated main tuning dial one day!

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1GMX on May 08, 2015, 09:26:36 AM
Need fractional to small value caps.... Coax, nominally 20pf/foot (you can find
the exact amount for what you have).  So a 1" piece is 20/12 or 1.6pf. 

The usual twisted wire "gimick" also works and if you have a capacitance
meter you can set it.  Generally for regens small coupling is better as it
reduces the antenna loading on the tuning and increases the Q of the tuned circuit.

If the circuit you published is where you are, more or less, the distance
from the plate coil of the first tube  (RF amp) to the second (tuned coil)
can be increased with the same improved Q and more stable tuning at
the expense of gain which likely with a RF amp ahead will not be a problem.

Hint if you can make the coil such that the RF coupling, main tuning,
and tickler are adjustable for position you may find better optimization.


Allison



Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1GMX on May 08, 2015, 10:25:02 AM
Part 2...

Screen bias regeneration control and throttle capacitor.   The answer is both.

The detector is grid leak.

http://www.angelfire.com/electronic/funwithtubes/Grid_Leak-1.html

Short form when the grid  has a positive signal there will be current flow (maybe very small)
and the capacitor in the regen circuit will acquire charge and in the negative part of the cycle
act as a bias.  Its the early approach to self biased.  Since a regen is either oscillating or not
that self bias helps to keep things sorta tamed down.  There is a magick point for the R and C
that is generally best determined by experiment as for a given tube and circuit there is wide
variation possible and a wide acceptable range of values.  Hint is the currents are small so
the resistor used as the discharge path should be large if that is the case then the C must be
small but there is a balance there.  If to large you get problems with controlling the electron
stream in the tube (they have finite leakage due to electrons that collide with the grid).  If the
C is too small you can't couple RF to the tube well enough.  the Why on that is the grid
at low frequencies is a capactitor (two plates separated by an insulator and vacuum is a
good insulator) and that means the grid input has a finite capacitance.  Its best if the coupling
cap C is about 10x more than the grid internal capacitance (from the data sheet 1AD4 about 4pf)
so a 50pf to 100pf cap is about right.  Since the resistor discharges that cap there is a time constant
and acceptable range for that.  Too high and you get audio tone, too low and there is signal distortion.
Start at 1M and 100pF as starting point.  Use a good low loss Ceramic (C0G) or dipped silver mica
for lowest loss.

Same site and different page:  http://www.funwithtubes.net/

Now, controlling a regen.  Three popular ways, control the gain of the device (screen bias),
the feed back path (tickler with throttle capacitor) and mechanical adjustment of feedback
(rotating or moving the tickler) relative to the grid coil.  First, they all work.  One of the great
engineering head banging exercises is optimizing a regen, to many variables, some you
can control.

Using pentodes, there is a relationship to screen voltage to plate(anode) voltage. 
That being, lower screen voltage lowers available gain and also lowers the strong
signal handling ability.  Increasing plate voltage increases signal handling (overload).
So for regen work a low screen voltage make the tube lower in gain and a bit tamer. 
Higher plate voltages allow bigger signal without distortion.

Throttle cap or mechanical feedback adjustment.  An amplifier becomes an oscillator 
when the ratio of gain to feedback exceeds 1.  The more gain in the amplifier the less
feedback needs to oscillate. A regen is an oscillator with a total gain (at signal frequency)
of just less than 1 (like .9999998).  At that point the signal is amplified greatly but there is
not enough total feedback to sustain oscillation.  So for CW the gain is more than 1,
for SSB more than 1, and for AM just barely 1.  We can control the feedback (assume
fixed gain devices) by reducing the means needed to achieve some measure of control
of the signal fed back.  A throttle cap is one way.  Moving the coil or turning it 90 degress
will reduce what is coupled back.

Now we put the two last paragraphs together.  With more screen voltage you get more
gain and returned signal but, only to a point then gain gets too high and regeneration
does not transition softly to oscillation but instead pops sharply.   Even the lowly battery
tubes have more than enough gain to oscillate vigorously so controlling feedback and
gain you can achieve a measure of control and stability.  So the answer is, use both,
and experiment.  Vacuum tubes are adjustable gain devices (either by bias in any
form or even lowering the heater current) and you have a coil for feedback plus
random wires that also contribute feedback.  So there are a few "knobs" you can
tune for best results.
 
So when you mix all of that plus mechanical layout optimization seems hard but experimentally
its very achievable.   The very best regens are mechanically strong with good RF layout
as well as operating point selection by bias and other means.


Allison


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 08, 2015, 10:26:12 AM
Need fractional to small value caps.... Coax, nominally 20pf/foot (you can find the exact amount for what you have).  So a 1" piece is 20/12 or 1.6pf. 

The usual twisted wire "gimick" also works and if you have a capacitance meter you can set it.  Generally for regens small coupling is better as it reduces the antenna loading on the tuning and increases the Q of the tuned circuit.

If the circuit you published is where you are, more or less, the distance from the plate coil of the first tube  (RF amp) to the second (tuned coil)
can be increased with the same improved Q and more stable tuning at the expense of gain which likely with a RF amp ahead will not be a problem.

Hint if you can make the coil such that the RF coupling, main tuning, and tickler are adjustable for position you may find better optimization.

Allison

Thanks! The RF stage is not built yet. First, I will be experimenting with a new form factor for the coil: reducing the diameter from 1.25" to 1" to try to get improved Q (because it will make the diameter/length ratio of the tank coil closer RZP's suggested 1:1).

The 1.25" diameter pill bottle has been easy to experiment with because I have been winding the coils and securing the ends directly onto the outside of the form with Scotch tape, then running the leads horizontally to the nearby terminal strips (I have not been drilling holes in the form or running the leads vertically down the inside). This makes it extremely easy to experiment with (1) number of turns and (2) spacing between them. It is sub-optimal in the sense that these horizontal leads (parallel to the turns in the coil itself) must be adding phantom inductance to the coil, but at the moment I'm just trying to get orders of magnitude for number of turns and spacing between them, before making better coils on better forms.

I ordered some 1" polycarbonate (Lexan) pipe, hoping to use the same "trick" for experimentation -- RZP point out that it's very lossy, but I was hoping to use 1" Millen 45005 forms for the final coils and he says they themselves are lossy (because mica-filled) in about the same order of magnitude as the Lexan. So perhaps the Lexan is appropriate for experimentation after all. But now I'm not so sure. The main problem with the Millen forms is that they only have about 1.5" useful height, which is a problem considering I need to wind three coils on them (tickler, tank, coupler from the RF amp). I'll need to use thinner magnet wire (so far, it's been 20AWG) to get all the coils to fit, and won't have much leeway to move the coils apart from each other.

I wonder how important the 1:1 D/L ratio is ... right now it's more like 1.5:1, on the 1.25" pill-bottle form. I have lots of Amphenol 1.25" polystyrene forms; once the basic coil parameters are "figured out" experimentally on the pill bottle, the final coil versions could be on an Amphenol form. Not only are these excellent forms, but using a plug-in coil would enable me to add more bands later (specifically, 20m). One way of getting the 1:1 ratio would be to space-wind the form, eh? Using a length of string or hookup wire for the spacing, and then removing it afterwards?

Yet another possibility would be to use lengths of Air Dux coil. I have a fairly large variety of these in all sorts of wire thicknesses, pitches, diameters and lengths.

Still pondering all this ... will probably experiment with various options. As I said earlier, it seems that building radios often boils down to: "winding coils."

73 de Martin, KB1WSY



Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 08, 2015, 10:38:53 AM
Screen bias regeneration control and throttle capacitor.   The answer is both.

That's what I had pretty much already decided -- I will install both the screen voltage control, and the throttle capacitor. I'll pass on the mechanical/variometer approach for the time being.

Thank you for your detailed explanation of the workings of the grid-leak detector.

First, they all work.  One of the great engineering head banging exercises is optimizing a regen, too many variables, some you can control.

It is indeed mind-boggling. Ironically, it was once a rite of passage for most hams to build a regen set as a "beginner's radio" when in fact the workings of the circuit are very complex. You can build a one-tube radio that combines RF amplifier, oscillator, detector and AF amplifier in a single tube. The component count is low but I wonder how many novice builders really understood what was going on -- and how many of them managed to get the decent results that are possible with a carefully built and optimized regen.

Thank you also for your detailed explanation of the wonders of regeneration.

One day I'll find a book on the great legal battle between DeForest and Armstrong ... the technical arguments must have been quite interesting!

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: WB6BYU on May 08, 2015, 12:18:28 PM
Switching small capacitor values gets difficult because the switch wiring itself adds capacity.  A better
approach is to mount a small variable capacitor on insulated stand-offs with an insulated shaft to a knob
on the front panel.  Getting a cap where the stray capacitance across it may be the biggest problem -
a neutralizing capacitor (two parallel flat plates, one attached to a screw thread to adjust the spacing)
might do the job.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on May 08, 2015, 01:32:27 PM
The L/D ratio is very non critical from about 0.75:1 to about 2.5 :1 and you won't notice the difference.

Consider the antenna as a low resistance in series with a capacitor if it's under 0.25 wavelengths long. Now at one frequency, a series RC circuit has an equivalent RC parallel circuit, so you can see that the antenna with the series capacitor looks like a shunt capacitor and resistor across the detector tuned circuit. The smaller the series capacitor the smaller the effective shunt capacitor and the less the detuning.

Coil former losses can generally be ignored in receivers unless Q is very important, but a lot of the pre-WW2 coil forms had ribs along their length so the coil winding was effectively spaced away from the form to improve Q......The 'kid's crystal set' of the 1920's with a cats whisker and lead pyrites crystal used the cardboard oatmeal drum to wind the coil on.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on May 08, 2015, 01:36:32 PM
I omitted to mention that assuming Qs of 75 in the RF stage grid and plate (probably optimistic) any plate load over about 70kohm for the pentode should be stable. It's plate impedance is about 500kohm, so the damping effect of the plate impedance on the detector should be pretty negligible.

Alison, do you know who first established the stability criteria for tuned tube amplifiers? (H<2)


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 08, 2015, 01:41:18 PM
The great grid-leak experiment:

Keeping the 100pF grid-leak capacitor and varying the grid-leak resistance:

--470kΩ = narrow band of useful regeneration on the regen pot, very faint signals audible.

--1MΩ = similar to above, narrow regeneration range, slightly more gain on signals.

--2.2MΩ = much more vigorous oscillation, broader range of oscillation, more signals audible, somewhat "rougher" oscillation threshold (this is the resistor I've been using all along, up 'til now).

--4.7MΩ = even more vigorous oscillation, and yet more signals, additional roughening in the threshold.

--6.8MΩ = the most signals, the strongest oscillation, roughest threshold; I listened for audio distortion and didn't hear any, but the caveat is that there wasn't a lot of activity on the band here and no really strong signals that would invite overload/distortion.

Unfortunately reception conditions weren't great. There was a loud broadband cyclical whooshing sound that sounded man-made and located nearby; and it was the early afternoon, not a strong time for 40m propagation.

I've decided to leave the 6.8MΩ resistor in place. I rather suspect I'll end up going back to something closer to the original 2.2MΩ value, but it's worth experimenting.

It's tempting to put a potentiometer in that place ... presumably it ought to be close to the tube with short leads, and with a shaft extender to reach the front panel.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 08, 2015, 06:28:48 PM
Adding a variable-capacitor regeneration throttle

I added a "variable capacitor throttle" for extra control over regeneration. The capacitor is basically a variable RF bypass between the choke-end of the tickler and ground:

(https://lh3.googleusercontent.com/-BttTQiSrlqA/VU1es-CBCVI/AAAAAAAAE30/ozGU4CZUafQ/s800/Sumbini_regen_throttle.jpg?gl=US)

(I have also updated the main schematic that appeared a few pages back --  http://tinyurl.com/lva95cm (http://tinyurl.com/lva95cm) --  to add this new capacitor.)

I used a 140pF variable because that's what I had on hand ... but it seems to be about the right value in this application.

It provides precise additional control over regeneration. When used in combination with the screen-voltage control on the detector tube, it makes a big difference to ease of reception and particularly, adds considerable control over (1) the signal-to-noise ratio and (2) controlling overload. When copying a weak signal, I keep the screen regeneration control low and close the capacitor plates for maximum "threshold of oscillation" sensitivity. With a stronger signal, I increase the screen regeneration control and open up the capacitor. This is excellent for avoiding "pulling" of CW signals, and for clean demodulation of strong SSB signals.

The presence of the capacitor also provides additional, and welcome, overall gain (G3RZP had already suggested adding at least a fixed capacitor at that point in the circuit).

Edited to add: So far the only downside of the throttle capacitor is that adjusting it has a noticeable "detuning" effect. I am not sure whether anything can be done about that -- which means that calibrating the main tuning dial will be difficult (I think there will have to be a "bandset" or "calibration" control on the front panel because the tuning points will change when regeneration is adjusted with the capacitor). I believe this is a known issue with regenerative sets. Perhaps adding the extra isolation of an RF stage will help???

(On the other hand, the screen regeneration potentiometer doesn't seem to have any detuning effect.)

After using the new "capacitor throttle" configuration for a little while, I changed the value of the grid leak bias resistor back from its "experimental" value of 6.8MΩ back to its original value of 2.2MΩ. I was finding that the larger-value grid-leak resistor was now a source of too much noise.

Today's other major improvement was replacement of the 20pF antenna series capacitor with a small-value "gimmick" cap (two wires twisted together), which had the following effects:
(1) Eliminated almost all of the problems with stability and hand capacity effects. It is now possible, for example, to tune in an SSB signal and hear a normal voice without "Donald Duck" making an appearance every time I move my hands toward or away from the dial, and I can listen to a long QSO without needing to "touch up" the tuning. Also, the cords on my high-Z headphones are no longer affecting the tuning or the regeneration when the cords are moved or when I grasp them with my hands.
(2) Enabled grounding of the set, which had originally been impossible because it choked off oscillation. Grounding has caused a drop in sensitivity, but also led to an obvious increase in stability and additional reduction of hand-capacity effects. Grounding also eliminated a "microphonics" audio feedback problem I was having in the audio stage (this problem appeared when I switched from resistor load to a 7-Henry choke load), and solved several other problems with "touchy" components.
(3) Eliminated the problem I was having with restricted tuning range.

I'm very excited by the progress so far. Now it's time to work on the inductors and build the RF stage.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 09, 2015, 04:35:50 AM
Some questions about the 1AD4 tube and about the B+ supply:

(I have edited this post to fix the link to the 1AD4 data sheet. The link was correct, but the host website now seems to be having issues, so I've now linked to a different but equivalent version on a different site.)

Question 1:

According to the data sheet (http://tinyurl.com/pm4pn56 (http://tinyurl.com/pm4pn56) -- this link has now been fixed), this tube's suppressor grid is composed of "two deflector plates," one of them connected to the negative side of the filament supply, the other one connected to the positive side. Der leitende Überzug ist mit dem Anschlussdraht 3 verbunden. g3 besteht aus zwei Platten, die mit je einem Heizfadenende verbunden sind (adding random German because it sounds so nicely technical). For simplicity, I have schematically drawn these tubes as if they are traditional-suppressor types, internally connected to only one side of the filament. Can someone explain the advantage of the 1AD4's suppressor-"grid" configuration? Tnx.

Question 2:

Concerning the B+ power supply. For the "desktop" (non-portable) version of this receiver, I'll be switching from miniature low-capacity (55mA/h) 23A 12V disposable batteries to high-capacity (600mA/h) rechargeable 9V Lithium-Ion batteries. Currently there are four 23As wired in series. With new 23A batteries, this yields a B+ voltage under load of about 51V.

What I have found is that the receiver's performance deteriorates sharply once the batteries have discharged down to about 42V. This could be a voltage issue; or I guess it could be that the discharged batteries are struggling to supply a steady current??

A 9V LiOn battery has a nominal voltage of about 7.4V and an "almost discharged" voltage of about 5V. Presumably however, when fully charged it's more like (say) 8.5V, at least for a short time. I'm thinking of using seven of these cells. This would yield nearly 60V at full charge, about 52V at the "nominal" 7.4V and about 35V when nearly discharged. The alternative would be to use only six batteries, which would yield nearly 45V "nominal" but probably wouldn't take all that long to dip below the 42V below which my set's performance deteriorates.

So, looking again at the 1AD4's data sheet (http://tinyurl.com/pm4pn56 (http://tinyurl.com/pm4pn56) -- this link has now been fixed). It specifies a "typical plate voltage" in Class A amplifier operation of 45V. Depending on the manufacturer, the "absolute maximum" plate voltage is either 90V or 100V.

Am I right that, therefore, putting voltages as high as about 60V on the plate (at the top of the LiOn battery's charge cycle) would be absolutely fine? Safe for the tube, and not likely to deteriorate the receiver's performance (in fact, quite the opposite)?

Question 3:

With the B+ supply that I have now (maximum of 51V, using low-capacity miniature batteries) I have never been "zapped" even though I've made no special effort to avoid putting my fingers across the supply while the set is on -- the B+ has its own on-off switch which is a cheap miniature knife switch with exposed metal lever and contacts. I wonder whether increasing to a maximum of 60V (and using batteries with 10 times more capacity) will make any difference. Am I courting electrocution by being so cavalier? I promise I won't sue you, whatever your answer is!

Why batteries?

Edited to add: Perhaps you are wondering why I'm using a battery supply for the shack-bound receiver, rather than just building an AC-powered PSU. Well, several reasons. (1) Later on, a portable version of the set will be built, so keeping the battery supply is a useful "proof of concept." (2) Batteries should help eliminate hum issues, especially since the 1AD4 is a directly heated tube. (3) There's something cool about a battery-powered tube set and besides, it will be "portable enough" to be hauled to a picnic table on Field Day or whatever, and I can monitor the bands during power cuts! (4) If I started using an AC PSU, there would be no reason not to use standard indirectly heated tubes, which have higher power consumption but also higher performance and are easier to design circuits for!

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on May 09, 2015, 05:06:44 AM
The usual requirement for marine radios was that anything over 50 volts had to be accessible only by the use of a tool such as screwdriver.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: W1BR on May 09, 2015, 06:34:49 AM
I've had good luck with battery voltages as low as 9 volts (for the detector) on some of my sets.  "Smallest" tubes I've used are type 1G4, 1H4 and some '30's.  Battery powered regens are cool.

Pete


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 09, 2015, 06:55:48 AM
I've had good luck with battery voltages as low as 9 volts (for the detector) on some of my sets.  "Smallest" tubes I've used are type 1G4, 1H4 and some '30's.  Battery powered regens are cool.

Pete

Yes, I've heard several people mention their success with low plate voltages. So I'm a bit puzzled that I seem to need the full 45V to get decent performance. At one point I deliberately lowered it to 36V (by using three 23A batteries in series instead of 4) and the receiver stopped working altogether -- no oscillation, at any point on the screen regeneration control.

It's a bit frustrating, in the sense that several of the design guides I've consulted state that you'll get better performance overall if you drive the detector tube with a lower plate voltage -- this is supposed to have been one of the big regen-radio design tips in the 1920s. For a while I thought my failure with low plate voltages was because of the "modest performance" of the 1AD4 compared to cathode-type tubes, but then I realized that the tubes used in the 1920s were a lot wimpier than my 1AD4s.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1GMX on May 09, 2015, 05:11:42 PM
Q1, suppressor grid.  In the 1ad4 it was designed as a hearing aid tube and its a pentode.
The grid give more power gain, also better isolation from control grid (G1) to plate.

Q2 voltage.  Higher is not an issue the tube will accept it.

     Note if you having troubles at 42V and below that means you need a turn or two more on the tickler.

Q3 threshold for shock is in that region for most (varies with skin resistance and resulting current flow).
      above about 60V start to be careful.  Also with bigger batteries the total current possible can be
      serious.

NA4 (not asked) lower voltages are possible and some tubes work better than others down there.
         Obviously you need more feedback and many users use a large inductance audio choke for
         the plate resistor in your version as it has less DC resistance and high impedance at audio.
         A typical choke might be several dozen Henries.  You may try a tube audio output from a AA5
        (primary side is usually 5Kohms at audio), others have used ignition coils, pulls from old radios and
        small (physical) 220 to 6.3v transformers. I have a 50 henry choke from a seriously messed up
        BC221 and that works well.    In the old tube days this was called impedance coupling and it
        relied on the high AC impedance.


Allison



Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1GMX on May 09, 2015, 05:17:03 PM
Both lexan and mica filled are not lossy.  I've swept them on my 4191A to 1ghz
and they are pretty decent as everything is lossy there so less so is easy to see.

For HF even PVC (the white stuff) is fine and easily found at home despot.
The thinner wall CPVC is slightly lossier but the thinner wall makes up for that.
By lossier at UHF where I can see it to a fine degree. At HF it would be hard
to measure.

If you want to go to extremes more air the better but mechanical support is a must.
Look at spider wound and basket weave.

Allison


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 09, 2015, 08:50:09 PM
NA4 (not asked) lower voltages are possible and some tubes work better than others down there.
Obviously you need more feedback and many users use a large inductance audio choke for the plate resistor in your version as it has less DC resistance and high impedance at audio. A typical choke might be several dozen Henries.  You may try a tube audio output from a AA5 (primary side is usually 5Kohms at audio), others have used ignition coils, pulls from old radios and small (physical) 220 to 6.3v transformers. I have a 50 henry choke from a seriously messed up BC221 and that works well. In the old tube days this was called impedance coupling and it relied on the high AC impedance.

Interesting, thanks ... I'll see what I can find in the junkbox. I know that I have an old filament transformer somewhere; it's not particularly small but it could be used experimentally. In my local electronics store today I saw a small Hammond tube output transformer with a 10KΩ primary and I almost bought it so I could play around with it as a plate load and/or use it as an output transformer for a future power audio stage. It was a low-current transformer, about 3/4" wide. However I choked on the price ($18).

Just to be sure I understand, I believe you are talking about the plate load in the *detector* stage, right? I can see how that would increase the feedback. (As you know, I'm already using a 7H choke as the load in the *audio* stage.)

The receiver is working beautifully at the moment ... had a lot of fun monitoring the band tonight and heard lots of DX. So this idea of lowering the B+ is more in the nature of an experiment, rather than a "necessary fix." The builders from Days of Yore seemed to believe that running lower plate voltages (on the detector) was advantageous in a regen.

Both lexan and mica filled are not lossy.  I've swept them on my 4191A to 1ghz and they are pretty decent as everything is lossy there so less so is easy to see.

To start with I'll probably go with the 1" Lexan pipe.

Here's a view of the receiver and my patented "helter skelter" experimental construction technique. At the back is the "power supply": left to right, "C" batteries (about 5 volts negative bias from four NiMh AA-cells, which I will be using to bias the RF stage); "B" batteries (the empty space is where I'll put the 9V rechargeables, replacing the four tiny 23A batteries); and the "A" battery, which is a rechargeable NiMh D-cell. The power supply wouldn't be out of place in the 1920s, apart from the advances in battery technology and the ubiquitous plastic replacing bakelite and metal. The new regeneration "throttle" capacitor is just in front of the power supply. The screen regeneration pot is in the lower-center of the picture. The empty space to the left of the coil is where I'll put the RF stage.

(https://lh3.googleusercontent.com/-L4k80_uq2io/VU7TP1Ps6uI/AAAAAAAAE4U/OoznkAuZcC8/s640/Submini_regen_8.JPG?gl=US)

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1GMX on May 10, 2015, 06:35:39 AM
Is the large inductor for the plate load for the detector.  I have the latest schematic you posted.


Allison


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: N3QE on May 10, 2015, 08:24:04 AM
The suggested circuit is a no-no. It won't put bias on the tube because of the DC path to ground through the coil. It also shunts the signal path.

No, it's not obviously wrong. 1AD4 grid bias at 45V B+ is indeed often simply stated to be 0V.

In most every "real world" 1.5V filament tube battery radio, the grids are shown at DC ground in the schematic either by inductor or resistor. But usually neither or at most one of the filament ends is at ground. It often takes some inspection of the filament wiring to find out that the filaments form a circuit in themselves with voltage drops along the chain and sometimes resistor networks to get correct bias at each stage. Usually in multi-tube radios there are RF and AF bypasses for the filaments too (although in some cases they do not bypass the filament at some stages specifically to get some specific degeneration relative to the next stage - now that's complicated but they saved one capacitor in the design!)

This is a "feature" of many 1.5V directly heated battery-supply tubes. Correct grid bias relative to the heater on the spec sheet, is often 0V or or a multiple of -1.2/-1.5V for series connected filament chains.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 10, 2015, 09:37:41 AM
Is the large inductor for the plate load for the detector.  I have the latest schematic you posted.
Allison

Not sure whether you are asking a question ... the only large inductor in the current set is the 7H choke on the plate in the audio stage.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on May 10, 2015, 02:18:23 PM
If you don't put a negative bias on the grid, then you have  very limited dynamic range on strong signals, because you really don't want the grid driven positive  - positive grid means grid current means distortion and likely gain compression - and you can't get an RF gain control function.. 

Getting round it with a grid leak leads to detection in the RF stage, lower selectivity and more cross-modulation.

RF gain control by varying RF stage screen volts with zero bias still leaves the grid going positive with all that entails....


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 10, 2015, 03:18:31 PM
If you don't put a negative bias on the grid, then you have  very limited dynamic range on strong signals, because you really don't want the grid driven positive  - positive grid means grid current means distortion and likely gain compression - and you can't get an RF gain control function..  

Getting round it with a grid leak leads to detection in the RF stage, lower selectivity and more cross-modulation.

RF gain control by varying RF stage screen volts with zero bias still leaves the grid going positive with all that entails....

That indeed is the message I got from the previous discussion about the RF stage (not in detail, but the outlines).

If you go back to the previously linked data sheet for the 1AD4 (http://tinyurl.com/pm4pn56 (http://tinyurl.com/pm4pn56)):

Voltage on Grid 1 for S = 10 microAmps/Volt: Typical is -3.8V. Limit value, -7V.

Yes, it seems to be true that many of these "battery" tubes operate with zero-bias, or very low bias, in a number of situations.

However -- and maybe I got the wrong end of the stick -- but what Peter seemed to be saying back then is that if you want tube-level control over RF gain, you'll need to bias the tube with an explicit supply. I am guessing that, otherwise, you are limited to "passive" RF gain control methods such as resistors or capacitors in the antenna circuit. Plus, Peter's point about dynamic range (makes sense: restricted operating voltage = less dynamic range).

Meanwhile I have a question for Peter and others: Now that I have added a -5V battery source for bias, would anything be gained by applying that bias to other stages (not just the RF gain stage)? I assume we should leave the finely-balanced grid-leak detector alone, but what about the audio stage(s)? I assume adding additional bias would reduce the gain but increase the controllability of those stages -- and that might be a good idea in a regen, for stability, even if I have to add additional audio stages.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on May 11, 2015, 02:03:38 AM
You may well find less overload problems with some bias on the AF amplifier grid. If you get distortion on strong signals, it may either be RF overload of the detector or overload of the AF amplifier if the grid starts going into grid current on positive peaks. A small amount of grid current can be acceptable, but it means that the input impedance starts to drop on signal peaks and that means the load on the previous stages is changing - which can reflect back there to give distortion. Which is why  (such as in modulators for AM transmitters) when Class AB2 and B audio amplifiers were used a lot, it was common to use a step down transformer to drive the grids and negative feedback on the driver stage to give a low output impedance to the push-pull stage.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 11, 2015, 05:15:26 PM
Incidentally, what material is the coil former? Even at 7MHz, some plastics aren't very good, especially coloured ones. A quick and nasty check is to put a glass of water and the coil former in the microwave oven and zap it for 30 seconds or a minute or so to see if it gets warm. If it does, that tells you that it's lossy at 2.4GHz: if it stays cold, you can be pretty certain it's not lossy at 7MHz.

So, today I finally found a second pill bottle with the same form factor as the first one. I was visiting some relatives in a retirement home and it only took 30 seconds to find one in a dustbin! (Removing the pharmacy instruction labels took a lot longer, but never mind.)

I stuck it in the microwave with a glass of water (pictured below) and ran it on High Power for one minute. The water was very hot, but the pill bottle was still room temperature, exactly the same as when I put it in.

(https://lh3.googleusercontent.com/-07JWyH6XFA0/VVFG0_o1bYI/AAAAAAAAE4o/fX8lRhCwvr0/s640/Submini_regen_microwave.JPG?gl=US)

So I'll be using the new pill bottle for the experimental RF stage. I like these particular pill bottles because they are exactly 1.25", the same as my collection of virgin Amphenol forms. I figure I'll use the pill bottles to mess around with, and then use the Amphenol forms for the "final version" of my receiver, transferred from the breadboard to "proper construction."

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 11, 2015, 06:13:29 PM
NA4 (not asked) lower voltages are possible and some tubes work better than others down there.
Obviously you need more feedback and many users use a large inductance audio choke for the plate resistor in your version as it has less DC resistance and high impedance at audio. A typical choke might be several dozen Henries.  You may try a tube audio output from a AA5 (primary side is usually 5Kohms at audio), others have used ignition coils, pulls from old radios and small (physical) 220 to 6.3v transformers. I have a 50 henry choke from a seriously messed up BC221 and that works well.    In the old tube days this was called impedance coupling and it relied on the high AC impedance.

The detector choke-load experiment

If you've been following this thread (not necessarily recommended!) you'll know that I've been puzzled by the difficulty of getting these little 1AD4 tubes to oscillate at anything lower than the "nominal" B+ of 45V. The "old-timers" from nearly a century ago apparently recommended running a regenerative detector at low plate voltages, among other things because it is supposed to provide smoother regeneration control.

Tonight I replaced the 10KΩ detector plate resistor with the 10KΩ primary of a very small tube audio output transformer (Hammond model 144Q). This was at the instigation of KB1GMX and also because Charles Kitchin, N1TEV, uses just such a trick in his "junkbox special" regenerative receive -- his design is solid-state (http://www.arrl.org/files/file/Technology/tis/info/pdf/9811qex026.pdf (http://www.arrl.org/files/file/Technology/tis/info/pdf/9811qex026.pdf)).

It works! Until now I needed a B+ of at least 42V to get any oscillation going [this statement is wrong; see next post]. After replacing the resistive plate load with the aforementioned Hammond inductor, I started lowering the B+ voltage (by removing 12V batteries from the series chain):

--Original B+ voltage of 46V on the DMM: good reception using that little inductive load, in fact it seemed a bit better than with the resistive load, but not dramatically so.

--Removing one battery, the B+ falls to 34 volts on the DMM. Wow, I can get oscillation, in fact it's almost on the same spot on the screen-regen dial as it was before! Now, is the reception "better" than before? I'm not sure; perhaps. I think it is "calmer" but just as sensitive. A fair amount more listening is needed, to be sure.

--Removing two batteries, the B+ falls to 23 volts on the DMM. If I now advance the screen-regen dial almost all the way, I can get oscillation going, but it's "hair-trigger" and only fractionally below the "loud squeal" point. Reception: I can hear some stations, but overall, not great.

This is very interesting! Further experimentation needed!

Note: This was a crude experiment because I lowered the B+ not just on the detector, but on the audio stage too, simply by removing batteries. Next thing I'll try is putting a potentiometer in the B+ supply to the detector tube, but keeping the full 46-to-48V B+ on the audio stage.

It is also tempting to try an inductor with more "oomph." In other words, not this baby output transformer, but something bigger. Kitchin used the primary of a 6.3V heater transformer.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 11, 2015, 09:14:30 PM
The detector choke-load experiment (continued)

Here are the various loads tested today. From left to right:
(1) A 10K resistor (the original load in this circuit).
(2) The primary of a Hammond 144Q tube output transformer, published impedance 10,000Ω, the DC resistance on my DMM is 620Ω.
(3) The primary of an old tube output transformer from the junkbox, DC resistance 390Ω.
(4) The 117VAC primary of an old power transformer from a tube radio, DC resistance 26Ω.

(https://lh3.googleusercontent.com/-flprUNmjv4s/VVF6Roq77GI/AAAAAAAAE44/R-5YYrwJgNk/s912/Submini_regen_transformers.JPG?gl=US)

Contrary to what I said in my previous post, even with the 10K resistor load the tube *does* oscillate at the lowered B+ voltages (34V and 23V). What seems to have happened is that the addition of the screen "throttle capacitor" a few days ago improved not only control over oscillation, but also increased the tube's ability to oscillate in the first place. I had not noticed this at the time, because I didn't test it with reduced B+ at the time.

Anyway, to summarize: I believe that any of the three tested inductive loads are better than the 10K resistor in my tests. Regeneration control is a bit smoother and there is noticeably more gain. SSB signals in particular have very good audio quality and are easy to tune and very stable, which is no mean feat for such a simple set (I'm not particularly interested in SSB as a mode, but it's one of the useful receiver tests).

I wouldn't call the improvement a "slam dunk" -- time will tell. I think (2) the little Hammond transformer does about as well as (3) the much bulkier older output transformer. As for (4) the old power output transformer, it's not noticeably better or worse than the other inductors although there is perhaps a loss of gain.

The experiment is a success: an inductor is better than the resistor. I am leaving the little Hammond transformer in the circuit, replacing the 10K resistor.

So is the set's performance better if you have a lower B+ (for example 34V instead of 46V)? For sure, regeneration control is very smooth -- but the difference compared to normal B+ is fairly subtle. I think what I'll do is put a potentiometer in the B+ feed to the detector tube, allowing experimental variation of the B+ on that tube without reducing B+ to the other stages (the audio stage, and the future RF stage). I will mess around with various settings of that pot in the future and see whether this mantra about "lower B+ on the detector provides better performance" is borne out in practice.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 11, 2015, 10:44:50 PM
Lowered B+ voltages, controlled with potentiometer

I added a 50K pot in the B+ supply to the detector tube. This allows the detector plate voltage to be set anywhere between 15V and 45V, while keeping the full 45V for the other stages in the set. To my surprise, the set still oscillates vigorously with only 15 volts on the detector plate, although achieving this does require setting the screen regeneration control near the top of its travel, and adjusting the throttle capacitor to get oscillation. Reception of signals is possible at that voltage. At a somewhat higher voltage (between 20V and 25V) the set performs pretty well; and if I'm not mistaken, the S/N ratio on certain strong signals is actually better at this low B+ compared to the 45V "default."

A couple of things:
(1) To humo(u)r the late, great Regeneration Old-Timers I think I may leave this detector B+ voltage pot in place. It will be fun over the next few months to compare "full B+" reception with their suggested "lower B+" setting. The number of knobs on this set is rapidly increasing....
(2) I am now optimistic that I could (later) build another version of this set, this time using the 2E32 subminiature pentodes rather than the 1AD4s used in the current set. I had originally started on a 2E32 project because that tube has modest power requirements: a 50mA filament current (half as much as the 1AD4) and a "normal" B+ of only 22.5V, but I could not get the tube to oscillate, come what may. I am now fairly sure that was because I had not included a capacitor in the "throttle capacitor" position. The 2E32 has more modest performance (transconductance of 500µMo , compared to 2000µMo for the 1AD4) but it would seem ideal for a portable set.

(https://lh3.googleusercontent.com/-yCJDfErBtgY/VVGYX0xTaYI/AAAAAAAAE5I/v5sNgzF5384/s640/Submini_regen_low_voltage.jpg?gl=US)

Time to go to bed.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 12, 2015, 08:16:50 AM
The usual requirement for marine radios was that anything over 50 volts had to be accessible only by the use of a tool such as screwdriver.

Today I deliberately put the thumb and index finger of one hand across the B+ supply (nominal 48V). I couldn't feel anything. Then I pressed my fingers harder into the terminals and felt a very slight tingling. The 23A batteries I'm using now are very low-capacity (55mA/H). I'll soon be switching to a bank of 7.4V Lithium-Ion rechargeable batteries, with 7 in series giving about 52 volts. (Actually they are marketed as 9V batteries, but the nominal voltage is 7.4 for LiOn). Compared to the 23A's, these have 10 times more capacity: 600mA/H. I will repeat the experiment, with the new batteries, carefully and touching the terminals with the back of my fingers so that they bend away from the terminals if I get zapped!

I was wearing rubber flip-flops as it happens (=irrelevant since one of my fingers was grounded anyway!) and my hands were dry. I suspect that under a rather unlikely set of circumstances, 48V could be dangerous. In a marine setting however, with all that salt water around, I would definitely be taking precautions. (I come from a family of sailors.)

Edited to add: Actually what I'm more worried about is fire! With the really messy construction technique I'm using, a short across one of the three power supplies can't be ruled out. It already happened a few weeks ago, and melted a 1.5V AA plastic battery holder. I can imagine that 50VDC shorted to ground, with high-capacity batteries, could generate a lot of heat. The radio is built on a wooden breadboard and sitting on a wooden workbench. My only safeguard against fire is to make sure that I never leave the radio on when I leave the shack, so I have been getting a little paranoid about making sure that those little knife switches are in the "off" position before leaving the room.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: WB6BYU on May 12, 2015, 03:13:28 PM
A fuse or circuit breaker on the battery packs would, of course, reduce the danger of a short circuit.

I like using the PTC Thermistors that are sold as current limiters / self-resetting fuses.  Something like this:

http://www.digikey.com/product-detail/en/0ZRC0025FF1E/507-1332-ND/1560187

for example will pass 250mA but if the current approaches 500mA will switch to a high resistance state
that limits the current to a safe value.

Yes, it looks funny to see what appears to be a disk ceramic capacitor in series with your battery supply...


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 13, 2015, 04:58:56 AM
Something is screwy here

Before proceeding with construction of the RF stage, I need to solve an issue that's been swept under the carpet until now.

The main pill-bottle tank coil has a diameter of 1.28" (including the diameter of the 20AWG wire), 14 turns, with a length of 0.7". If you use Wheeler's formula:

L = (Radius2 * Turns2) / ((Radius * 9) + Length * 10)

= 6.8µH

The trouble is that I calculated the LC circuit to use a 10µH inductor. I could understand if there were a small discrepancy, but this is a very large error. With the coil described above, I am bringing in a range of about 6980KHz to about 7150KHz. This just isn't right!

Could one of you good souls do me the favor of checking my calculations? The first one to check is the inductance calculation (above). The second one is to check whether I've made an error in setting up the capacitance network.

(https://lh3.googleusercontent.com/-FyAruqruBO4/VVMu1KRzyOI/AAAAAAAAE5Y/ZVsWxlTLnEY/s512/Submini_regen_large_layout_schematic_cap_network.jpg?gl=US)

As you can see from the schematic excerpt, there are four capacitors involved, in a parallel and series combination. The fixed capacitors are silver mica. The variable is a Hammarlund MC50S, straight out of its original box, with the part number stamped on the porcelain. It has a meshed capacitance of 50pF and I am assuming an unmeshed capacitance of 5pF.

Based on those assumptions, I get the following values for the capacitor network:

--Maximum 51.7pF.
--Minimum 48.5pF.

For parallel capacitors I'm using C = C1 + C2. For two series capacitors I'm using (C1 * C2) / (C1 + C2).

The final calculation is LC equation for frequency. With C = 51.7pF and L = 10µH, I get frequency = 7000KHz. If I stick in the actual putative value for L in my current build (14 turns or 6.8µH) I get f = 8488KHz (!!!!!).

Formula used:

f = 106 / (2π * √LC) where units used are KHz, µH, and pF.

(I have done all these calculations on paper, and also checked them with online calculators.)

If my calculations are indeed correct, my proposed troubleshooting checklist is in this order:

(1) Double-check the wiring (again).

(2) Check the solder joints for continuity (again).

(3) Replace the pill bottle with an Amphenol 1.25" form, just in case there's something screwy about the plastic material.

(4a) Edited to add: remove all of the fixed capacitors. Connect the variable capacitor across the coil and mesh it fully to 50pF. Check the reception frequency with my signal generator (sig-gen frequency double-checked on another receiver). Tuning will be non-linear but it's just a test.

(4b) Replace the mica capacitors; I suppose it's possible one of them is way off-value (I don't own a capacitance meter, and my grid-dipper only provides approximate values when used as a capacitance reckoner).

(5) Edited to add: physically rebuild the radio so that the tuned circuit is separate from other components. For instance, in the current build, some of the capacitors in the LC circuit are very close to the plate choke and the plate bypass capacitor (and the LC circuit shares a grounding lug with the bypass cap).

(6) Any other suggestions, if none of the above works?

BTW the antenna is currently connected to the "hot" end of the tuned circuit, via a gimmick capacitor (two wires twisted together, two turns).

Many thanks!

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 13, 2015, 03:12:37 PM
So I've now done all of these things:

--Double-checked the wiring.

--Checked the solder joints for continuity.

--Replaced the pill bottle with an Amphenol 1.25" form, just in case there's something screwy about the plastic material in the pill bottle. Made a new coil on the Amphenol form, with 18 turns and 0.75" length (which should yield 10µH).

--Removed all of the fixed capacitors. Connected the variable capacitor across the coil and meshed it fully to 50pF. So now we have a bog-standard LC circuit with a single inductor and a single capacitor.

Checked the reception frequency with my signal generator (sig-gen frequency double-checked on a digital receiver). Result: the set is now tuning a range that begins at 5.9MHz (when it should be around 7MHz). So there is nothing wrong with my capacitance calculations apparently, because substituting a known 50pF capacitor (replacing the capacitance network) led to a similarly screwy result.

I also removed the "gimmick" capacitor from the antenna line, and it made no appreciable difference to the frequency.

I am really, really puzzled.

I am convinced that my coil must be roughly 10µH (and I have ordered a fancy B&K LCR meter so that I can check this for sure tomorrow!). I am shunting it with 50pF. So why is the set tuning at about 5.9MHz? What is the dynamic inside that regenerative set that can shift the frequency down by more than 1MHz?

The upside is that I am enjoying all sorts of exotic shortwave broadcast stations!

To be continued....

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: WB6BYU on May 13, 2015, 03:17:55 PM
You might try the Hamwaves inductance calculator, which is supposed to be much more accurate than
Wheeler's formula:

http://hamwaves.com/antennas/inductance.html


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 13, 2015, 03:36:24 PM
You might try the Hamwaves inductance calculator, which is supposed to be much more accurate than
Wheeler's formula:

http://hamwaves.com/antennas/inductance.html

I tried it, but "an error occurred when solving for beta" (whatever that means).

I was already aware of the hamwaves calculator. Fact is though, my current experimental results are way off from what any formula yields, even if Wheeler's is only an approximation.

I have wound quite a lot of coils in the past three years and never had this kind of error. So my thinking is that the inductance calculation is (approximately) correct, but there is some stray capacitance somewhere in the circuit that is lowering the resonant frequency. The hunt for that capacitance is ongoing.... To get 5.9MHz at 10µH would imply a capacitance of 73 pF. The coil is currently shunted with 50pF, so somewhere in my radio's LC tank circuit, there is an extra 23pF (approximately). At least that's what I think.

Thank you for your suggestion though!

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: WB6BYU on May 13, 2015, 04:54:12 PM
All coils have some self-capacitance, and this effect is worse when the turns are close-wound
because the capacitance between adjacent turns increases (especially as the air dielectric between
the turns is replaced with shellac or some other form of insulation that has a different dielectric
constant.)  Typical recommendation is to maintain about 1 wire diameter spacing between turns
(typically by winding the wire with a second one, or a piece of string, as a spacer and then
unwinding it when the wire is secured.)

You also have to include the input capacitance of the tube - should be given in the data sheets
as the capacitance from grid to cathode, but also to the other grids.  This can be increased due
to the "Miller effect", though I'll admit that I don't understand it or where it applies.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 13, 2015, 05:12:08 PM
On a totally different subject, I think I am beginning to understand better the old timers' advice to run the regenerative detector tube with a low B+. Although I have seen various versions of this advice, it often seems to pertain particularly to *AM* reception.

This evening I was listening to "Radio Alexandria" on 7490 kHz as a technical test. They finished their broadcast with an extended section of Beethoven's "Eroica" symphony. (I had never heard this station before, but for what it's worth, it seems to be characterized by a combination of "prepper," "environmentalist" and somewhat religious themes.)

I set the "regeneration throttle" capacitor to somewhat below the oscillation level -- as is recommended for AM -- but it still sounded distorted (in real life I am in the music business). Then I lowered the detector-tube B+ down to a very low 15V (as compared to a nominal 46V) and boosted the regeneration a bit (using the throttle capacitor) and the music fidelity improved dramatically.

So, if only because I enjoy listening to short-wave broadcasts, I think it's important to keep some kind of control over the B+ on the detector tube.

Whether this has anything to do with improving overall "communications receiver" performance (as opposed to lessening harmonic distortion on AM, which is not of much importance to most hams) I am not sure.

Edited to add: later this evening I heard a Rachmaninov piano concerto on 7320kHz. Extremely distorted at the full B+. Once again, when I reduced the detector tube's B+ all the way down to 15V and re-adjusted the regeneration throttle capacitor, the music fidelity dramatically improved. Alright, it's not "hi-fi" but the difference was substantial.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1GMX on May 13, 2015, 07:16:21 PM
Miller effect is (in terminally rough form) grid to plate capacitance multiplied by the gain of the tube.
For multi-grid tube that is a small number as the purpose of the screen grid is to isolate the control
grid from the plate.  In most tube, transistor and especially FET (mos or not) it represents a variable
capacitance with the problem of undesired negative feedback.

For those tubes the input C is roughly 4pf mostly structural.  Miller C is very small for the most part.

The Wheeler equation is a bit fussy as it does not seem to work well with other coils in proximity
(mutual inductance).   Also the error tends to be greater if there is stray capacitance (leads and
other).  Add to that your capacitors are likely 5 or 10% tolerance units.  So error will happen.

if you get "beta" error then he coil length for the diameter of the coils is impossible example
a coil of .1" diameter wire and 10 turns cannot be a single layer .5" long.   Also possible if the
outside diameter is not larger than the form plus twice the wire diameter.  Basically two solid
objects can't logically be in the same space.  That and division by zero.

The AADE LCII is a good LL-C meter for measuring caps and coils.  When I've had problems
like that measuring the parts usually found something was not as expected.  Typical is marked
caps were not even close. 

Grid dipper can be accurate if the reference coil is known value and the actual frequency is
checked with a receiver. 

If the tickler winding is wound over the grid winding then you have more capacitance.
If the RF stage has a coil to couple to the grid winding that adds physical capacitance.

You can figure out that missing capacitance (assume for the moment the coil is as calulated)
by working backward from where you are to where you should be to get the excess added
capacitance.  Hint if the total capacitance is about 51pf than an additional 10 pf (tube input
plus other coils) would move you down about the right amount.

Do not forget the random wire running all over the breadboard.

RE: regenration and old timers.  You run regeneration ONLY for AM.  For CW and SSB
you require oscilation making the radio an autodyne (direct conversion) receiver.  Two
different critters with different operating criteria.

Now the distortion could be plain overload, enough gain and too much signal.  The other is
enough regeneration and you get sidebands being deleted from oddly, too much selectivity.
Add to that a regen is NOT a linear detector.

Increasing the feedback winding (turns) and spacing it further away can make for running
with less screen grid at low voltages.  FYI the feedback winding (tickler) is also a possible
load on the grid coil lowering selectivity and gain.  Counter intuitive but its coupled to the grid
and has the tube plate as a load.

Like I said before, its a great engineering exercise to try and optimize a regen as you have
many controls and just as many things that do whatever they want.  Result, banging head
against wall.  Lots can be learned.


Allison


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: VE3LYX on May 13, 2015, 07:52:20 PM
A simple radio project seems to be getting awfully complicated.
Build an oscillator. Under power it so it just below oscillation point and install a set of earphones in series with the B+ or B-. You will have a working regen. Raise the voltage and remove the phones, sub in a key and you have a nice CW transmitter. It really is that simple. After that you can add extras you want like an audio AMP, a filter etc. A RF amp to a regen is totally unnecesary. Nice to talk about but of no real purpose. A regen when done right will out hear any radio you currently own. It is very easy to overload them antenna wise and an Rf amp just adds to that problem. I am very partial to the twinplex twin triode design but have currently in shack 8 regens in service. Most fun was the POW regen built from garbage dump stuff. Also have some solid state ones. Clough's patent (1927)using an absorption loop to control regen is another very interesting design. Biggest problem with most is regen pot failure. Throttle cap method works but is not as nice as a twinlex with pot control. 


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 14, 2015, 05:11:37 AM
if you get "beta" error then he coil length for the diameter of the coils is impossible example a coil of .1" diameter wire and 10 turns cannot be a single layer .5" long.   Also possible if the outside diameter is not larger than the form plus twice the wire diameter.  Basically two solid objects can't logically be in the same space.  That and division by zero.

Here's what I entered into hamwaves (http://hamwaves.com/antennas/inductance.html (http://hamwaves.com/antennas/inductance.html)):
Diameter 32.5mm
Length 19mm
18 turns
Wire diameter 0.81mm

I'm always getting the "beta error." But I don't see anything wrong with the values entered.

When I've had problems like that measuring the parts usually found something was not as expected. Typical is marked caps were not even close.  

When my new LCR meter arrives today I will check everything. Coil on its own, coil in circuit, with tickler mounted, without tickler mounted. Capacitance in various connected/disconnected scenarios.

Thankyou for listing the numerous "culprits" for extra inductance and capacitance! It will be interesting to track them down.

Ultimately I'm just interested in knowing what's going on; the set is working and that's what counts.

A simple radio project seems to be getting awfully complicated.  

That's why I started this new thread about building a "high performance" regenerative radio. The intention is to use all possible techniques toward that goal, even if the resulting set is more complex than a typical regen. It is getting far away from the other thread's theme of a "portable hiking radio" and that is done on purpose. I have learned an enormous amount of radio and electronics in the process.

A RF amp to a regen is totally unnecesary. Nice to talk about but of no real purpose.

I have heard many opinions on that. I don't expect it to improve performance in any obvious way apart from lowering susceptibility to antenna movement in the wind and providing a steady load to the detector. Plus, elimination of stray radiation from the oscillator.

Edited to add: I am also interested in finding out whether an "active" RF gain control (controlling the grid bias) will outperform "passive" gain limitation in handling strong signals.

The only way I will find out for sure, though, is to build it! There's nothing like hands-on experience.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1GMX on May 14, 2015, 05:57:27 PM
Here's what I entered into hamwaves (http://hamwaves.com/antennas/inductance.html):
Diameter 32.5mm
Length 19mm
18 turns
Wire diameter 0.81mm

I'm always getting the "beta error." But I don't see anything wrong with the values entered.<<


I had to enter a frequency to make it happy and I used 7mhz.  Result 9.6uh give or take.

If you out in the boonies the RF amp is optional.  If you ahve people that you want to annoy with
wideband regen his noise its optional, If your using it for CW (oscillating) and want to annoy
people its optional.  Regens radiate RF.  The RF amp is not there for gain (never hurts) but
to isolate the detector from the antenna.

That and the RFamp can be used as frequency converter later!

Allison



Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: VE3LYX on May 14, 2015, 07:07:17 PM
not there for gain  (never hurts) qte
Actually it does. The absolute No 1 reason many (100s of) regens dont work or perform properly is too much signal either by an RF amp or too much antenna. There is a concept the a regen is a step down from modern rxs in its ability to hear. Actually it will give even the best of them a run for the money. It is however a difficult radio for some to run and I have met folks who simply cannot operate one.  
However the other comment building it to see what happens I wholeheartedly endorse. Famous engine builder Smokey Yunick once said "One good experiment beats 1000 expert opinions." And that applys here as well. Go for it. Be honest with your findings (don't lie to yourself) and have fun. Don't be afraid to retrace your steps should you find yourself down a path of no potential.
don


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1GMX on May 14, 2015, 07:53:34 PM
Yep, after years of building them I still do because of that bang for the buck.

The key factor for RF amp is allowing the regen to be loose (barely) coupled
to the RF source.  Then the net gain is small but isolation is a useful attribute
as then the antenna size or tuning is then not a factor.   Bad design or failed
experiment does not mean its wrong.

The other critical factor is still the fact that we already have enough RFI and a regen
oscillating or on the verge will radiate.  Granted its low power but around here there
are people within less than a mile of me and a milliwatt does carry even on a short
piece of wire.

The other way I like to play with them is running the detector at say 1700-1800khz
fixed a simple effective IF and use a tunable converter ahead of it.  You get the
selectivity advantages or regen and stability while being able to tune upper HF.

Also Smokey would have loved CADD and modeling, alas they were in the future,
but he could experiment now.

In the end, after more than 50 years of playing with them, when I need a receiver
I do think about it and then remember why it was interesting but not what the task
called for.  I still remember trying to get WABC NY 770khz 40 miles away with
WGSM 740khz less than a mile away.  No regen was good enough to sort that out
and I tried all I came across. I would repeat that process with transistors starting
around '65. The first successful radio that could was a regen at roughly 455khz
with a converter before it and a audio amp after (6sa7 and  6SN7).

Three of those 1ad4s could easily be a superhet (self oscillating converter,
regen if, audio amp).  Its the next step in receivers.   Look at what AA1TJ has done.

With regens there are many dead ends and many pockets where they shine. 
Its appeal is why after nearly century we still use them both commercially and
experimentally.  There are places where they fit though most of those are technically
superregens.  They were the building block leading to modern receivers and
often a part of them.  Have fun and experiment, but do not limit yourself.


Allison


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 15, 2015, 03:18:49 AM
Three of those 1ad4s could easily be a superhet (self oscillating converter, regen if, audio amp).  Its the next step in receivers.   Look at what AA1TJ has done.

With regens there are many dead ends and many pockets where they shine. Its appeal is why after nearly century we still use them both commercially and experimentally.  There are places where they fit though most of those are technically superregens.  They were the building block leading to modern receivers and often a part of them.  Have fun and experiment, but do not limit yourself.

My next receiver will be a superhet. If anything they are easier to understand, because each stage has a single function!

I have the parts for the ARRL RS-3 (regenerative superhet). I also have the parts for the W6TNS "Novice Q5er" converter along with a beat-up old BC-453 to restore. Or I might branch out and build an entirely self-designed superhet with subminiature tubes ... based on previous designs of course and with a lot of help from you kind folks, if you can bear it. I've already learned an enormous amount of tube theory, just building the regenerative set. This is despite having built a fair number of things in the past; those were all either kits, or entire ARRL projects, so no "designing" was involved.

The first order of business however is to build the best regenerative receiver I can build.

What I am finding now is that I have a much better "feel" for the purpose of individual components, and an appreciation of appropriate component values -- while still committing numerous "clangers," some of them dangerous! Where I need a lot more knowledge is in some of the basic RF theory: impedance, reactance, and so forth. (I have read quite a lot about these things but it seems to go in one ear and out the other.) Theory and practice need to go hand in hand.

The other way I like to play with them is running the detector at say 1700-1800khz fixed a simple effective IF and use a tunable converter ahead of it.  You get the selectivity advantages or regen and stability while being able to tune upper HF.

In your opinion, are pure regens less suitable for upper HF? The first regen that I built (the ARRL 3-transistor set from 1968) works fine on 20m, although this did require a fair amount of optimization of the coil and capacitors (when I built it exactly as described by ARRL, I could not get it to oscillate on 20m, or alternatively it would motorboard wildly; and antenna loading was tricky).

When the basic regen circuit has been settled and tested, I will be adding a 20m coil (the antenna I'm installing this summer, in the limited space available, is a 40m/20m trap dipole). The plan is to use 6-pin coil forms in the detector, enabling all of the frequency-setting capacitors to be mounted inside the coil form. The RF stage will use 4- or 5-pin coils. What I will basically end up with is an old-fashioned "TRF" with a single RF stage and a regenerative detector. Once I've got all that working, I'm hoping to improve the audio side, adding more amplification, a gain control, and an audio filter for CW. I might even build a 100kHz crystal calibrator (some kind of routine calibration tool is pretty essential with a regen, especially a multiband one). Once all that is done and the design has been finalized, I'll build a "proper" version of this radio, using good construction techniques and in a more compact footprint with a nice tuning dial and controls.

(Also, at some time in the future, it would be cool to build a "minimalist" miniature set for portable use, probably just two tubes and squeezed into a small case. It would use lower-power, low-voltage 2E32s, which would require only a single AA battery for filaments and a couple of tiny A23s for the B+ or perhaps just a couple of larger 9V batteries. The main challenge is miniaturizing the tuned circuit ... but that's a subject for the other thread.)

By the way, based on my limited testing so far, the two-tube version of my regen (with no RF stage) out-performs my ARRL "3-transistor" regen on 40m. In particular, it has even greater sensitivity and smoother regeneration. However the comparison isn't really fair because an enormous amount of optimization and tweaking has been done on the tube set; in comparison, my ARRL build has only minor alterations compared to the published design.

I can also see a big advantage to using a regen as a fixed IF. A regen requires a lot of skill to optimize for each frequency, so once you have it "stable" (well, as stable as a regen can be!) it makes a lot of sense to leave it alone and use a converter to do the tuning!

not there for gain  (never hurts) qte
Actually it does. The absolute No 1 reason many (100s of) regens dont work or perform properly is too much signal either by an RF amp or too much antenna. There is a concept the a regen is a step down from modern rxs in its ability to hear. Actually it will give even the best of them a run for the money. It is however a difficult radio for some to run and I have met folks who simply cannot operate one.  
However the other comment building it to see what happens I wholeheartedly endorse. Famous engine builder Smokey Yunick once said "One good experiment beats 1000 expert opinions." And that applys here as well. Go for it. Be honest with your findings (don't lie to yourself) and have fun. Don't be afraid to retrace your steps should you find yourself down a path of no potential.
don

I agree about "too much signal." That's why it's useful to have various methods of attenuation available.

I've been retracing steps frequently ... sometimes it's frustrating because you get something working really well, then you try to "tweak" it and it gets worse, and then you can never quite get back to where you were before!!!

The cool thing about using these subminiature tubes with their own leads, and low voltages, is the ease of experimentation. One of these days I will smash one of them with a hammer so that I can admire the industrial construction feat (these tubes are still plentiful).

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1GMX on May 15, 2015, 05:39:51 AM
I agree tubes seem to outperform bipolar trasistors and usually fets.  I believe
the transistor problems is way too much gain.  I've tried the three transistor
and it was not satisfactory. 

My feeling in regens is that HF selectivity is not adequate.  If the regen
is oscillating its a direct conversion receiver with the inherent limitation no
rejection of the opposing sideband.   

The RS3 is a fun radio.

Like you said if you break the problem of reception into a modular one then
optimizing becomes easier.

Allison


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: W1BR on May 15, 2015, 05:43:53 AM
[q
Edited to add a quick question: I dithered about the polarity for the 1µF electrolytic on the RF Gain wiper. I've now "fixed" it so that the positive plate goes to ground ... which I think must be correct since the bias is negative?

MDM

73 de Martin, KB1WSY


Also remember that the filament polarity affects the tube bias. 

Pete


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 15, 2015, 07:04:30 AM
Today I used my lovely new LCR meter to get to the bottom of the "screwy tuned circuit" issue. I completely dismantled the tuned circuit and did the following tests. Note that when using the leads on the meter, I started out by zeroing out their capacitance (using the CAL button).

--The fixed mica capacitors, in isolation, all measured within their tolerance (i.e. plus or minus 5%).

--The variable capacitor had a maximum of 51.1pf and a minimum of 7.1pF. I had assumed 50 and 5 -- good enough.

--When I wired the fixed capacitors and the variable into a network, in isolation from the radio, it had a maximum capacitance of 53.0pF and a minimum of 50.0pF. My calculations had yielded 51.7 and 48.5. Again, good enough, and well within the tolerance of the components.

--I tested the tank coil, after disconnecting it from the radio. It is mounted on an Amphenol polystyrene form. It has 18 turns, a diameter of 1.28" including the diameter of the 20AWG wire, and a length of 0.75". It measured at 10.9µH with the (disconnected) 3-turn tickler mounted (close-wound, directly under the tank). After removing the tickler, the tank coil measured at a slightly lower 10.5µH (it was fun to get direct evidence of the effects of mutual inductance). According to Wheeler's equation, the coil should be 10.0µH. So again, "good enough" (the difference is probably partly caused by the inductance of the leads).

--Edited to add: The meter also shows that the tank coil has a Q of "0.55" -- I presume that is a ratio and that this would conventionally be rendered as "55".

Finally, I tested the capacitance between the input of the grid-leak detector and ground. At this point, the tuned circuit is dismantled, so there is nothing between that point and ground. The meter showed a capacitance of 20.6pF and I find this extremely interesting because it accounts for the screwiness I've been discussing: it agrees almost exactly with my calculation that there was something like 23pF 21pF of stray capacitance messing things up. But is the testing methodology valid? Surely the capacitance of the circuit is different when it is "powered down" and there are no voltages on the tube?

I believe the next step is to rebuild the detector stage entirely, using a more "linear" layout from left to right. I'll eliminate the "criss-cross" wiring and run a 14AWG copper-wire ground bus down the middle of the circuit (in the current build, my grounding points are haphazard). Maybe I'll finally discover a wiring error, despite checking my work several times already. I am determined to get to the bottom of this -- unless one of you experts thinks that a 20pF "input capacitance" for the grid leak is normal?

One possibility that just occurred to me is a defect in the tube, causing a large increase in inter-electrode capacitance. I took it straight from its NIB packaging and don't own a tube tester (and anyway, few tube testers are configured to test these little tubes).

Hmmm ... a fancy and expensive LCR meter just to confirm what I knew already! (Still: You can never have too many pieces of test equipment.)

(https://lh3.googleusercontent.com/-kFKbjpo1wk8/VVX8gzkv59I/AAAAAAAAE58/6BStipTlgvE/s640/Capacitance_meter.jpg?gl=US)

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 15, 2015, 02:41:25 PM
So I rebuilt the detector from scratch, but with the same circuit. This time I built it from left to right, without any arbitrary "wires crossing over" and generally with much shorter signal paths. I also used a "new" 1AD4 tube from my stock, just in case the previous tube was defective.

But when I powered it up, with the aforesaid 10.5µH coil and about 50pF of capacitance, it was more than 700KHz off (it was tuning around 6200KHz instead of around 6950KHz). This is almost as "bad" as in the previous build.

So what I've done is remove 16pF of shunt capacitance, so that there is now about 34pF across the coil. In that configuration, it is tuning the 40m band, starting at around 7000KHz.

The set is working nicely (as before) but I am quite disappointed not to have been able to identify/correct the stray capacitance. In the new configuration, when I measured the (powered-down) capacitance at the input of the grid leak (prior to connecting the tuned circuit) it was about 12pF.

It looks like I may have to write off this issue as one of the "mysteries of the universe" and just get on with building the RF stage now. Unless anyone has any other suggestions....

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 15, 2015, 04:55:00 PM
On the positive side of the ledger:

I splurged and bought two low-current Hammond filter chokes, 60-Henry each.

I installed them as the plate loads in both the detector (replacing the small audio transformer primary that I was using as a load) and the audio amplifier (replacing an old 7H filter choke).

Substantial improvement!

In the detector, regeneration begins earlier and is spread over a larger range. There is substantially more control, especially over strong signals. Thankyou KB1GMX (at least I think it's him who suggested using a large-value choke in this application).

In the audio amplifier, there is substantially more gain (thank you G3RZP, who first suggested using a choke load in this stage).

73 de Martin, KB1WSY



Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1GMX on May 15, 2015, 07:25:23 PM
Martin,

Just passing on info I got back when from other old hands.  All of the best regens
tube or transistor had a choke or transformer to get high sensitivity though there were
a few that used active devices to create the same effect.

The reason it works is the plate resistance of tube is very high so the higher the
load resistance (or impedance) the greater the available gain.  the differnce is at 1khz
the choke looks like 370,000 ohm load (impedance)  without the DC resistance
where the resistor was 10K.  The plate resistance of the tube is about 500K so you
get most of the signal rather than loosing it in the tube.  To visualize it just think of
a AC voltage generator with 500,000 ohm resistor feeding the 60H coke VS 10 K resistor.
If the generator puts out 1V the choke has .425V across it, where the 10K resistor has
.0196V.  That is about 21.6 times more signal.

Since your into the experiment... you can break up a regen to a point in to modular sections...
Start with a gridleak detector than add another tube running as a Q-multiplier.
You can get circuits out of the Handbook.  That's  case for a externally excited regen (for AM)
and its run to oscillating for CW or SSB then its really a product detector or cirect conversion
receiver.


Allison


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 16, 2015, 03:49:43 AM
More progress. Until now, I've been connecting the antenna directly to the "hot" side of the tuned circuit, via a "gimmick capacitor" (two wires twisted together, a couple of turns). I did this because in the previous build of this radio, I could not get separate antenna-coil coupling to work.

This morning, following yesterday's total rebuild and the addition of a choke load to the detector plate circuit, I had another go. I removed the gimmick capacitor, then added a 2.5-turn antenna coil about half an inch above the tank coil. I connected the bottom end of the coil to ground, and the top of the coil (the end farthest from the tank coil) to the antenna.

Success! Reception is good, and if I'm not mistaken it is a bit less susceptible to overload than it was before -- perhaps not surprising now that the detector is no longer directly connected to a 20-meter-long longwire antenna.

The other good news is that after making the above changes, I needed to add back to the tuned circuit about 6pF of additional fixed capacitance in order to bring the bottom of the frequency range down to about 7000KHz. The capacitance of the tuned circuit at the bottom of its frequency range is now around 41pf, which is "only" about 8pF "wrong" for my 10.5µH coil (instead of the previous figure of about 14pF).

I'm glad to say goodbye to the "gimmick capacitor" which always struck me as, well, a gimmick ... and a source of various "issues" including a narrowing in the tuning range.

I will now proceed with building the RF stage. I will leave the above-mentioned, loosely coupled "antenna coil" in place and will re-purpose it as the "plate coupling coil" from the RF stage, and see what happens. It may need to be adjusted, but it's a good start.

The reason it works is the plate resistance of tube is very high so the higher the load resistance (or impedance) the greater the available gain....[snip]

Thank you for the detailed explanation. The various types of load, and the methods of interstage coupling (resistor, choke, transformer) are explained in the literature but often rather opaquely: they tell you the merits and de-merits of each alternative, but often don't go into the "why."

I'm surprised that choke coupling wasn't more common. I suspect it was, in part, because high-value inductors were large, heavy and expensive (and they still are!).

Since your into the experiment... you can break up a regen to a point in to modular sections... Start with a gridleak detector than add another tube running as a Q-multiplier. You can get circuits out of the Handbook.  That's  case for a externally excited regen (for AM) and its run to oscillating for CW or SSB then its really a product detector or direct conversion receiver.

There you go again, giving me ideas ... this project is getting more Baroque by the day! For the moment I'll keep it narrow -- building the RF stage and optimizing the "basic radio" -- before branching out and adding more stages.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 16, 2015, 05:40:01 PM
Major surprises today.

I added the RF stage, and grid-biased it following G3RZP's instructions.

Huge improvement in the receiver's performance!

A reminder: this is all in the 40m band.

(1) Stability is now very good (it was already "good" but there is a still a major improvement now). No hand capacity effects (unless you put your hand "inside" the set near the detector coil). You can put your hand on the antenna wire and shake it, without audible effect. You can copy CW with no tone variation and SSB demodulation is great -- you can listen for several minutes without touching the dial.

(2) There actually is an improvement in usable RF gain (=somewhat improved sensitivity).

(3) The RF gain pot give good control over strong signals, and it has absolutely no very little "de-tuning" effect unlike the detector/regen controls. In fact it is so useful that I have changed the bias battery. It was originally 4 NiMh AA cells (about 4.8V) but I've replaced it with a Li-Ion "9V" cell (actually 7.4V) to give even more control over strong signals. (Edited to add: This modification is still under assessment. It adds only a small degree of additional attenuation.)

(4) In a change to the previously posted schematic, I made the RF stage tunable. By "peaking" the RF stage, substantial additional gain is available (if desired).Edited to add: I am not sure whether it is worth modifying the receiver to install a two-gang tuning capacitor and attempting to implement accurate "tracking." Additional bands will be added later -- 20m and 80m -- which is one reason to consider implementing "tracking," despite the difficulty of implementation.

(5) I am not totally sure about this, but there may also be a moderate improvement in selectivity.

By the way, the coupling in both the RF stage (antenna coil) and the detector (tank coil) is very loose. The antenna coil is a couple of turns about 1/2" 1" above the 18-turn RF-stage tank coil. The plate coupling coil between the RF stage and the 18-turn detector tank is also a couple of turns, 1/2" above. Is it possible that this, on its own, is improving selectivity?

Edited to add: I think I've figured out how the improvement in selectivity happens. With the gain-controllable RF stage, it is now possible to set regeneration to the threshold of oscillation (= most sensitive, and most selective, point on the dial) even when copying strong signals. This only works because, with the RF gain control, you can now attenuate the strong signals that otherwise, would be intolerably distorted or strong enough to cause "blocking." So: At the very least there is an improvement in selectivity when copying strong (or moderately strong) signals, and that is noticeable on a night like tonight with heavy CW contest activity in the lower part of 40m.

I will post an updated schematic once the current "tweaking" phase is over.

This set now has six controls (leaving out the power switches) from left to right:
--RF Peaking capacitor (small vernier dial).
--RF Gain pot.
--Main Tuning capacitor (large reduction drive/dial).
--Screen Regeneration pot.
--Detector Plate Voltage pot.
--Regeneration Throttle capacitor.
... and it doesn't even have an AF Gain control yet.

All the controls are useful, amazingly enough. The challenge is to select the right permutation for the signal at hand ... it has always been said that more skill is required to operate a regen, compared to a modern superhet.

(https://lh3.googleusercontent.com/-n_70AzPDya0/VVf7O5p5JtI/AAAAAAAAE6Q/VRexfZmxtBE/s912/Submini_regen_9.JPG?gl=US)

Famous engine builder Smokey Yunick once said "One good experiment beats 1000 expert opinions." And that applies here as well. Go for it. Be honest with your findings (don't lie to yourself) and have fun. Don't be afraid to retrace your steps should you find yourself down a path of no potential.
don

Don, as I said, this substantial improvement in performance was a total surprise. My purpose in adding the RF stage was more a case of "hygiene." Big surprise.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 17, 2015, 04:50:06 AM
In the end, no "tweaking" was required. The new RF stage worked without needing modification. I tried using a higher-voltage grid bias battery by the improvement in attenuation wasn't substantial so I've kept the original 4.8V supply.

The set now broadly conforms with Charles Kitchin's design principles for of a "high performance" regenerative receiver (http://www.arrl.org/files/file/Technology/tis/info/pdf/9811qex026.pdf (http://www.arrl.org/files/file/Technology/tis/info/pdf/9811qex026.pdf)) except that I've used a tuned RF stage instead of his broadband amplifier.

In the future:
--Aditional stage(s) of audio amplification, allowing an AF gain control and a small speaker (or modern low-impedance 'phones) as a listening option.
--An audio filter. It looks like Kitchin uses passive filtering (switching capacitors into the audio line) but I want something more sophisticated, namely an active filter with adjustable "boost" frequency and passband (very useful, plus I get to learn about filter theory). There are several circuits in old ARRL and RSGB publications -- the Audiofil, the Selectojet, and others. Seems like there are at least two approaches: audio "tuned circuits" on the one hand, and phase-canceling circuits on the other (am I right about this?). It strikes me that there are at least two advantages to an audio filter in a regenerative set: (1) allowing discrimination between closely spaced signals, especially CW ones; and (2) filtering out the regeneration "rush noise" when operating on the high-sensitivity threshold of regeneration and copying weak signals.
--Adding several more bands: the 20m and 40m 80m ham bands and a couple of SWL bands (the latter, because the audio quality of AM reception is excellent if you greatly lower the plate voltage, and I'm a BBC World Service addict). This will be done by winding new coils and changing the values in the capacitor network (eventually, the capacitors will be mounted inside the coil forms).
--However KB1GMX has a good point in advising the use of a frequency changer instead (in effect creating a superhet with a regenerative tunable IF). I have a 3.5MHz crystal which could be the basis for an 80m converter (tuning "backwards," subtractively from 7MHz to reach 3.5MHz) and a 20m converter (using the 7MHz harmonic of the crystal, adding to 7MHz to reach 14MHz). Or I could follow his suggestion and "retune" the regenerative receiver down to a lower "IF" (1.75MHz or 3.5MHz) which presumably would give greater selectivity while still being high enough to avoid images on 20m. I guess the next question would be whether to convert the current RF stage to be the mixer of the frequency changer; or instead, to build an "outboard" frequency changer to place in front of the RF stage.
--Edited to add: Silly me! I actually have a crystal for exactly 7000MHz. It's an FT243 transmitting crystal in my collection, but I'm a General and thus barred from using that frequency (and probably wouldn't risk being so close to the band edge, even if I were an Extra). So that's a perfect building block for a very simple 20m converter, for starters.
--A calibration system. This will probably just be a one-tube 100KHz oscillator. I will add a new, small-value variable "Calibration" capacitor to set the band edges.
--Transmit-receive circuitry, probably just a small reed relay.

When this process of continual experimentation/improvement is "finished" I will build a "proper" receiver in a nice handmade wooden case, in a much smaller form factor and with better layout and short, logical signal paths. Ideally I will design and make my own PCBs for this, although maybe I'll just stick to point-to-point wiring for the first iteration. A PCB build would be much more compact, apart from the controls, which are numerous and take up a lot of room (necessitating a substantial front panel).

Here's the latest schematic (this is exactly the current build):

(https://lh3.googleusercontent.com/-V4lSHlRuRzo/VVh_j3lCFUI/AAAAAAAAE6g/fmOzp4CP9a4/s912/Submini_regen_large_layout_schematic_09.jpg?gl=US)

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 17, 2015, 06:55:11 AM
I've devoted a bit more thought to the idea of building a 7MHz oscillator (I have a crystal) as the basis for a 20m converter. In addition to 40m, I'm mostly interested in 20m -- when I start transmitting, it will be 40m, with 20m following soon after -- after I have built my second transmitter.

Looking over the circuits for converters, they often follow a 3-stage topology: RF amp, oscillator, mixer.

Since I already have an RF amp I presume it would be logical to put the mixer in between the existing RF amp and the detector. Alternatively I could convert the existing RF amp into a mixer, but then I would lose the advantages of having a good, gain-controllable RF stage (or would I?). If I leave the existing RF amp in place, I will still need to adapt it to cover 20m as well as 40m -- either with a new coil/capacitor combo, or with a large-value variable capacitor swing to capture both bands on one coil.

So my thinking is: use a strapped 1AD4 as a triode Xtal oscillator, and then inject that signal into the receiver with a new 1AD4 pentode mixer stage just ahead of the detector, for use when receiving 20m. Add circuitry to "bypass" the mixer when using the set for 40m reception.

The next challenge is figuring out how to do a Hartley oscillator with a directly heated triode. I'm working on it ... will build and test the oscillator first (it can also serve as a cool band-edge calibrator!).

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 17, 2015, 08:04:38 AM
For Peter (G3RZP). Three weeks ago, in a different thread, you and I discussed the problem that the voltage on the AF stage grid was varying when the regeneration control setting was changed. This was an indication that RF was leaking into the audio stage.

Specifically, these were the grid voltages at the end of April:
--Regen control at maximum resistance (50K) and no regeneration: -0.4V.
--Regen control in controlled oscillation (about 20K): -0.8V.
--Regen control shorted and oscillation a loud squeal: -3.0V.

Today I checked again, after the total rebuild of the detector stage and the installation of a large-value choke load in the audio stage. The new grid voltage values are as follows.
--Regen control at any setting, except the maximum setting where "squeal" sets in: a rock-steady -0.34V.
--Regen control shorted and oscillation a loud squeal: -1.2V.

I am assuming that there is nothing unusual about the spike in voltage while "squealing" because that's a natural result of extreme *audio* (not RF) feedback. So I think it is safe to say that the old problem of RF reaching the AF grid is fixed. I did pay more careful attention to bypassing during the rebuild, including installing a thick 14AWG tinned copper wire as a ground bus alongside the terminal strip, and keeping capacitor leads shorter.

73 de Martin, KB1WSY




Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 17, 2015, 09:08:47 AM
So I've been reading all about Colpitts oscillators in QSTs from the 1920s -- from a series called "How Our Tube Circuits work" by Robert S. Kruse. Great stuff. I also looked at the standard circuits in "Understanding Amateur Radio" (ARRL 1963).

Trouble is that right around the time that crystal oscillators became widespread, so did indirectly heated cathode tubes. So I couldn't find a *directly* heated Xtal oscillator circuit.

However I've noticed that in a number of applications, a "cathode" current was developed from a filament tube by tapping the middle of a capacitive divider across the filament. So I've improvised and here's the circuit. Wish me luck, I'm building it, unless someone jumps in with a sharp rap across the wrist:

(https://lh3.googleusercontent.com/-tQW2BbzxVzI/VVi7Xr9NtmI/AAAAAAAAE7M/hLBOijRoutA/s512/Colpitts_02.jpg?gl=US)

73 de Martin, KB1WSY



Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1GMX on May 17, 2015, 10:53:01 AM
Look at what AA1TJ did in his QF721 Superhet RX.  There is both a crystal controlled converter
(product detector) and a free running (VFO) mixer/converter there.  the tubes he used are close
to the 1AD4.

If you float the pentode screen grid it can be used as a second control grid or a intermediate plate
for RF feedback.  or you can do it with two tubes (osc mixer) or a 1R5 (pentagrid converter tube).

Look here for that:
http://aa1tj.blogspot.com/2012/05/mystery-tube-20m-superhet-receiver.html

Also look at the hikers portable for that approach: QST-sept-1950

With the right values the circuit you show will oscillate.  FYI the plate connection for one end of the circuit can be
the screen grid with a 2mh choke and the RF off the plate.  The easy way to get both sides of the heater/filliment
isolated from battery/ground is to use a bifilar choke (pair 10t on ft50-43) and put a cap across the heater side and the
battery connection on the other.   

The other way to make an oscillator is like the regen aka Armstrong oscillator with feedback coil on plate
and the tuned grid circuit. 


Allison


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 17, 2015, 02:51:17 PM
With the right values the circuit you show will oscillate.
Allison

I built it and did oscillate, but only intermittently (and most of the time, it didn't oscillate at all). Time to start playing with the component values. As you said (and as the ARRL literature explains) these values are tube-specific. I will try the whole range they suggest.

(When it did oscillate, it was deafening in the 'phones, which is only to be expected.)

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 17, 2015, 03:35:10 PM
OK we have liftoff. Vigorous and stable oscillation. It required increasing the value of one of the two capacitors that's across the crystal (from 220pF to 470pF) at which point it works. The value of the second of those capacitors seems to be less critical: it oscillates anywhere between about 6pF and 16pF.

Now that I have a working 7000KHz oscillator let me see what I can do with it....

Hey, that was quite good really. I designed my own circuit (combining a classic Colpitts with a method I had spotted for using a capacitive divider across the filament). When it didn't work, I played with the component values as suggested by ARRL, and with encouragement from KB1GMX, until I got it working.

Now my confession. Half way through this I screwed up and inadvertently shorted something to ground. Spectacular puff of light-blue smoke and strong smell! But I couldn't figure out which component had emitted the smoke. After a lot of head-scratching and testing, it turned out to be the RF choke ... fortunately I had one spare choke of that value still left in the junkbox!

Updated to add schematic with the component values that actually worked. The Colpitts oscillator circuit is from "Understanding Amateur Radio" (ARRL 1963) adapted by me to work with a directly heated tube. The component values are within a fairly large range of values suggested by ARRL, "depending on the tube used," and were determined experimentally after quite a few false starts.

(https://lh3.googleusercontent.com/-kij6OHr-rPw/VVk8-XmnuuI/AAAAAAAAE7c/PumN1iWGnLE/s576/Colpitts_03.jpg?gl=US)

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 18, 2015, 01:52:38 AM
Can the RF stage also be a mixer?

This morning I'm scratching my head a bit, so it's time to ask yet more dumb questions.

For the moment I'd like to limit the multi-band capabilities to 40m and 20m. For 40m we can operate "straight through." For 20m it would be interesting to use a crystal-controlled converter, as discussed in previous posts (this could output either 7MHz and work additively, or the overtone at 21MHz and work subtractively). This would be instead of winding a new set of coils for 20m and running the set "straight through" as a 20m regenerative receiver. Possible advantages of using the converter:

--Superior performance: It is said that the performance of "straight regen" deteriorates at the higher HF frequencies: sensitivity, selectivity. By using a converter I end up with, in effect, a superhet on 20m.

--Mechanical simplicity: No need to change the detector coil. As for the RF stage, it should be possible to "peak" both 40m and 20m with the existing tuning range, simply by sweeping the capacitor to the other end of the scale.

--On the other hand, with a 7MHz or 21MHz crystal, there would presumably be an annoying "band edge" crystal overtone on 14000KHz and it could splatter quite far into the band (the regenerative detector easily gets overloaded). I also have a crystal for 6800KHz and could use it instead; that way, the tone would be out of band, but I'd have to shift the detector's tuning band upwards by 200KHz so that it starts at 7200KHz (this would not be hard to do; I can remove about 3pF of capacitance in the tuned circuit). Alternatively, I could build a 7MHz tuned circuit at the oscillator output, to suppress the 14MHz harmonic, but would it provide enough harmonic attenuation?

Because, for the moment, I'm only interested in adding one "frequency converted" band (20m) and have already built a working Colpitts 1AD4 crystal oscillator, I was trying to figure out whether the Xtal oscillator output could be injected directly into the RF stage, thus avoiding the need to add a mixer tube (involving more construction, and possibly additional mixer noise).

I didn't completely understand the advice given by KB1GMX a couple of posts ago. I did consult his references (the blog post and the QST hiking set from 1950) and also looked at several converter circuits from old ARRL books. Is he saying that I can inject the oscillator output directly into the screen of the RF stage? Or is he saying that I should "float" the screen on the *oscillator* (turning it back into a pentode, instead of a strapped triode) and use *that* as a combined oscillator/mixer tube?

If he means that the RF stage is usable as a mixer, how should the RF-stage screen circuit be modified? Here is the current build:

(https://lh3.googleusercontent.com/-xDA69K_hdMg/VVml3VXl0hI/AAAAAAAAE70/kotxxliHDZw/s512/Submini_regen_large_layout_schematic_RF_10.jpg?gl=US)

In several published circuits for converters/mixers, the oscillator output is injected into the mixer's screen, and there is an RF choke (usually around 1mH) connected between the screen and the plate -- or sometimes, connected between the other side of the screen dropping resistor and the plate. The screen is also bypassed with between 10,000pF and 1,000pF, depending on the circuit consulted.

Will this work and if so, why? What is the purpose of the choke? I am assuming it is to prevent the oscillator output from getting into the B+ supply?

For what it's worth, I've already experimented with injecting the oscillator into the RF-stage screen, and it didn't seem to "work" (it did not shift the receiver's tunable frequency). However I made no prior changes to the RF-stage screen circuit; so it's not surprising it didn't work.

One fascinating aspect of building radios is that it's not sufficient to understand each "modular stage" in isolation: you also need to figure out how to connect them together!!!

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1GMX on May 18, 2015, 04:10:02 PM
Martin,

The choke isolates the screen at RF from the DC supply allowing it to be a source or
to inject a signal there.  Think of Dual Gate FETs.  The screen also controls current flow so
modulating it at RF (mixing) works and it is somewhat isolated from the control grid (G1).

Allison


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 18, 2015, 06:19:39 PM
Martin,
The choke isolates the screen at RF from the DC supply allowing it to be a source or to inject a signal there.  Think of Dual Gate FETs.  The screen also controls current flow so modulating it at RF (mixing) works and it is somewhat isolated from the control grid (G1).
Allison

Thanks. Still not clear whether this is a discussion of direct injection into the *RF Stage* as opposed to a separate mixer tube. I will give it a go tomorrow -- direct injection into RF stage, with choke isolation from the B+ supply.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1GMX on May 18, 2015, 06:21:20 PM
Martin,

For the moment I'd like to limit the multi-band capabilities to 40m and 20m. For 40m we can operate "straight through." For 20m it would be interesting to use a crystal-controlled converter, as discussed in previous posts (this could output either 7MHz and work additively, or the overtone at 21MHz and work subtractively). This would be instead of winding a new set of coils for 20m and running the set "straight through" as a 20m regenerative receiver. Possible advantages of using the converter:

It can be done but likely going to be messy with switching.

--Superior performance: It is said that the performance of "straight regen" deteriorates at the higher HF frequencies: sensitivity, selectivity. By using a converter I end up with, in effect, a superhet on 20m.

Sensitivity is good on regens up there.  Selectivity, its more or less a case of every time you double the frequency you
double the selectivity. That's for the regen case (AM reception) for the oscillating case its a DC receiver and the actual selectivity
is the audio band pass.  

--Mechanical simplicity: No need to change the detector coil. As for the RF stage, it should be possible to "peak" both 40m and 20m with the existing tuning range, simply by sweeping the capacitor to the other end of the scale.

Thats been done in the past and can work well.


--On the other hand, with a 7MHz or 21MHz crystal, there would presumably be an annoying "band edge" crystal overtone on 14000KHz and it could splatter quite far into the band (the regenerative detector easily gets overloaded). I also have a crystal for 6800KHz and could use it instead; that way, the tone would be out of band, but I'd have to shift the detector's tuning band upwards by 200KHz so that it starts at 7200KHz (this would not be hard to do; I can remove about 3pF of capacitance in the tuned circuit). Alternatively, I could build a 7MHz tuned circuit at the oscillator output, to suppress the 14MHz harmonic, but would it provide enough harmonic attenuation?

That's a problem of using a 7Mhz radio for 14 or 21.... harmonics are a problem.
The only solution is to move the IF.  

Because, for the moment, I'm only interested in adding one "frequency converted" band (20m) and have already built a working Colpitts 1AD4 crystal oscillator, I was trying to figure out whether the Xtal oscillator output could be injected directly into the RF stage, thus avoiding the need to add a mixer tube (involving more construction, and possibly additional mixer noise).

Look at the AA1TJ design.  There are two tuned circuits in the grid one for the RF and one for the Osc, the osc has a coil
off the plate (armstrong) and the IF is in series with that.  The problem is that harmonically related frequencies don't work.


I didn't completely understand the advice given by KB1GMX a couple of posts ago. I did consult his references (the blog post and the QST hiking set from 1950) and also looked at several converter circuits from old ARRL books. Is he saying that I can inject the oscillator output directly into the screen of the RF stage? Or is he saying that I should "float" the screen on the *oscillator* (turning it back into a pentode, instead of a strapped triode) and use *that* as a combined oscillator/mixer tube?

Use the pentode as a pentode there are two grids you can control.  Or just use a 1r5 (helptode/pentagrid tube) as a
crystal controlled converter.  It works well at 45V and the filament is 50ma.


If he means that the RF stage is usable as a mixer, how should the RF-stage screen circuit be modified? Here is the current build:

Hint: Allison is a XYL, not  OM. ;)  

Yes, you can make the RF amp into a mixer but its awkward.   The context is when you want to go to superhet
that can be a stepping stone using the regen as an IF.  Suggested IF would be 2.5-3mhz as thats high enough to go to
20m and would work for 80m and 40M as well.  Obviously the converter would have to tune all of those.

Hope it puts an image in here...
(http://www.eham.net/home/allison/Desktop/converter.jpg)

Allison


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1GMX on May 18, 2015, 06:28:11 PM
Can't post an image.

Email works I'm in QRZ.

Allison/Kb1GMX


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 18, 2015, 06:36:19 PM
Hint: Allison is a XYL, not  OM. ;)  

Mortifying apologies. I've been confusing you with the ARRL Allison, who I saw striding past me in a corridor at the Hartford convention center at the ARRL centennial a year ago. Definitely of the male chromosome but has a different callsign (that Allison was the co-author of an equipment review in the current QST) ... but perhaps I'm even more confused than I thought and I got the wrong end of the stick, and you are all the same Allison, and I mis-read the nametag. Apologies again!

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 18, 2015, 07:10:33 PM
It can be done but likely going to be messy with switching.

Yes I've been thinking about that. But in a 2-band iteration perhaps not too hard.

Sensitivity is good on regens up there.  Selectivity, its more or less a case of every time you double the frequency you double the selectivity.

Do you mean that negatively or positively? Double the frequency, double the passband, or halve it? I presume you mean, negatively, i.e. double the passband. This doesn't happen with heterodyning ... you keep the same selectivity even if you double the frequency by addition.

That's a problem of using a 7Mhz radio for 14 or 21.... harmonics are a problem. The only solution is to move the IF.  

Yes, I've started thinking about that. Moving the detector's range to start at 5MHz or lower for instance. I'd have to find some new crystals, but that's not a big deal. Then, I could use the converter for both 40m and 20m. At which point it would no longer be a "high performance regenerative receiver," but a superhet with a tunable, regenerative IF.

Use the pentode as a pentode there are two grids you can control.  Or just use a 1r5 (helptode/pentagrid tube) as a crystal controlled converter.  It works well at 45V and the filament is 50ma.

Yes, you can make the RF amp into a mixer but its awkward.   The context is when you want to go to superhet that can be a stepping stone using the regen as an IF.  Suggested IF would be 2.5-3mhz as thats high enough to go to 20m and would work for 80m and 40M as well.  Obviously the converter would have to tune all of those.

So I'm moving toward adding a proper mixer tube, after the RF stage.

I am actually quite interested in trying *both* options:
--The straight-through 20m regen (achieved through winding new coils).
--The 20m achieved through conversion (adding an oscillator and mixer).

Then comparing the performance of the two options. I'm still sort of fascinated by taking the "pure regenerative receiver" through its paces.

Can't post an image.

Yes, it's a pain, you have to host the image on a third-party service (Picasa in my case), then copy the url, and paste that link into eham. It's fussy, and even if it works, there's no obvious control over the image size as displayed in QRZ.

Edited to add: Speaking of antique subminiature tubes, do you have $47,000 to spare? If so, see http://tinyurl.com/n8tyour (http://tinyurl.com/n8tyour).

It says, "LOOK AT THE COOL  TUBE AMPLIFIER YOU CAN MAKE WITH THESE TUBES."

Seventy-five thousand of them? That's a lot of amplifiers. I suppose there is a market ... if you are imaginative. These little tubes are popular with a subset of guitar-audio buffs.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 19, 2015, 11:29:17 AM
While we are still on the subject of the oscillator, on further usage it turns out to be less reliable than I thought. Sometimes it oscillates, sometimes it doesn't, and it is very touchy about which crystals will oscillate and which won't. My crystals are vintage FT-243 transmitting types (the originals, not the retrofitted modern crystals in an old casing). They work fine in my transmitter, but that's with much higher voltages.

So again, here's the schematic of the version that (intermittently) works. It required a lot of playing around with the values of the two capacitors that are across the crystals. The B+ in this radio is about 52 volts.

(https://lh3.googleusercontent.com/-kij6OHr-rPw/VVk8-XmnuuI/AAAAAAAAE7c/PumN1iWGnLE/s576/Colpitts_03.jpg?gl=US)

FYI the plate connection for one end of the circuit can be the screen grid with a 2mh choke and the RF off the plate.  

Can you elaborate? I don't fully understand the exact circuit change you suggest (what is connected to what?). If it involves using the screen (instead of strapping the pentode as a triode), I am hopeful that it might provide more amplification and therefore more reliable oscillation. My apologies again for being dense, I'm still very much a beginner with this stuff.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1GMX on May 19, 2015, 03:09:32 PM
The oscillator,

First use the screen grid and connect that to B+.  The 470pf is a bit large and the 10PF may be a bit small.

What that circuit wants is the signal from the plate to the grid to be large enough to oscillate at the
series resonance of the crystal.  If the 470 (plate cap) is too large you effectively short out the RF
and if the grid cap is too small you do not get the right loading of the crystal and it may take off
at other frequencies.



Allison


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 20, 2015, 04:28:57 AM
First use the screen grid and connect that to B+.  The 470pf is a bit large and the 10PF may be a bit small.

What that circuit wants is the signal from the plate to the grid to be large enough to oscillate at the series resonance of the crystal.  If the 470 (plate cap) is too large you effectively short out the RF and if the grid cap is too small you do not get the right loading of the crystal and it may take off at other frequencies.

Allison

Concerning the screen circuit modification, is this what you meant?

(https://lh3.googleusercontent.com/-XkvEjcHAyEA/VVxd9Cf1e2I/AAAAAAAAE8I/b7s4zyPEAag/s912/Colpitts_04.jpg?gl=US)

I have replaced the small cap with a 5-40pF air trimmer, and the big cap is a test jig with two alligator clips soldered in so that I can quickly try any value of fixed capacitance.

With the modified circuit (pentode, as above), I cannot get oscillation at any value of the big capacitor. I tried a range between 220pF and 680pF and each time, swept the small trimmer through its range.

With the original circuit (pentode strapped as triode), oscillation starts at around 430pF and ends at around 600pF, with a "sweet spot" at around 470pF. At that value, the small trimmer is about half-meshed, so let's say it's 20pF or so. With those values, several of my crystals usually (but not always) oscillate, while other crystals won't trigger.

Suggestions? I'm happy to continue the experiment. Not yet sure what I will do with this oscillator once it's reliable. For the moment, it makes a neat calibration device for the receiver. But I'm probably stepping back a little bit on the "frequency converter" project for the time being: I think I'll keep the basic regenerative design and build some coils for 20m first.

If performance on 20m in "straight-through regen" mode is disappointing, I will probably forge ahead with the frequency converter idea and incorporate it into a second copy of this set (I want to keep the original "pure regen" build as an example of how far one can go with such a design). If I do go ahead with a second set with a frequency converter, I will probably design the detector to tune a lower frequency and then use the converter for both 40m and 20m. I have about 20 of the 1AD4 tubes, so lots of raw material for experiments.

Meanwhile, I am learning lots about Colpitts oscillators....

Thanks again.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 20, 2015, 04:57:42 AM
Adding an AF gain control

With optimal settings for RF gain and regeneration, some signals were painfully loud in the headphones. So I added an AF gain control. However there are issues (described below).

The existing circuit had a fixed 470K grid bias resistor on the AF tube, which yielded a steady grid voltage of minus 0.34 volts regardless of the setting of the detector's regeneration control.

I decided to use the 4.8V bias battery (already employed in the RF stage) to add some variable bias to the AF tube.

This is the circuit I ended up building:

(https://lh3.googleusercontent.com/-ktBe-aylpas/VVxzdpptTEI/AAAAAAAAE8g/IxJY-z0rqVY/s720/Submini_regen_AF_Gain_01.JPG?gl=US)

Explanations:

--The 1M pot was used because that's what I had in the junkbox; eventually it should be replaced with a 500K pot.

--The 68K resistor in series with the pot was added so that, at maximum volume, there was still a bit of negative bias on the tube. Its value was determined experimentally: it was the right value to yield the same grid voltage as the original, fixed-gain circuit (minus 0.34V), at maximum volume.

--The 1µF capacitor was added to reduce pot noise.

--The 10K resistor is there because ... it was part of a similar "gain control" circuit that G3RZP designed for the RF gain circuit so I thought it was probably a "good idea." I have no idea why it's there, but I assume it helps to equalize voltages or something?

As built, the AF gain functions very nicely, going from maximum volume down to nearly zero in about half the sweep of the 1M pot (hence, the need to replace it with a 500K pot in future). During rotation, the grid voltage varies from minus 0.34V at maximum volume to about minus 5V at minimum volume. So far, so good.

However there is now noticeable audio distortion on all signals, especially strong ones. It is a similar sound to what you get if the detector is overloaded, but it is happening even when the detector isn't overloaded (it is now very easy to eliminate detector overload, simply by adjusting RF gain).

Why?

My first thought is that I maybe I have neglected some audio decoupling/bypass issues?

A couple of other, much less important questions:

--As it happens, the 1M pot from the junkbox has an audio taper. Is that appropriate? It does seem to yield a smooth gradient of volume control.

--The presence of the 1µF capacitor makes a noticeable difference to the audio tone; it seems "thinner." I suppose this is acting as an ersatz "audio filter"?

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on May 20, 2015, 10:52:18 AM
When you vary the bias, you vary where the signal sits on the tube grid volts to plate current curve. This curvature produces harmonics, so you want it biased where there is least curvature and biggest range of signal before limiting. In an RF amplifier this doesn't matter, because the plate tuned circuit rejects the harmonics, but in an AF amp, it does. So you should couple through the .01mFd capacitor to the top of the volume control. The slider goes to the grid, and the bottom of the control is bypassed with a capacitor - I would use 4.7mFd but you might get away with 1 mFd. The bottom of the pot should have a resistor to ground and a resistor to the bias voltage: it might be easier to use something like a 10 k pot across the bias supply with the slider to the  bottom of the volume control. Having established the bias that gives least distortion, choose two resistors as a potential divider to give that voltage. They can be pretty high values - in the tens of kohms for the smaller one.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 20, 2015, 11:59:50 AM
When you vary the bias, you vary where the signal sits on the tube grid volts to plate current curve. This curvature produces harmonics, so you want it biased where there is least curvature and biggest range of signal before limiting. In an RF amplifier this doesn't matter, because the plate tuned circuit rejects the harmonics, but in an AF amp, it does. So you should couple through the .01mFd capacitor to the top of the volume control. The slider goes to the grid, and the bottom of the control is bypassed with a capacitor - I would use 4.7mFd but you might get away with 1 mFd. The bottom of the pot should have a resistor to ground and a resistor to the bias voltage: it might be easier to use something like a 10 k pot across the bias supply with the slider to the  bottom of the volume control. Having established the bias that gives least distortion, choose two resistors as a potential divider to give that voltage. They can be pretty high values - in the tens of kohms for the smaller one.

Do you mean like this?

(https://lh3.googleusercontent.com/-h_yPxQqpKNw/VVzYvqwckDI/AAAAAAAAE8w/4siXTWySMe8/s720/Submini_regen_AF_Gain_02.JPG?gl=US)

Asking because doesn't the resistor across the capacitor undo the capacitor's bypassing effect?

Also not totally sure whether you mean the pot should be a low-ish value (10K) or whether you are suggesting an alternative circuit in which the pot would be across the bias supply and be 10K.

In the above diagram, have I installed the electrolytic the right way round?

Finally, something I don't understand. I have read about the tube curves and the importance of setting the bias so that the tube operates in a linear portion of the curve. What I don't understand is that the old circuit (with a fixed 470K bias resistor) biased at -0.34V had no obvious audible distortion, but the volume control that I built and optimized to have the same -0.34V at maximum volume, produced obvious distortion at maximum volume (and lower volumes too, by the way).

I think I will initially optimize it for a maximum-volume bias of -1.0V. That's because I've just tested it and found out that there is no audible difference in volume between -0.34V and -1.0V. Am I right that this is evidence that the curve is non-linear in that portion (it has flattened out considerably)? During testing, I found that the effect of the volume control only began at the -1.0V point and then volume changed fairly smoothly as you go through the -2V to -4V points.

As for audio vs. linear taper, I just tested both of them, and found that -- strangely enough -- the linear taper produced a more audibly "proportional" response, to my ear. But of course, I haven't optimized the circuit yet...

You and Allison are being most patient with me. It may not seem like it, but I'm learning a lot. Thank you.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 20, 2015, 01:21:43 PM
So I built the circuit according to the schematic in my last post -- which is my visual interpretation of Peter's voltage-divider circuit.

In my implementation, the pot is 500K, resistor "X" is 150K and resistor "Y" is 470K. It seems to work rather well, although there's not enough 40m traffic on the air right now (mid-afternoon) to assess whether the distortion issue has improved. I will report on that later -- and if the distortion is still noticeable, I will try different values of the X/Y resistors. [See update later in this post.]

As a volume control it works nicely, with a very smooth gradation between maximum volume and silence. Again, linear taper, strangely enough. Maybe it's just my ears that are odd!

I am however even more puzzled than before. If I put a VTVM on the grid, the voltage between maximum and minimum volume hardly budges at all: it stays right on minus 1.2V. If that is the case, how are we able to obtain that huge swing in volume without varying the bias voltage on the grid?

Edited later in the evening to add: I have now been able to monitor many strong SSB signals and the distortion is no longer audible. I did increase the value of resistor "X" from 150K to 220K, to bring the grid bias into what seems to be the start of the more "linear" part of the curve. With "Y" at 470K and "X" at 220K, the bias has settled at -1.6V.

Conditions are noisy right now and this receiver, when all the controls are "optimized for gain," has enormous volume into the headphones, even with only a single audio stage. So it's great to have plenty of options for control. At maximum gain, strong signals overload heavily, but that is totally controllable with the RF gain control. With a strong SSB or CW signal, I have to turn down both the RF and AF gain considerably and sometimes increase the regeneration (with consequent loss of sensitivity and selectivity, but with a really strong signal that's not an issue).

This set now has seven dials (not counting the power switches). Fun!

73 de Martin, KB1WSY



Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: WB6BYU on May 20, 2015, 02:47:40 PM
That circuit does NOT adjust the gain by varying the bias on the tube:  if the grid current is zero there
will be no DC voltage drop through the pot regardless of setting.

Instead it acts as a standard volume control - that is, and adjustable voltage divider on the audio
signal coming in through the  .01 cap.  That audio develops a voltage drop across the pot relative to
ground (through the 4.7uF, and, yes, that is the correct polarity.)  With the wiper at the top of the pot
there is maximum audio voltage applied to the grid.  At the other end of travel very little audio voltage
is developed between the grid and ground.  The net effect is to vary the audio voltage applied to the
tube, rather than the gain of the tube itself.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 20, 2015, 02:56:05 PM
Instead it acts as a standard volume control - that is, and adjustable voltage divider on the audio signal coming in through the  .01 cap.  That audio develops a voltage drop across the pot relative to ground (through the 4.7uF, and, yes, that is the correct polarity.)  With the wiper at the top of the pot there is maximum audio voltage applied to the grid.  At the other end of travel very little audio voltage is developed between the grid and ground.  The net effect is to vary the audio voltage applied to the tube, rather than the gain of the tube itself.

Thank you for the clear explanation! Another yawning gap in my knowledge has been filled.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1GMX on May 20, 2015, 04:36:54 PM
>>>Suggestions? I'm happy to continue the experiment. Not yet sure what I will do with this oscillator once it's reliable. For the moment, it makes a neat calibration device for the receiver. But I'm probably stepping back a little bit on the "frequency converter" project for the time being: I think I'll keep the basic regenerative design and build some coils for 20m first.

First at 7mhz a 220 pf cap is a reactance of about 100ohms so compared to the tubes plate resistance its near a dead short for RF.
I'd expect more like 10-80pf and the other cap make it variable in the same range.  Look at other oscillator circuits some work better
with low gain tubes. 

>>If performance on 20m in "straight-through regen" mode is disappointing, I will probably forge ahead with the frequency converter idea and incorporate it into a second copy of this set (I want to keep the original "pure regen" build as an example of how far one can go with such a design). If I do go ahead with a second set with a frequency converter, I will probably design the detector to tune a lower frequency and then use the converter for both 40m and 20m. I have about 20 of the 1AD4 tubes, so lots of raw material for experiments.

yes, it gets worse with higher frequency as a regen, this is the mode used for AM typically.  If
its oscillating for CW or SSB no difference as its operating as a DC RX (autodyne converter, RF in mixes
with self oscillation and the resulting audio difference is heard.).

In the early days of CB (early 60s ) there were a few regen (actually superregen) CB sets (also the heath tower and sixer
back when AM was king on 6 and 2M) and typically for AM the receiver was very sensitive but you could never tell if the
person hear was only your channel or 5 channels up or down (as much as 60khz).   

My first aircraft receiver, one transistor supperregen, when set for 120mhz it could hear everything from
about 118 to 124 easily with a 10 inch whip and as far as 40 miles away easily.   A later one rand the regen
at 20 mhz and I and made a simple converter circuit from Airband to 20mhz.. That was about 50khz wide for
any channel good enough as Aircraft radios of the day were on 100khz spacing.  As you go up regens get
wider.


Meanwhile, I am learning lots about Colpitts oscillators....

[Indeed.   Don't forget Miller, Pierce, Hartley, Vacker, Seiler, Clapp and likely others less known.

Allison


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 20, 2015, 06:46:56 PM
First at 7mhz a 220 pf cap is a reactance of about 100ohms so compared to the tubes plate resistance its near a dead short for RF.... I'd expect more like 10-80pf and the other cap make it variable in the same range.  Look at other oscillator circuits some work better with low gain tubes. 

I have just tried the values you suggest but the oscillator is mute.

Don't forget Miller, Pierce, Hartley, Vacker, Seiler, Clapp and likely others less known.

Yes, it is probably time to make their acquaintance. Actually I already know Pierce (the Xtal oscillator in my transmitter) and Vackar (the oscillator in my solid-state regen).

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on May 20, 2015, 11:43:18 PM
Quote
Don't forget Miller, Pierce, Hartley, Vacker, Seiler, Clapp and likely others less known.

However, the Pierce, Vackar, Seiler, and Clapp are all basically versions of the Colpitts. Hartley is different, as are the Miller (tuned plate tuned grid) and its version the tickler - untuned plate or untuned grid - (although this is related to the Hartley), the Meissner, Franklin and the cathode coupled, and then there are the negative resistance tube oscillators - the dynatron and the transitron.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 21, 2015, 02:48:53 AM
Peter and Allison are such name-droppers!

Colpitts (1872-1949) was a Harvard graduate who was research branch chief for Western Electric, the precursor of today's blandly named Lucent. His oscillator dates to about 1918-1920 (sources vary). Ralph Hartley (1888-1870) was a colleague of Colpitts who filed a patent for his own oscillator in 1915. Both of these fellows, or the teams they headed, went on to achieve significant advances in tube amplifiers, multiplex technology, and other fields. Hartley made a contribution to mathematics with the Hartley Transform.

The only reason I tried the Colpitts first is that I'm lazy and didn't want to design and build an inductor! That turned out to be a false economy apparently.

I'll probably try a Hartley next although, as Peter and I discussed a while back in another thread, with a directly heated tube I'll probably have to float the filament. The discussion is here: http://tinyurl.com/l6vhh7b (http://tinyurl.com/l6vhh7b). Or maybe I could try the same trick that I used with the Colpitts, using a capacitive divider on the filament.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 21, 2015, 03:45:02 AM
Moving Forward: Adding 20m, Audio Filtering, an Audio Power Amplifier

Oscillator experiments aside, I'm interested in completing this "high performance regenerative receiver" to add the same features that Kitchin has in his solid-state set, although my tube version could end up being somewhat more advanced.

--Adding 20m (and eventually other bands, including SWL broadcast): I will do this by winding new coils and setting up new capacitor networks. Eventually the capacitors will be moved inside the coil forms (6-pin for the detector, 5-pin for the antenna coil). I may mount air trimmers inside the forms, for calibration, but I suppose that could degrade the Q because of the metal?

--Audio filtering: Kitchin uses a simple passive filter by switching capacitors into the audio circuit. I'm thinking of building an active audio filter that will either boost a particular audio frequency, or reject it, and also have adjustable gain/rejection. That circuit is the Selectojet Selectoject, which can be found in many publications; I found it in the 1963 Radio Amateur's Handbook. It's a phase-shifting circuit using a couple of dual-triodes. The challenge is adapting it to directly heated, low-voltage subminiature tubes, a self-inflicted limitation but rather fun too; the only submini dual triodes I'm aware of are (1) cathode-type tubes with 6.3V filaments and (2) use a B+ of more than 100V. I could use four 1AD4 tubes, instead of two dual triodes -- but at the cost of an extra 200mA of heater current compared to the dual tubes. (The Selectojet would add another three controls to my set, which already has seven controls. Airbus cockpit, eat your heart out!)

--An audio power amplifier. Yes, I know that I've been complaining that the headphone volume was too loud (prior to adding the AF gain control). However, at the other end of the spectrum, very weak signals often get the best S/N ratio when I set the controls for very quiet levels and at that level it is a strain to copy the code. So a bit more audio boost would be good. A couple of other considerations: I would like to be able to use low-Z headphones if desired, and I would like to add a small speaker (to show off my set to non-hams, and for SWL broadcast use, even if I only manage to get a few hundred milliwatts of speaker audio). Am I right that the best way to get high output for a relatively low level of distortion is to build it in class B push-pull? Yes, I know that the transformer(s) are an extra expense but I think it's worth it to get considerably better efficiency than a standard, one-tube Class A amp (plus I have an old, large push-pull output transformer in the junkbox). Again, the challenge is adapting to directly heated tubes. I have found several circuits on the Internet and will share my ideas later.

--Calibration oscillator: I could add a band-edge calibrator (using a 3.5MHz crystal and its harmonics). Or it could be classic 100KHz marker. First of all however, I find to have to build a reliable 1AD4 oscillator!

This morning I heard CO7EH calling CQ Asia. From his QRZ page he is in Camaguey, Cuba and is using a 600W linear built by another ham, CO7SP -- I assume he's using the linear in CW, not just SSB, because he says CW is his main mode.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 21, 2015, 06:19:00 AM
For the Selectoject, which is designed for two dual-triodes, the answer may be to use four 2E31 pentodes strapped as triodes. They only take a 50mA filament current. The B+ is 22.5V (half of the radio's main B+ potential) which I could get either with a resistive voltage divider, or by tapping my series string of seven 9V (actually, 7.4V) batteries three or four batteries along (but that risks having uneven discharge of the batteries). The mutual conductance is a modest 500µmos (only a quarter as much as the 1AD4) but presumably that's irrelevant in an audio filter, where we don't necessarily need any net gain.

The other possibility would be to use 1V6 triode-pentode tubes and strap the pentode, thus creating a dual-triode. But I don't think that would work because these are designed as mixer/converter tubes rather than voltage amplifiers; and presumably, also, the two halves of the tube would be unevenly "matched" which probably doesn't work well with the Selectoject's phase-shifting circuit. The other specs are good though: it's a low filament current, and the B+ is the same as for the 1AD4 (45V nominal).

Now, it's time to figure out how to convert the ARRL Selectoject circuit for directly heated tubes ... regardless of which of the above tubes I end up using.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 21, 2015, 07:55:05 AM
How does a phase splitter work?

The ARRL manual provides no explanation of how the Selectoject works. However, the two sections in the schematic are labeled "phase shifter" and "amplifier." Looking more closely, it appears that the principle is to take a "copy" of the incoming audio, shift its phase by 180 degrees, and then feed it back onto the original signal. By "picking off" a frequency, and depending how you wire it, you can either attenuate or boost that particular frequency.

But how does the circuit actually work? The basic phase-shifter circuit is described thus in Langford-Smith, "Radiotron Designer's Handbook" (and I can tell that the ARRL Selectoject circuit is closely related):

(https://lh3.googleusercontent.com/-uFuloHfND0A/VV3vVCf5jiI/AAAAAAAAE9E/US50AR7Qb6Y/s1024/Selectoject_01.jpg?gl=US)

Once again, I "get" some of the general principles but I still need some help! This is a "degenerative" circuit in which the load is spread to both the cathode and the plate. If you make the two resistances equal, you will succeed in shifting the phase by 180 degrees. But why? How does it work?

73 de Martin, KB1WSY



Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: WB6BYU on May 21, 2015, 08:05:25 AM
Consider three resistors connected as a voltage divider across a 4V source:  RL on each end, and a variable
resistor Rv in the middle.

When Rv is 0 ohms, the voltage drop across each RL is 2V.  The voltage at the bottom of the upper RL is
2V (as low as it can go) and the voltage at the top of the lower RL is also 2V, but that is the highest
value it can reach.

Now when Rv is 2 x RL, the voltage drop across each RL is 1V.  Now the voltage at the lower end of the
upper RL is 3V and on the upper end of the lower RL it is 1V.  Both resistors have seen a shift of 1V, but
one has increased and the other has decreased:  the voltages are out of phase with each other.

Use a tube or transistor for Rv and you have your phase splitter.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 21, 2015, 08:40:05 AM
Use a tube or transistor for Rv and you have your phase splitter.

Excellent explanation, I get it now, thanks!

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 21, 2015, 08:58:51 AM
Analysis of Selectoject circuit

Here is the schematic:

(https://lh3.googleusercontent.com/-AIEQZzYCUYo/VV37w52uCmI/AAAAAAAAE9U/ykCqJ-fENTQ/s800/Selectoject_02.jpg?gl=US)

Credit: Radio Amateur's Handbook, 1962/1963, reproduced with permission from ARRL.

Following on from the previous discussion about the voltage divider.

For V1A: By varying R12A you vary the amount of phase shift between the two signals. With R12A = zero you will get a 180-degree shift (because R2 and R3 are equal). As you increase the value of R12A, there will be an increasing imbalance between the two resistors and therefore lessening amounts of phase shift.

Having figured that out, I got all tangled in knots figuring out what the other tubes do.

I can see that V1B is also a phase splitter. But I'm not sure how the ganged R12A/B functions as a tuning control (??).

What about selectivity control R11: does it work by varying the amount of coupling between the original signal, and the out-of-phase signal?

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on May 21, 2015, 10:02:50 AM
V1A and V1B provide a phase shift between 0 and 90 degrees, the amount of which at any frequency is varied by R12A and R12B.  This variable phase signal is applied to V2B, and because of cathode follower action, to V2A when S1 is in the selective amplifier/oscillator position. The output of V2A when in position 1 is then applied to the AF input and at a frequency where the total phase shift is 180 degrees and the gain is enough (set by R11), it oscillates. With the gain reduced by R11, the gain is peaking at the frequency where the phase shift is 180 degrees and so is varied by R12A and R12B.

With the switch in position 2, the frequency where the phase shift is 180 degrees sees the input signal amplified by V2A and the phase shifted signal amplified by V2B but 180 degrees out of phase. So the signals cancel to give a notch. The width of the notch depends on the rate of change of phase shift with frequency.

It's not so easy with directly heated tubes, because you can't lift the filaments above ground without  separate filament supplies for each tube. Using extra tubes to get the in phase and out of phase signals is difficult because the loading on the tube plate varies with adjustment of the frequency control.

For a peak function with limited tuning range, you can use a ladder network around a single stage: it might even be possible to adapt a Wien bridge network, but I would have to think a bit more about that.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: WB6BYU on May 21, 2015, 10:05:40 AM
Quote from: KB1WSY

...By varying R12A you vary the amount of phase shift between the two signals...



Now you have to think in the frequency domain.  The RC circuit of C4 and R12A will have a phase shift
that varies with audio frequency.  Given that the resistor and capacitor are driven out of phase,
there will be some frequency (determined by the setting of R12A) where the phase shift from the
input to V1A to the input to V1B is 90 degrees.  When C8 and R12B are set to the same values then
V1B will also have a 90 degree phase shift at that frequency.  That gives a 180 degree phase
shift at a specific frequency determined by R12.  Other frequencies will pass through the stages,
but with different amounts of phase shift.

What happens after that depends on the setting of S2.  In one position the out-of-phase signal is
combined with a reference signal with 0 degree phase shift:  when the gains are identical, the
two signals cancel only on the frequency where the phase shift through the upper network is
exactly 180 degrees
.  That causes a null in the output at that specific frequency.  In the
other setting, the output is applied back to the input with the appropriate phase to give a peak
response at the desired frequency but reduced response at other frequencies - a variable
frequency peak.  With enough gain it will oscillate (just like any other regenerative circuit.)


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 21, 2015, 12:55:12 PM
Thank you very much WB6BYU and G3RZP for your explanations. I had to read them several times, even though they were very clear, because the action of the Selectoject is quite complex really.

It's not so easy with directly heated tubes, because you can't lift the filaments above ground without  separate filament supplies for each tube. Using extra tubes to get the in phase and out of phase signals is difficult because the loading on the tube plate varies with adjustment of the frequency control.

All sorts of possibilities:

(1) Trying to build the Selectoject with directly heated tubes but I agree that it's unlikely!

(2) Caving in, and using subminiature cathode-type tubes for the audio filter and for the future audio power amplifier. This particular receiver is not designed to be portable (although the core "three-tube" version that has been built so far could indeed be re-purposed as a small portable radio eventually). If I decided to add cathode tubes to the "shack-bound" version of the receiver, the power supply needs would get a bit Byzantine. There's the existing supply for the 1.2V filament tubes and their 52V B+, and then there'd have to be a 6.3V filament supply ... and a higher B+ supply as well, usually above 100V. If I were adding cathode tubes, it would make more sense to retrofit the existing three tubes by replacing them with subminiature cathode-type tubes ... and the whole project completely changes course (and at that point, why stick to subminiature tubes?).

(3) Building an outboard version of the Selectoject, with an AC-powered supply, with 12AX7s, and connecting cables to the regenerative receiver (it is designed to fit in between the detector and the first audio stage). Unwieldy.

(4) Giving up on the phase-shift system and using an inductive audio filter instead. There is an old ARRL circuit called the Audiofil that uses high-value inductors (about 5H each) in combination with a double-triode amplifier. The Audiofil is a single-gain, single-frequency CW filter that peaks at around 700Hz and has a passband of about 500Hz to 900Hz. It's nothing like as flexible as the Selectoject but surely much better than nothing, and I'm pretty sure it would be easy to build it using directly heated tubes. Maybe I could figure out a way to build an Audiofil with adjustable frequency and selectivity, which are the best features of the Selectoject.

(5) Defaulting back to Kitchin's simple capacitor-type passive audio filters. Again, better than nothing.

(6) Some other kind of active audio filter that will work with directly heated tubes! G3RZP mentioned a ladder network (I've heard of those but don't know anything about them) or a Wien bridge (over the Danube?).

(7) Using the solid-state version of the Selectoject instead (ARRL produced one). But that's cheating!

I just realized that it might be fruitful to search for old *audio amplifier* designs. They sometimes had filters for scratches on 78s and so forth ... like the one in my Dad's Quad II preamp. If I go back far enough I might find directly heated versions??

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 21, 2015, 02:10:51 PM
Here's a recent design for a "voltage controlled [audio] bandpass filter" using an EF86 pentode.

http://www.cgs.synth.net/tube/vcf.html (http://www.cgs.synth.net/tube/vcf.html)

I only vaguely understand the article but basically it's a variable-Q audio filter, with a "tune" control to set frequency. Because it's aimed at audiophiles and musicians, the audio frequencies are higher than what we'd want in a ham CW filter -- or at least that's how I understand it. But surely that's just a matter of adjusting a few component values. If it works with an EF86, it could possibly be adapted for a directly heated 1AD4....

Am I right that this is an interesting possibility?? (The context of the article is audio synthesis, so I may well have got the wrong end of the stick.)

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1GMX on May 21, 2015, 02:44:00 PM
The easier filter is what was the rule of that day.   There were 88mh toroids available and
they were run in various combinations to make LC based filters.  These days miniature
inductors are available in that range (1-1000mH) that can be used.

Most Opamp or active filter based  designs are not suited for directly heated tubes
with out a lot of contortions.

A good dual triode is 3A5, makes a good final too.

Allison


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on May 22, 2015, 12:58:49 AM
For things run off batteries, passive filters have a major power consumption advantage....


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1GMX on May 22, 2015, 07:02:29 AM
Here's a recent design for a "voltage controlled [audio] bandpass filter" using an EF86 pentode.

http://www.cgs.synth.net/tube/vcf.html (http://www.cgs.synth.net/tube/vcf.html)

That is a simple twin T audio filter with screen grid gain control. 

Likely you would have to use two stages for enough CW selectivity.   Using 2 2e26
would cost 100ma heater and 3-5ma of plate current.

Allison


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 22, 2015, 07:14:01 AM
For things run off batteries, passive filters have a major power consumption advantage....

Good point. Right now this project is facing contradictions/illogicalities going forward.

As built so far, it "makes sense" from a design point of view: it's a 3-tube, battery-powered set with a total filament current of 300mA, providing autonomy of up to 20 hours if you use a couple of rechargeable 3,100mA/H AA-cells in parallel (actually probably more like 12-15 hours in real life) and non-rechargeable A23 batteries for a 48V B+ (bring along several sets, they are very small and cheap). This 3-tube configuration is all that's needed for the future "totally portable, hiking-friendly" version of the radio; you could even leave out the RF section but IMO it provides such a large performance advantage that it would be worth including. You'd also need to reduce the number of controls to minimize size. The only remaining challenge for this "hiking/travel" version is getting the detector tuned-circuit right, in a much smaller form factor.

Going further in the direction of small size and low power consumption, I am hopeful of building a version of the above "hiking/travel" set that would use 2E31 tubes. These have much more modest power/voltage needs: 150mA total for the three filaments, and a 22.5V B+. This could be two rechargeable 3,100mA/H AA-cells for the filaments, and three 9V (actually, 7.4V) rechargeable Li-Ion batteries for the B+. The autonomy would be a theoretical 41 hours, probably more like 30 hours in practice. The challenge with the 2E31 is the much lower transconductance of 500µmos (the 1AD4 is 2,000µmos). In my early experiments I could not get a 2E31 regenerative detector to oscillate, but now that I have much greater experience, I am hopeful of more success. (I may have to include a high-Henry inductor as the detector plate load, which is a pain from a size/weight point of view!!).

The performance of the current build is very good and, IMO, meets the original goal of "high performance." It is noticeably better than the ARRL 3-transistor regenerative receiver that I built a couple of years ago. This is not only because (together!) we've optimized every little detail, but also because every parameter that counts is controllable, hence the profusion of dials! Very exciting, and great fun.

However the new features I'm aiming for are not only for operating convenience; they would actually make the set's performance even higher. The biggest improvement would be greater selectivity. With careful adjustment of regeneration, plate voltage and RF gain, good "isolation" of signals is already possible, but it could be better. My first instinct, because I thought it would be easy, was to add an audio filter -- and that would still be easy, if we keep to passive methods.

Speaking of selectivity, a little while back Allison mentioned a Q-multiplier; does that make any sense in a regenerative set? (Some Q-multipliers are, themselves, regenerative and I'm a little wary of juggling two regen circuits in one set!)

What we would end up with, after adding all sorts of new features, is a shack-bound radio with, perforce, the kind of power consumption that requires large banks of batteries. At that point it makes more sense to build an AC power supply. So why stick to subminiature tubes? Other than the novelty factor, and the ease of experimentation with these "leaded, socketless" tubes, there's not much point.

On the other hand, homebrew projects don't necessarily have to make coherent sense ... as long as the builder gets what s/he needs, and the needs can be very specific and not "commercial" in nature.

After this long disquisition, I have one question for G3RZP: am I right that the problem with building the Selectoject is the directly heated filaments? I'm asking because I've found four or five types of subminiature twin-triodes that are directly heated (no need to use four separate strapped pentodes). For instance the 5967 twin-triode (1.25Vfil, 45V B+, 2000µmos); or the 5968 -- they are scarce nowadays, but not unobtainium. There is also the 3A5 suggested by Allison, which is not subminiature but is otherwise similar and has a 1.4V filament.

If we go for cathode types, there's lots of subminiature types that would be suitable, but then we have to go for higher filament voltage, higher filament current, and higher B+

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 22, 2015, 07:57:40 AM
That is a simple twin T audio filter with screen grid gain control.  

Likely you would have to use two stages for enough CW selectivity.   Using 2 2e26
would cost 100ma heater and 3-5ma of plate current.

Allison

Wherein I learn more about filters. It looks like:

--A twin-T (http://en.wikipedia.org/wiki/RC_oscillator (http://en.wikipedia.org/wiki/RC_oscillator))

--or a Wien-bridge (http://en.wikipedia.org/wiki/Wien_bridge_oscillator (http://en.wikipedia.org/wiki/Wien_bridge_oscillator))

are possibilities, although again we hit the challenge of the directly heated tube. However, they might be more doable than a phase-shift circuit.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 22, 2015, 08:46:09 AM
For a twin-T circuit, I found a patent filed in 1944 and granted in 1950. The schematics show cathode tubes but I would think it could be adapted for filament tubes. I'm still wading through the circuit description. There are several versions, both 1-tube and 2-tube. They are for a fixed audio frequency but presumably they could be made variable. I think the fixed frequency is because one of the key applications they had in mind was for multiplex filters in telephony.

https://www.google.com/patents/US2495511 (https://www.google.com/patents/US2495511)

To see the schematics, click on the graphics at the top of the patent page.

This patent filing includes an excellent discussion of the issues, as well as good descriptions of the circuits of other inventors. (Despite the typos and typographical formatting errors resulting from OCR scanning.)

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 22, 2015, 10:26:21 AM
To make sure I understand the twin-T circuit:

(https://lh3.googleusercontent.com/-s6JHDp9h1TQ/VV9jMZsileI/AAAAAAAAE9k/LCmgWEwk5TQ/s358/Filters_01.jpg?gl=US)

In the CRC filter (components 1, 2 and 3) the output is taken across the capacitors and it therefore acts as a high-pass filter: lower frequencies are shorted to ground.

In the RCR filter (components 4, 5 and 6) the output is taken cross the resistors and it therefore acts as a low-pass filter: higher frequencies are shorted to ground.

Hence the "Twin-T" after the shapes of the two superimposed filter schematics.

If we wanted to vary the peak bandpass frequency ... what would we vary? Would we use a dual-ganged resistor or capacitor and if so which values would we want to control? It seems to me that every component in this circuit has effects on frequency.

(Edited because initially, I got the high-pass and low-pass the wrong way round....)

Edited to add: It might be interesting to make both parts of the Twin-T adjustable independently, instead of ganged. That way you would have an extremely flexible filter, although requiring a lot more skill to operate -- for instance it would become possible to have to high-pass frequency higher than the low-pass frequency, resulting in ... extreme attenuation of everything!

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 22, 2015, 12:41:07 PM
I just figured out the clinching reason why it's unlikely a directly heated subminiature twin-triode would work in the Selectoject. The tube only has a single filament! So there's no way you could have two "cathodes" except conceivably if the filament were center-tapped -- and the filaments on subminiature tubes aren't.

So if we're interested in an active audio filter with adjustable frequency and attenuation, it would seem that if anything is possible with directly heated tubes, it would be something like the Twin-T or Wien Bridge, with any luck using no more than two tubes.

Or a Selectoject using four separate triodes instead of twin tubes. But as Peter said, "It's not so easy with directly heated tubes, because you can't lift the filaments above ground without  separate filament supplies for each tube. Using extra tubes to get the in phase and out of phase signals is difficult because the loading on the tube plate varies with adjustment of the frequency control."

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 22, 2015, 01:11:26 PM
I found a web page that has several tube audio filter circuits:
--Two-tube cascaded high and low pass.
--One-tube Wien Bridge (it's a notch filter but presumably there's a way to turn it into a peaking filter).
--Twin-triode, Twin-T filter.

It's here: http://www.seekic.com/circuit_diagram/basic_circuit/fast_clamp.html/index149.html (http://www.seekic.com/circuit_diagram/basic_circuit/fast_clamp.html/index149.html).

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 22, 2015, 10:48:28 PM
I built a "Twin-T" audio filter

This evening I built a filter based on this circuit: http://www.cgs.synth.net/tube/vcf.html (http://www.cgs.synth.net/tube/vcf.html). I didn't have very high expectations but it actually turned out a bit better than I expected. Here's the circuit, as built, after a bit of tinkering with component values:

(https://lh3.googleusercontent.com/-kyZZz8DRfY4/VWAOEYEu1XI/AAAAAAAAE90/i8bqwMlZD9o/s912/Filters_02.jpg?gl=US)

In the original design that I copied, only one of the resistors in the filter network was variable: the one that I have marked "high pass control" (the author of the design calls it the "tune" control). When I originally built the circuit, I found that this control had some effect, but not much (except that at the low-ohm end of its travel, it cut off sound entirely). It was only when I made the other resistor into a 500K variable that control over filter frequency became apparent.

Basically, the one I've marked "low-pass control" behaves like a supercharged version of the Treble control on a stereo. If you have two CW signals close to each other and one of them is much higher in pitch than the other, rotating this control toward the lower-resistance end of the scale somewhat attenuates the highest pitched signal (and also eliminates almost all of the regenerative "hiss" and atmospheric noise). The other control, which I have marked as the "high-pass control," also attenuates noise and other signals, but the effect is subtler and harder to describe; and in the middle of its rotation, it actually boosts the volume of the wanted signal.

I also tested the filter on strong SSB signals that were close to each other, and there was a perceptible but not substantial attenuation effect of the unwanted signal.

The gain control on the screen has an interesting effect. At one end of the scale (with the wiper at ground potential) the background noise is quiet and the effect of the aforementioned filter controls is moderate. As you rotate the control, increasing the voltage on the screen, there is the onset of a "woosh" so clearly some oscillation/regeneration is going on. This gets louder, then toward the top of the scale, gets quieter again until (at the very highest screen voltage) there is mild "motorboarding." The effect on the filter controls is complex. Just past the threshold of oscillation, the gain is greatly boosted. As you go higher up the scale, the boosting effect falls off. I am not sure whether the extra gain makes the filters "sharper" or not.

Am I satisfied with this filter? Not at all. It doesn't cut in until very close to the dial-travel edges of the filter controls. It provides only moderate attenuation of neighboring signals -- useful at the margin, but not impressive. Also, it would be nice to have a single "audio frequency" control rather than playing with separate high-pass and low-pass controls. (By the way, I've labeled these filter controls with a question mark because I'm guessing what their function is, from my very recently acquired knowledge of Twin-T filters, and from their behavior in practice.)

In these tests, I inserted the filter between the detector and the AF stage. (Just as an experiment, I also tested the filter as an adjunct *after* the AF stage but that was not at all satisfactory: too much input gain from the AF stage causing overloading and early "motorboarding" of the tube; attenuation with a series resistor on the input didn't work terribly well either.)

Allison suggested that a Twin-T circuit of this type would require two stages to provide enough CW selectivity, and it looks like she's right! I now need to figure out what that would look like. I have plenty of examples to look at (cited in my earlier posts). But clearly, it can be done with directly heated tubes. (Unless my ears are playing tricks on me. Because switching in a filter often coincides with a drop in signal volume, it can be difficult to detect whether adjacent signals have actually become filtered out, or whether they just seem that way because the overall volume of all signals is quieter.)

Edited to add: Tomorrow I'll do the math (which, for a Twin-T, is fairly complex) to figure out whether the component values in the filter are optimal. This one was designed by someone in the field of music synthesis; it may be that it's optimized for frequencies higher than the 700-800Hz we like to see in an ideal CW tone. OTOH maybe it doesn't matter since it's adjustable with the controls anyway.

Bedtime.

Updated the next morning: In a bit of housekeeping, I noticed that as built, the screen voltage would always be higher than the plate voltage! So I rewired the top of the screen pot to connect it to the junction of the plate and the plate load resistor. But this made the pot extremely noisy, for some reason (despite the 1µF bypass on the slider). So instead, I added a 470K resistor between the top of the pot and the B+ supply. The schematic has been updated (you may have to clear your browser cache to get the updated schematic, if you've already viewed this page today).

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 23, 2015, 05:55:13 AM
I tried doing the math "by hand" but worried about mistakes -- most of the results I got were absurd. Probably requires more sleep.

However to my astonishment, there are plenty of online calculators for a Twin-T filter. I plugged in the values from my filter, as built, and got a default center frequency of about 800Hz, which is nearly perfect for CW (I used this calculator: http://earmark.net/gesr/opamp/twint_bpf.htm (http://earmark.net/gesr/opamp/twint_bpf.htm)).

Today I'm going to figure out how to build a better filter. This probably involves adding a second stage. This may require deciding on a center frequency and sticking to it, rather than having a variable frequency control? (No great hardship for CW.) For a while I thought I could keep a single tube and increase selectivity of the filter by adding a second filter in the feedback loop ("stagger-tuned filter") but then realized that the stagger-tuned circuit is not really intended to increase selectivity per se, but rather to achieve other effects.

One possibility that's gnawing at the back of my mind is that the circuit I built isn't really functioning properly as an active filter; perhaps the effects I'm getting are just the result of the passive network on its own. However, there is evidence of fedback (the rushing sound when I adjust the gain) so presumably there is at least a partial active effect, but perhaps it's not optimized properly.

To be honest with myself, after building a good-performance two-tube Twin-T filter, I really should also build a passive filter using 200mH to 500mH inductors, as suggested by Allison, then compare the two. It seems to me that the main advantage of active filters is that it's easier to add controls for center frequency and gain/bandwidth; but it's not clear how essential those controls are, if all you want is a nice sharp CW filter. I'll see if I can find toroids in the range she mentioned, when I go to the electronic-component store this morning. (At least the math for those simple LC filters is easier.)

After checking the various passive circuits online ... whoa! A whole new ballgame I know nothing about. I had no idea how elaborate these filters can be, and it looks like it would be helpful to learn how to wind and adjust toroids (or learning about telephone loading coils!!). Dithering between using a published design, or designing one from scratch. Could start with a really simple, home-designed one, just for kicks.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on May 23, 2015, 06:50:04 AM
A number of Marconi receivers used two coupled tuned circuits with inductors for an audio CW filter.  They were tuned by fairly large mica compression trimmers in conjunction with large stacked foil and mica capacitors.

This allows some variation in bandwidth by varying the coupling factor.

There is little point in going for much selectivity at audio merely because the instantaneous dynamic range of the regenerative detector is limited...


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 23, 2015, 09:11:18 AM
My local electronics-component store had toroids, but only low-value ones (max 1mH).

However I remembered that I have a couple of small Radio Shack transistor push-pull output transformers (form factor about 1/2" on each side). According to my super-duper new LCR meter, the inductance on each side of the center tap is 175mH which is in the ball part for an audio filter.

I should be able to make a high-pass and a low-pass filter. Connecting the two filters in series should make a bandpass filter. (Edited, after looking at the circuit choices.)

By my calculations that's about 0.9µF for a high-pass filter at 400Hz, and 0.18µF for a low-pass filter at 900Hz.

Cool. Can't wait to see if it works.

(I am also still experimenting with active filters!).

There is little point in going for much selectivity at audio merely because the instantaneous dynamic range of the regenerative detector is limited...

That may be the case, but at the very least I found that having a sharp-ish low pass filter to cut off the regenerative mush and atmospheric noise made quite a difference on a weak signal (picking it out from the mush) and on a strong signal (just making it pleasanter to listen to). That of course has nothing really to do with selectivity per se, and I presume I could achieved it with an extremely simple passive device (a capacitor somewhere in the audio circuit!).

The other issue, as I found out last night on a busy CW night, is that the selectivity of the set is already rather good; I actually had trouble finding signals that were annoyingly "on top of each other" and when they were, the audio tones tended to be so close to each other that any audio filter would have had trouble splitting them.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 23, 2015, 02:38:22 PM
A discouraging day :(.

I built a passive LC bandpass filter, using those little Radio Shack transformer primaries (about 150mH) and some capacitors. Unfortunately the insertion loss is such that it is not usable or even assessable: the volume is just too low after the filter is inserted -- either between the detector and the AF stage, or between the AF stage and the headphones. I suspect, among other things, an impedance mis-match but most of all, it probably won't work until I build an additional stage of audio amplification: there is currently only one stage and these baby pentodes aren't exactly bursting with power.

I had enormous trouble with the math for anything other than a very simple LC circuit: basically a LPF and a HPF in series, because it was easy to do the math. Anything more complex -- such as a pi-network or T-network bandpass filter -- was completely beyond me even though I have all the equations in the ARRL books. The online calculators weren't much help either. But eventually I will understand all of this! Things that really tripped me up were (1) I already knew that L = 150mH but plugging that into the calculation was almost harder than letting it decide all the values from scratch; and (2) source impedance considerations. It would be much easier to just build a published design, but I'm stubborn ;D.

Concerning the active, Twin-T pentode filter that I built yesterday. On a hunch, today I disconnected the B+ from the filter and found that the effect of the two controls was about the same as when the tube is actually active. In other words, those "filter" effects I described in an earlier post are apparently only the results of the passive action of those Twin-T RC filter circuits, so it's not surprising it's not working too well. This is strange because with the B+ connected, I do hear a roar in the headphones when I advance the screen gain control. Apparently, even though there is feedback, it doesn't seem to be having any audible effect on the filter circuit. I wonder if this is because there's something I don't understand about using a directly heated tube in this application (schematic here: http://tinyurl.com/lkbtbhy (http://tinyurl.com/lkbtbhy)). Or maybe it's just that it won't work properly until I add a second tube with a second filter stage.

Anyway, I will continue my research with both passive and active filters. Failed experiments are often better learning experiences than successful ones ... the past week has seen a number of disappointments, apart from adding that AF gain control!

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on May 23, 2015, 02:45:56 PM
Martin,

Cascading HPF and LPF sections to get a BPF works provided that there is a fairly large frequency separation between the cut off frequencies i.e. the bandwidth is large. This is because the filters rely on an impedance mismatch out of band to work and if the two cut off frequencies are too close, both are cutting off at once.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 23, 2015, 03:24:42 PM
Martin,

Cascading HPF and LPF sections to get a BPF works provided that there is a fairly large frequency separation between the cut off frequencies i.e. the bandwidth is large. This is because the filters rely on an impedance mismatch out of band to work and if the two cut off frequencies are too close, both are cutting off at once.

Thanks Peter. The two frequencies I'm choosing are 400 and 900 Hz. That's certainly a large percentage difference and very low desired Q, if I understand it correctly.

During my Internet searches I found that another ham (KA7OEI) had made the same discovery as I have, concerning Radio Shack baby output transformers. He built an SSB audio filter using two of them, and then a somewhat better filter using five of them (his blog entry is here: http://ka7oei.blogspot.com/2013_01_01_archive.html (http://ka7oei.blogspot.com/2013_01_01_archive.html)).

For what it's worth, with these two 150mH transformers, I'm using 1.0µF for the high-pass and 0.22µF for the low-pass. I calculated them as if these are two completely separate LC circuits, so didn't account for the interaction between them, if any. Maybe I'll built a KA7OEI filter next ... and adapt it for CW.

(An apology to readers: I changed this post several times after posting it, after realizing I'd made some mistakes.)

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 24, 2015, 05:19:23 AM
Back to Square One

Starting from first principles, I built a one-stage RLC bandpass filter:

(https://lh3.googleusercontent.com/-NP-aRCnn3LQ/VWHABUPiPJI/AAAAAAAAE-k/KJ9oQ7iHFnQ/s800/Filters_04.jpg?gl=US)

It works! There is noticeable overall attenuation when it's switched in, but I did find that a weak CW signal (daytime 40m) "popped out" a bit against the atmospheric noise background. The effect is modest but obviously that can be improved by adding additional filter stages. Figuring out the optimal values for the components in the stages is the tricky bit. (I already know that L = 150mH for all my inductors, since I'm using Radio Shack output transformers.) I need to experiment both with RLC bandpass, and/or with HPF and LPF sequences.

Really the only way to assess a passive filter properly is to include a stage (or two) of amplification in the circuit. At least initially, you want a gain control, because you want to set gain such that when the filter is switched in, there is no change in overall volume. Once the desired gain is figured out, the gain control could be replaced with an appropriate fixed gain value. The more filter stages you add, the more amplification you need.

The ARRL Audiofil from the 1960s is exactly that: an RLC filter combined with two stages of amplification (double triode), one before the filter and one after.

Edited to add: So, in fact, there is a "power penalty" regardless of whether the filter is active or passive. An active filter obviously requires power; and a passive filter attenuates the signal thus requiring extra power for amplification.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 24, 2015, 12:18:45 PM
How to figure out input/output impedance

I'm using the ARRL filter-calculation software that came with the 2014 handbook to design a Chebyshev LC audio filter. Like any such calculation, it requires a knowledge of input and output impedance. If this is going between two tube stages, how do I figure out the impedance that the filter will see at input and output? I've been using an arbitrary value of 600 ohms but for all I know, it's way off.

Also, looking at the plot curves, is the "transmission" curve the relevant one for frequency response, or is the input impedance curve more revealing? (The former usually shows one or two relatively flat response peaks, the latter usually shows two or more wilder peaks, with the range of values I'm modeling.)

Tnx.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 24, 2015, 01:53:23 PM
So here's what I've been able to come up with. Wish I could have done the calculations "by hand" but they are beyond me. Instead I used the software that came with the ARRL 2014 handbook, which is called "Elsie" (a pun on "LC").

From my prior knowledge of high-pass and low-pass filters, I was very surprised that when you combine them in a Cherbyshev bandpass filter, the two resonances are very close together -- or at least that's how I got the best results. Strange -- if someone has an explanation for this I'd be grateful to have one.

Again, I'm using small Radio Shack transformer primaries. Inductance is either 150mH or 300mH, depending on whether you use one side of the center tap or both. I have six of these $3 transformers to play with.

Having no idea what impedance to work towards for use as an interstage filter in a tube receiver, I entered an arbitrary value of 600 ohms.

On the plot curves, the red curve is "transmission" response, the blue curve is "input impedance" response.

For a three-pole CW filter I came up with this:

(https://lh3.googleusercontent.com/-lm1lO4dtqYg/VWI3GarT46I/AAAAAAAAE-4/8WL7RnvF8K0/s781/Filter_Three_pole_schematic.jpg?gl=US)

(https://lh3.googleusercontent.com/-Abi_RylbM1c/VWI3Hurd-_I/AAAAAAAAE_M/zh09Dj95uoA/s1024/Filter_Three_pole_plot.jpg?gl=US)

For a five-pole CW filter:

(https://lh3.googleusercontent.com/-jfp993ihJI8/VWI3GhC1ZyI/AAAAAAAAE-8/eylmiGY-K7g/s1024/Filter_Five_pole_schematic.jpg?gl=US)

(https://lh3.googleusercontent.com/-wggB4P2LPRY/VWI3HLVSWUI/AAAAAAAAE_I/hmL8E1uzjPM/s1024/Filter_Five_pole_plot.jpg?gl=US)

If any of you experts see something fishy do let me know! Because my knowledge of what's going on is really basic, I may have chosen some really silly component values, or there may be values that would work better....

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: N3QE on May 24, 2015, 02:46:08 PM
I'm using the ARRL filter-calculation software that came with the 2014 handbook to design a Chebyshev LC audio filter. Like any such calculation, it requires a knowledge of input and output impedance. If this is going between two tube stages, how do I figure out the impedance that the filter will see at input and output? I've been using an arbitrary value of 600 ohms but for all I know, it's way off.

600 ohms to few kOhms is pretty convenient for AF filters and is similar to impedances in solid-state stages. 600 ohm is a very common interstage or line-driver transformer impedance.

Plate impedances in miniature tube stages is much much higher - 100K or more. Transformers can get you down to 600 ohms and back up. More realistically, you will probably put this type of filter after the final audio amp where you might be using a 600 ohm transformer for headphone driving (assuming you have 600 ohm phones). Or first step down to 600 ohms for filtering, then down to 8 ohms for speaker.

You can also design LC AF filters at 100K impedance level but inductances will be huge.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 24, 2015, 06:10:47 PM
600 ohms to few kOhms is pretty convenient for AF filters and is similar to impedances in solid-state stages. 600 ohm is a very common interstage or line-driver transformer impedance.

Plate impedances in miniature tube stages is much much higher - 100K or more. Transformers can get you down to 600 ohms and back up. More realistically, you will probably put this type of filter after the final audio amp where you might be using a 600 ohm transformer for headphone driving (assuming you have 600 ohm phones). Or first step down to 600 ohms for filtering, then down to 8 ohms for speaker.

You can also design LC AF filters at 100K impedance level but inductances will be huge.

Thanks. I need to pay careful attention to this. The AF stage is currently being driven into 10KΩ vintage headphones -- there is no output transformer. (A reasonably up-to-date schematic is here: http://tinyurl.com/mdjdf6m (http://tinyurl.com/mdjdf6m); it lacks only the AF gain control that was added last week.)

The ARRL "Audiofil" design from the 1950s/60s, which had a center frequency of 600Hz, used inductance values of 4.5 *Henry* for each coil. (It was preceded, and followed, by tube amplifiers stages.)

Mind you, that's not out of the question. I could ditch the 150mH components, find some bulky 4.5H inductors, and just build the "Audiofil." If not, I'll have to step down the audio with a transformer before feeding the "Radio-Shack transformer" filter. I have a small 10KΩ audio output tube transformer, but it has an output of 8Ω -- perhaps I need to find an "interstage"-type transformer.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 24, 2015, 06:51:38 PM
Or first step down to 600 ohms for filtering, then down to 8 ohms for speaker.

How about this. I have five of these 1,000Ω --> 8Ω Radio Shack output transformers.

Let's say that my tube plate output stage has an impedance of 100KΩ. If I used one of the RS transformers to step that down it would bring it down to:

100,000 * (8 / 1,000) = 800 ohms.

Which means I only need to redesign my "Radio Shack inductor" filter for 800Ω instead of 600Ω, and that is manageable.

(But perhaps I am being overly simplistic in assuming that the stepdown ratio is the only thing that matters; perhaps connecting a 1K winding to a 100K tube stage impedance is not a good idea?)

I can then use another RS transformer, installed backwards (with the primary used as secondary) to step it back up again into another tube audio stage. Or, I could put the filter at the output of the last audio stage and use a step-down transformer to get from 800Ω to 8Ω and switch to modern low-Z headphones.

Having used up one or two of my stash of five RS transformers, I'd then be limited to a 3-pole filter. Getting hold of any more of these $2.99 transformers is pretty much out of the question. They are "only available in stores" (not on the RS website) and in the past 24 hours I had to visit five RS stores to scrounge five transformers. I get the strong impression that RS is selling out all of its store-stocked inventory of "components" and won't be replacing many of them.

"Why do you want this?" (question asked at checkout today).

"I'm building a radio!"

"A radio??? Uh. I see. Well, perhaps if more people did that nowadays, we wouldn't be going bust!" (broad smile).

One has to wonder however whether this profusion of transformers isn't going to complicate things and affect overall performance. Mind you, in many tube designs I've seen, transformers were a good thing, not a bad thing: they tended to be found in the more expensive and better designs.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on May 25, 2015, 01:39:35 AM
Marti n,

One point that may make a BIG difference is the assumed Q for those transformer inductances. It's not likely to be very high.

I would look at making a coupled tuned pair for CW: parallel tune the 150mH with 0.27mFd for 800 Hz and use that as the plate load in the detector. In the AF stage grid, place another identical tuned circuit across the 1 Megohm volume control and couple the hot ends of the tuned circuits through a capacitor. You may need to put 'trim' capacitors across one or both 0.27mFd caps to  'trim' the frequencies to where you need them: while doing that unhook the grid tuned circuit and couple with a small - say 0.005 mFd - and trim the plate circuit for maximum response at the frequency you want. Then connect the grid circuit and trim that for maximum response at that frequency. Now increase the coupling capacitor until the output at the centre frequency just starts to drop and then back off a bit to a somewhat smaller value.

How narrow it is depends on the coil Q.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 25, 2015, 03:15:44 AM
Marti n,

One point that may make a BIG difference is the assumed Q for those transformer inductances. It's not likely to be very high. [snip]

How narrow it is depends on the coil Q.

Excellent, thanks -- I will try what you suggest. I don't think I have any high-value trimmers (more than a few dozen pF) in the junkbox but I can improvise by switching in a range of fixed caps across the bigger 0.27µF cap. My Eico-460 oscilloscope hasn't been restored yet, and I don't own an AF signal generator or a spectrum analyzer, so this will all have to be done "by ear." The nearest thing I have to an 800Hz "reference tone" is the modulated output from my Eico RF signal generator.

To get a 500KHz bandwidth at, say, 650Hz center frequency, I only need a Q of about 1.3, if my calculations are right.

My splendiferous new LCR meter does measure Q and Z, as well as L. For the Radio Shack transformer, L varies between 150mH and 175mH depending on which transformer I check. Here are the numbers for one of them (in all cases using "series" measurement system):

--At 1KHz sampling frequency: L = 165mH, Z = 1.4KΩ, Q = 1.09.

--At 120Hz: L = 499mH, Z = 430Ω, Q = 1.8.

--At 10KHz results are very odd: L = 4.1mH (??), Z = 4.3Ω, Q = 0.06. (Wild guess: could this have something to do with self-resonance? This frequency is within the audio range and I can just hear it on my headphones when I check the headphones on the same meter, but I suppose a cheap audio transformer is not expected to be have a flat response at that frequency.)

--At 100KHz: L = 538mH, Z = 384Ω, Q = 1.8.

These modern digital instruments do tend to rob the mystery out of things ... but save a lot of time. OTOH the huge variations caused by changes in sampling frequency indicate that one should take the results with caution.

I could buy some specialized inductors in the 100mH range (like these $1 coils from Mouser: http://www.mouser.com/ds/2/54/rl181s_series-58469.pdf (http://www.mouser.com/ds/2/54/rl181s_series-58469.pdf)) or wind my own toroids ... but first it would be fun to see what can be done with these little RS transformers.

Concerning Q, the ARRL article on building the "Audiofil" recommended removing the U-shaped metal bracket around the armature of the transformers and replacing it with a cardboard holder to fasten the transformers to the chassis. It said this would increase Q! (Not so easy with the little RS transformers; the bracket seems to be bonded to the armature in some way.)

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 25, 2015, 03:37:29 AM
Question: if a dual triode is directly heated and shares its filament between the two sections (with no center tap), how can each of the sections function independently? Assuming that the heater voltage provides some cathode "bias," do the signals for both sections of the tube somehow "share" a single virtual cathode without trouble, such as unwanted feedback/crosstalk for example?

I'm looking at the Audiofil ARRL circuit from the 1960s, where the pre-filtered signal is amplified in one section of a twin triode, and the post-filtered signal is amplified further in the other section of the tube. In the original circuit, it's a 12AX7 I think, with separate cathodes, each of them connected separately to ground through a resistor with a large-value parallel cap (25µF).

I can get hold of a subminiature, directly heated twin triode ... but am wondering whether it is usable. Otherwise, I could just use separate tubes at input and output, if I wanted to build the Audiofil.

This search for a nice homebrew CW filter is getting a bit obsessive ... I may end up building a half-dozen different circuits before I find the ideal for my receiver. Plus, as G3RZP rightly points out, a CW filter has only limited usefulness with a regenerative detector (but, I contend, still useful at the margin, if only to cut down on white-noise regenerative "chaos noise").

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 25, 2015, 07:59:00 AM
Modeling the ARRL Audiofil

If I plug in the Audiofil schematic into Elsie it's like this (those big inductors are the primaries of audio output transformers):

(https://lh3.googleusercontent.com/-wIIFY3cLFrg/VWM1tUda0dI/AAAAAAAAE_k/00JRhRWTtmY/s1024/Filters_08_Audiofil_Schematic.jpg?gl=US)

If you plot the frequency response, it differs radically depending on whether you include the first (1µF) capacitor in the chain (which, if I am not mistaken, is simply the plate bypass capacitor).

If you include the plate bypass cap in the modeling, the curve looks like this (BTW I assumed a Q of 6 for the inductors):

(https://lh3.googleusercontent.com/-FjxymxHsb9E/VWM1tLvbfZI/AAAAAAAAE_g/dUGNoj5EDXs/s1024/Filters_07_Audiofil_Plot.jpg?gl=US)

If you remove the plate bypass cap from the modeling (but is this kosher?) the model gives you a very different (and better) curve for a CW filter:

(https://lh3.googleusercontent.com/-mVlnys-Ti8Y/VWM24jaDhtI/AAAAAAAAE_w/CgYe0J-eQPk/s1024/Filters_10_Without_Bypass.jpg?gl=US)

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1GMX on May 25, 2015, 02:11:51 PM
Dual triode where the cathode is shared means both will be biased off that point.
if the cathode happens to be a directly heated type same difference for the most part.
generally if the cathode is at signal ground then it s less an issue.

Removing the plate bypass is allowed as is lowering the value.  Using a choke input filter
will also work but likely the choke will be a large value.  Hint start with a lowpass input structure
such as series L, followed by parallel C, If you want a peak at some frequency then add an
inductor across the C to resonate it. You can also add a series C to the input inductor to
get a series resonant circuit.  then you have series C, Series L, Parallel LC.  Since the grid
is nominally high impedance the terminiating value os the grid resistor (from 47k to 1meg
as needed/desired).  The source impedance is the plate impedance in parallel with the
plate resistor higher is better for gain and lower values of C.

To predict basic RC filter, series R parallel C...  1/f= t,   make RC=t  where R is the plate
(combined) resistance and C is the value to ground.  That will be the 6db point down the
curve with increasing frequency.  For a 100K plate source at 500hz C must not exceed
.002uf (2000pf).  For the case of the .1uf plate input cap you way too large and you see
a low output as a result.  For the sub 600ohm source that may work.   

Source and load impedance counts when working with filters.

If you use transformers or chokes on the plate resonate them with parallel
C for a peaked response.  For those tubes a large L and small C works best.

Transformer cores can be large inductance but often the core loss to eddy currents
is high (low Q).  There are trades to be investigated there.

The popular passive filter of the day was a peaked pair of parallel LC with top
coupled series C and series L.  Looks like a doubled tuned circuit at RF save
for its audio.  To get higher impedance at input use large L small C.

Allison


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on May 25, 2015, 04:45:43 PM
Martin,

the Q of those plate transformers appears to be too low to be of any use in filters. What's the DC resistance of the primaries?


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 25, 2015, 05:32:00 PM
the Q of those plate transformers appears to be too low to be of any use in filters. What's the DC resistance of the primaries?

They are not plate transformers, but transistor push-pull output transformers (why Radio Shack still sells them, and "only in stores" not online, is a mystery of the universe). The DC resistance is 35Ω on each side of the center tap, 70Ω total, according to my DMM. The primary impedance indicated on the packaging is 1KΩ.

I tend to agree with you that these transformers are more trouble than they're worth, having done more reading about the importance of Q. KA7OEI did use those same transformers successfully to build an audio filter: see http://ka7oei.blogspot.com/2013_01_01_archive.html (http://ka7oei.blogspot.com/2013_01_01_archive.html). He has what looks like a spectrum analyzer or 'scope trace to prove it. On the other hand, his filter was used in a transmitter audio application, a lot less sharp than a receiver CW filter. He speculates that a CW filter could be built with those RS transformers but didn't build one.

Myself, I've backed off on the idea of using those little RS transformers. OK that's $15 down the drain (and an awful lot of driving around the Boston area from one store to another!) but it seems like a lot of effort to build a filter based on them, with not much performance likely in return. Beyond the issue of low Q, there's the impedance mismatch since I'm using tubes, which to me is the clinching issue. I had got it into my head that inductors in the 100mH range were appropriate, without realizing that these are typical for solid-state applications (including the filters that KA7OEI built).

Am busy with my real job for the next few days but the next thing I'm likely to try is some version of the trusty old ARRL Audiofil (the filter at the heart of that design was analyzed a few posts back in this thread; schematic here: http://tinyurl.com/m2btxc2 (http://tinyurl.com/m2btxc2)). I've just ordered a couple of 4-Henry chokes, which if they are anything like the similar-value ones in my junkbox, should have a Q of around 6 (perhaps more if I remove the metal brackets as suggested by ARRL). Such a filter is quite lossy and has fairly wide skirts it seems, but at least it's a good impedance match for interstage tube usage. I'll put one tube stage of amplification in front and one behind. The inductors are bulky but, clunky though it is, it should work, I think. To save battery power, the filter in/out switch will also shut off power to the filaments when the filter is not in use.

I have a question about adapting cathode circuits for the directly heated tubes (either two pentodes, or a twin triode) that will serve as amplifiers before and after the filter. In the Audiofil circuit, the cathodes are connected to ground through a resistor in the 1K range with a parallel, large-value electrolytic (25µF). Where do I put that capacitor in a circuit using directly heated tubes? In the grid circuit below the grid leak resistor and in series with it? Should I also bias the tubes with the C battery, using a voltage divider circuit?

Some time in the next few days I'll post a suggested circuit....

I am still very interested in pursuing other audio filter concepts in the future (especially active circuits using phase shifters or twin-Ts), but must admit that the directly heated tubes are a very limiting factor. It should probably wait until I'm building a set with "regular" tubes, or be built as an outboard "accessory" with regular tubes.

Allison: Thank you for your post about designing filters. It's all coming together for me now, albeit rather fuzzily.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on May 26, 2015, 04:33:45 AM
I don't understand this.

Quote
--At 1KHz sampling frequency: L = 165mH, Z = 1.4KΩ, Q = 1.09.

XL = 1037 ohms.

If Z = 1400 ohms then (XL2 + R2)0.5 = 1400 and then R is 940.5.

But R measures at 70 ohms, so Z should be 1039 ohms and Q = 14.8

The only explanation I can see for this anomaly is an awful lot of iron (and maybe eddy current)  loss.

Interesting. That might have been done on purpose to avoid resonances....But they aren't very much use for filtering.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 26, 2015, 01:04:31 PM
Adapting the ARRL "Audiofil" (1963)

Here's my first try at adapting the ARRL "Audiofil" CW filter for directly heated tubes.

(https://lh3.googleusercontent.com/-LiSbgBB9rtE/VWTOcomMtEI/AAAAAAAAFAE/Ubqjr6cOh-U/s912/Filters_10_Adapted_Audiofil.jpg?gl=US)

In the original design, the two tubes are two halves of a 12AU7 twin triode. I don't have any of those in directly heated form, so we're using two separate tubes. The overall gain of the filter needs to be about unity so that there is no overall change in volume when the filter is switched in and out. I will use either 2E31 tubes or 1AD4s (if the former work OK, they would be better because their filaments are 50mA while the latter tube is 100mA). If there is too much gain, I will strap the pentodes.

My main puzzlement is the biasing. In the original circuit, the cathode is connected to ground via a resistor (about 1KΩ) shunted with a large electrolytic capacitor. The grid bias resistors (RG1 and RG2) are only 10KΩ. I presume that I need to replicate those large bypass caps somewhere, but where?

Also, the 10KΩ they use for RG2 is part of the filter circuit too; it is the terminating impedance I guess. So presumably I need either to simulate that impedance somehow, or completely redesign the filter components for a much higher output impedance. (The original Audiofil has a passband between 400Hz and 900Hz, centered around 650Hz.)

There is also is the issue of whether I can leave out the plate bypass cap on the first amplifier. According to my modeling using the ARRL "Elsie" filter-design software, that capacitor could cause heavy attenuation and shift the whole curve to the left (lower frequency) -- although ARRL had a bypass cap there, so I guess I should try both with and without.

In the above design the filter is placed after the receiver's first AF stage. It could go immediately after the detector instead; however it would probably be good to keep a constant load on the detector, as provided by the existing first AF stage.

I won't be building this for another few days; I'm waiting for the chokes, which haven't been shipped yet.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on May 27, 2015, 02:11:59 AM
That 0.1 mFd will certainly shunt the output of that stage - its reactance is around 2.6k in the middle of your band.

Quote
In the original circuit, the cathode is connected to ground via a resistor (about 1KΩ) shunted with a large electrolytic capacitor. The grid bias resistors (RG1 and RG2) are only 10KΩ. I presume that I need to replicate those large bypass caps somewhere, but where?

The electrolytics are there to bypass the cathode resistor, as other wise, you would have lot of negative feedback and reduced gain. You don't have the resistor, so you don't need the electrolytics.

The 10k ohm load at RG2 in the original will be needed if the filter response is to be anything like right. If you need the high value for self biasing - and you may need to provide grid bias if signals are not to distort at high volume - then put a 10k in series with 4.7mFd across RG2.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 27, 2015, 04:01:50 AM
That 0.1 mFd will certainly shunt the output of that stage - its reactance is around 2.6k in the middle of your band.

So can I leave it out? My thinking is that there are already two 0.033μF capacitors to ground, later in the same network. Plus, if I use a pentode, there's some bypassing through the screen bypass (is that circuit totally independent?). I will try it with both options, and leave it out if that seems to work OK. Presumably, insufficient bypassing has an effect not just on this one tube, but on the entire radio, if audio gets into the B+ supply.

The 10k ohm load at RG2 in the original will be needed if the filter response is to be anything like right. If you need the high value for self biasing - and you may need to provide grid bias if signals are not to distort at high volume - then put a 10k in series with 4.7mFd across RG2.

Just to clear up an ambiguity, are you saying that I should *replace* RG2 with a 10K in series with 4.7μF?

If I do provide grid bias from the C battery, I presume it should be done in a similar way to the volume control on the AF stage, i.e. with a voltage divider? So I would put a separate, high-value resistor underneath the 10K and feed the bias through a resistor to the junction point between these two series resistors? What effect does that have on the nominal 10K impedance of the grid circuit -- I am assuming that the bias voltage acts as some kind of "divider" to keep the impedance at 10K, rather than increasing the impedance to the sum of the two resistors to ground?

Looking at the first tube now. If I do use pentodes, will the presence of the screen circuit on the first tube change the bandpass characteristics of the filter? I modeled the filter for an input impedance of 100KΩ (and an output impedance of 10KΩ BTW) but isn't the plate resistance of a pentode amplifier potentially much higher than 100KΩ? That would argue for strapping the first tube; or recalculating the filter network for a higher input impedance, but that could generate impractical inductance values. (Edited to add: the data sheet for the 1AD4 lists a plate resistance of 500KΩ in pentode use. See http://www.mif.pg.gda.pl/homepages/frank/sheets/138/1/1AD4.pdf (http://www.mif.pg.gda.pl/homepages/frank/sheets/138/1/1AD4.pdf)).

Thank you for your help again. The good news is that I had already been thinking of doing something along the lines that you are suggesting, which must mean I'm gradually learning stuff. I am optimistic that this filter will work OK and provide some useful filtering, although that may depend to a large extent on the suitability of the 4H filter chokes I ordered. They are not designed for audio and I suppose there could be some unhelpful resonances or eddy currents. Which would explain why the original ARRL design used the primaries of audio output transformers. (If the chokes are unsuitable, I can consign them to the junkbox for future use in power supplies, and try to find proper-value audio transfomers.)

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on May 27, 2015, 10:44:22 AM
I spent some time typing a reply - then the site went down and threw it all away!!!

Quote
So can I leave it out?

The 0.1mFd should be left out.

 
Quote
Presumably, insufficient bypassing has an effect not just on this one tube, but on the entire radio, if audio gets into the B+ supply.

Definitely. You want about 4.7mFd from B+ to ground.

Quote
Just to clear up an ambiguity, are you saying that I should *replace* RG2 with a 10K in series with 4.7μF?

Shunt the 470k with  10k in series with 4.7mfd

Quote
If I do provide grid bias from the C battery, I presume it should be done in a similar way to the volume control on the AF stage, i.e. with a voltage divider? So I would put a separate, high-value resistor underneath the 10K and feed the bias through a resistor to the junction point between these two series resistors? What effect does that have on the nominal 10K impedance of the grid circuit -- I am assuming that the bias voltage acts as some kind of "divider" to keep the impedance at 10K, rather than increasing the impedance to the sum of the two resistors to ground?

You bypass the bottom of the 10k to ground with about 2mFd. That effectively removes the bias resistors as far as AC is concerned.

Quote
If I do use pentodes, will the presence of the screen circuit on the first tube change the bandpass characteristics of the filter? I modeled the filter for an input impedance of 100KΩ (and an output impedance of 10KΩ BTW) but isn't the plate resistance of a pentode amplifier potentially much higher than 100KΩ? That would argue for strapping the first tube; or recalculating the filter network for a higher input impedance, but that could generate impractical inductance values.

The source impedance that the filter sees is the plate resistance in parallel with the 10k plate load resistor in parallel with the 22k screen dropper, so is about 6.8k. You should return the top of the screen grid resistor to B+ in both stages. Then you need to model the filter with a 10k source impedance. You might find it worth looking at the Amidon website - see what sort of ferrite pot cores they have for use at audio frequencies. It could be worth while looking at going from say 10k at the input to 470k at the output, but that might need too much inductance. However, a higher impedance than 10k at the second stage grid might be practical - I no longer have any modelling software, because it was all stuff working on DOS, and we don't have either a DOS emulator or a computer that runs DOS.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 27, 2015, 01:04:35 PM
Thank you very much Peter, much food for thought.

Modeling the original suggested circuit (which is very close to the ARRL Audiofil) after changing the source impedance to 10K (and leaving the output impedance at 10K), we get this:

(https://lh3.googleusercontent.com/-nCZTOQ9rybw/VWYhJe1e4LI/AAAAAAAAFAg/f5dIOtYssSA/s912/Filters_22_Audiofil_10K.jpg?gl=US)

I also modeled a more classic 3-pole pi-filter, using the two 4H chokes that I've ordered and a 7.5H choke from the junkbox (a lot of heavy iron!!!), and I get this:

(https://lh3.googleusercontent.com/-8loASTIviH8/VWYhJ4lvx9I/AAAAAAAAFAk/6i5xjN-JwT8/s720/Filters_21_3-pole.jpg?gl=US)

Anyway, still thinking about all this. Will follow up on your Amidon suggestion.

I no longer have any modelling software, because it was all stuff working on DOS, and we don't have either a DOS emulator or a computer that runs DOS.

If you have any recent ARRL handbooks, see if there's a CD in the back. That's where I found the "Elsie" filter modeling software.

Edited to add: I've just been doing some "what-ifs" with the conventional 3-pole filter and find that it's much less sensitive to source impedance than the original Audiofil design. To get a 500Hz bandwidth centered on 650, with the limits at 6dB down from peak response, I'd have to increase the middle inductor -- the top of the "pi" from 7.5H to 10H but that's just a detail since we already have the -6dB bandwidth at a reasonable 600Hz even with the 7.5H choke.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on May 27, 2015, 01:52:27 PM
I rarely design filters these days, so it is a good exercise to stir up the brain cells by digging out from the bookshelf the 'Handbook of Filter Synthesis' by Anatol I. Zverev and 'Simplified Modern Filter Design' by R. Geffe and do it the old way by actually calculating the values......Yes, it takes longer but reminds one of the 'why and wherefore'. Plus there are also tables in the ITT 'Reference book for Radio Engineers'...

As Dr. John McKown  (then of Motorola) stated at an international standards meeting, "Simulation is like masturbation: the more of it you do, the more you prefer it to the real thing."

There's a lot of truth in that, to my mind profound, statement.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 27, 2015, 01:57:21 PM
As Dr. John McKown  (then of Motorola) stated at an international standards meeting, "Simulation is like masturbation: the more of it you do, the more you prefer it to the real thing."

HIHI. Honestly, I tried to do the math myself ... hours and hours of messing around ... but I know what you mean.

In this case there are an awful lot of variables and although I have the formulae, I don't have a filter-design book, and the more general books I have (ARRL and so forth) don't give you step-by-step directions on doing the math. I mean, which variable do you attack first? For instance. If I get deeper into filters, I'd better get a book. I looked up one book on Amazon and it was $112 ... ouch.

There are tables in the ARRL manual, but they are for RF filters, not AF. (The manual also correctly points out that LC audio filters are not too popular nowadays, given all the other alternatives available.)

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on May 27, 2015, 02:07:25 PM
'Simplified Modern Filter Design' by Philip R. Geffe, Rider, 1963, Library of Congress catalog Number 63-11817, is usually available on sellers like Abe books fairly cheaply.

It keeps away from such 'niceties' as Laplace transforms.....and gives tables from which design becomes relatively easy if you have a calculator - remember them? Before computers? But it's  possible to design from them with a slide rule, but 5 or 6 figure log tables are better!

I think Zverev's 'Handbook of Filter Synthesis' would be a bit much for you - at times, it is for me....


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 27, 2015, 02:22:27 PM
'Simplified Modern Filter Design' by Philip R. Geffe, Rider, 1963, Library of Congress catalog Number 63-11817, is usually available on sellers like Abe books fairly cheaply.

Ok I just ordered it, $14 including shipping. Now I'll have no further excuse for onanism.

As for calculation tools, when doing "manual" design I use a desktop calculator some of the time, but also force myself to do some of the calculations on paper ... it's good mental exercise. I'm old enough that we had no electronic calculators at secondary school; the most advanced "automatic" tools were slide rules (which we were required to own) and log books. By the time I went to university (1975), the cheap electronic calculators had just started to flood the world.

There is no computer in my shack!!

Edited to add: Drawing schematics with pencil and paper is one of the most relaxing of all pastimes ... they can be things of beauty although mine are not. I notice that in the 1968 ARRL handbook, there is a credit to the "draughtsman" and he heartily deserves it. (Plus, he is the only non-ham listed in the credits.) Beautiful stuff: there are even a few "fold-out" schematics that must have cost a bomb to print and bind.

It's also interesting how you can draw two schematics of exactly the same circuit, but looking totally different (oscillators especially).

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 27, 2015, 03:22:10 PM
So here's what I'm going to build, I think. It's really unwieldy with its huge filter-choke inductors, but at least I understand the purpose of each component, even if I used software to calculate the component values in the LC network itself. I have reasonable hopes that it will work. The schematic incorporates G3RZP's suggestions, but only I am responsible for the behemoth result! A modern-day Boat Anchor!

I have removed any mention of the Audiofil because this filter now has nothing much left in common with it, other than using the Amp-Filter-Amp system.

(https://lh3.googleusercontent.com/-JrJ9TdqGktQ/VWZC-CCoMxI/AAAAAAAAFA0/1gce4LS5V2U/s1024/Filters_23_Three_Pole_10K_Full_Schematic.jpg?gl=US)

Quote
Presumably, insufficient bypassing has an effect not just on this one tube, but on the entire radio, if audio gets into the B+ supply.

Definitely. You want about 4.7mFd from B+ to ground.

There is already a 10µF, 100V capacitor across the B+ supply, just after it emerges from the battery bank (you suggested that cap a few weeks ago when we were talking about voltage regulation, not bypassing).

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on May 27, 2015, 10:19:05 PM
Quote
There is already a 10µF, 100V capacitor across the B+ supply, just after it emerges from the battery bank (you suggested that cap a few weeks ago when we were talking about voltage regulation, not bypassing).

If the supply has zero impedance, you won't get any AC signal impressed on it, and the regulation will be very good. But practical supplies don't have zero impedance, so regulation and decoupling of circuits go together. You probably want something like 0.01mFd across the B+ close to the detector to keep RF off the B+


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 28, 2015, 03:20:29 AM
For the CW filter, I may want to try 2E31 tubes first. They have much lower amplification than the 1AD4, but looking at the simulated response curve, it looks like we may need less than 12dB boost to compensate for filter loss and end up with unity gain. (OTOH I'm not expert at analyzing these curves; and they are based on ideal inductors so losses may be greater.)

If I do use two 2E31 tubes, what is the best way to get the required B+ of around 22.5V, dropped from the main B+ of 52V? I'm worried that if I use a dropper resistor on its own, it will become part of the source impedance for the filter -- is that a legitimate worry?

If, therefore, using a dropper resistor is a bad idea there are two other choices:

--Using a voltage divider, which presumably avoids the source impedance issue? I would probably want to switch out the voltage divider when the filter is in the "off" position, to avoid draining the 52V B+ battery uselessly. Or would this current draw be low enough not to be an issue in battery life? (Can use Ohm's law to figure out current draw; value of resistors however may need to be determined empirically first.)

--Or, I could tap the battery bank. It's currently 7 batteries of about 8.4V peak each or 7.4V in the bottom of their charge cycle. So if I tapped it three batteries along, I'd have about 25V, dropping to 22V in the bottom of the charge cycle. This seems like an ideal solution except that it would presumably result in uneven discharging of the 7 batteries. Perhaps that's not a problem since they are Li-Ion batteries and supposedly don't suffer from "memory effect"; and I can always randomly jumble the batteries every time I recharge them and place them back in the radio, so that overall, they get discharged at the same average rate over time.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on May 28, 2015, 06:12:20 AM
I would tap the battery.

I would measure the choke DC resistances and impedance, figure out from that the series resistance and put the numbers into the filter programme. You might find the results somewhat worrying.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 28, 2015, 10:14:44 AM
Today I was re-reading "The Impoverished Radio Experimenter" (Lindsay Publications, 2003, in 6 slim volumes). One of the projects is a TRF/Regen combination . He uses 6SK7s for the RF amp and the detector, and a two AF stages of 6C5s. Between the two audio stages he inserts a multi-pole LCR filter taken from "Solid State Design" by Hayward and DeMaw. The inductance values are low, for tube impedances (88mH shunt at the start of the filter, for instance). The plate load on the input is a 47K resistor to the B+, with a large capacitor (10µF) in series at the input.

The author states that the CW filter, as built, was "actually much too selective" and had high insertion loss. So he then suggests using an "old-timer's trick" for a CW filter: putting a capacitor across the detector plate load, which in his case is a 20H choke. He adds a capacitor for resonance at 700Hz, but strangely enough he gives no indication of whether it worked! Anyway I just tried the same "trick." My detector plate load is a 60H choke. I wired a 1000pF capacitor across it, which should resonate at about 650Hz. It made no difference to the audio at all!

I would measure the choke DC resistances and impedance, figure out from that the series resistance and put the numbers into the filter programme. You might find the results somewhat worrying.

OK we'll see.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 29, 2015, 05:57:12 AM
While waiting for the filter parts to arrive, I've been researching other issues.

A couple of my textbooks say that an RF amp is usually a remote-cutoff tube (also called "variable-mu"). To rehearse the issues, and for those who may be new to tubes: the term "mu" is another term for "amplification."

A variable-mu tube has the ability to reduce its overall amplification as the signal gets stronger, meaning that it is less likely to be driven into "cutoff." The "cutoff" is the point where the negative-voltage peak of a large input signal, superimposed on top of the negative grid bias, will cut off the signal.

For those who are interested in the internal construction of tubes: the variable-mu effect is achieved by using a variable pitch in the helically shaped control grid.

My existing RF amplifier tube is a 1AD4, which is a sharp-cutoff type. It seems to be performing just fine, but would it work even better (handling a wider range of signal levels) if I replaced it with a remote-cutoff type?

On another issue, I have just "discovered" another subminiature tube, the 6612 (or CK6612). It seems superior in many respects to the 1AD4 that I've been using. They are both directly heated RF pentodes using the same tiny form factor.

Value1AD46612
Fil. voltage1.251.25
Fil. current mA10080
Transcond. µmos20003000
Plate voltage nominal4530
Max plate diss. watts0.30.2

So I'm thinking of obtaining a few of these 6612s and experimenting. It seems I can get better performance, along with a moderate but useful drop in filament current and B+ voltage -- not a big deal in the "desktop" set but useful as I start thinking about the portable version. (As far as I know, the lower plate power dissipation isn't a big deal if you're not using the tube as a power amp?)

I don't know whether the 6612 is "sharp cutoff" or "remote cutoff" -- I probably ought to be able to figure this out from the graph plots in the tube data here: http://tubedata.milbert.com/sheets/138/6/6612.pdf (http://tubedata.milbert.com/sheets/138/6/6612.pdf).

On the other hand: my receiver is working very well. I wonder whether substituting a tube with 50% higher transconductance might make things worse, not better? (I'm remembering that some people think solid-state regens are inferior to tube because FETs have *too much* amplification.)

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on May 30, 2015, 01:36:03 AM
The 6611 and 6612 are sharp cut off tubes. One trick that helps make the cut off more remote is to feed the screen grid from a dropping resistor: as the control grid goes more negative, screen current drops and so screen grid volts rises, tending to keep the current up to some extent. It doesn't do a lot of good, but does give some improvement.

The main call for variable mu (remote or semi-remote cut off) was originally (late 1920s) to reduce cross modulation from strong local broadcast stations as the AGC reduced the gain of the receiver on a wanted station. The same problems can still exist, which is why there were even variable mu triodes for cascode stages - ECC189/6ES8. Many of the remote cutoff tubes were somewhat lower gm than their 'straight' equivalents e.g 6K7/6J7, 6SG7/6SH7 and their miniature derivatives 6BA6/6AU6. The 6BZ6 is a 'semi-remote cut off' tube, and so is a 'half and half', meant originally for use in TV sets where cross modulation could be a problem.

(I wish this damn computer wouldn't auto correct terms it doesn't understand, such as turning 'cascode' into 'cascade'!)

Where you see a plate current of 10 microamps at Vg = -3, you have a 'straight' tube.

Another rarely discussed advantage of the remote cut off tube is the effect on signal to noise improvement ratio.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 30, 2015, 02:32:28 AM
I'm building the filter today, as the Hammond inductors have arrived.

One slight puzzlement is that my LCR meter is indicating much lower inductance than the Hammond specification.

Hammond spec = 4H, measured = 2.45H.

Hammond spec = 10H, measured = 7.25H.

I get about the same values regardless of whether I sample at 1kHz or at 120Hz.

The Hammond data sheet says that tolerance is +/- 15%. It also says that units will exhibit higher inductance at lower currents and lower inductance at higher currents (these are designed as filter chokes). The LCR meter must be using a very low current, so if anything it should be reading higher than the Hammond spec, not lower.

I've decided to spec my capacitors in the LC circuits according to my measured inductance values and not the Hammond specs. If it turns out that the Hammond spec is correct and my measurements wrong, this will shift the response curve leftwards somewhat but it will still be OK for a CW filter. (I don't have any instruments to measure the result; it will all be done "by ear".)

Where you see a plate current of 10 microamps at Vg = -3, you have a 'straight' tube.

Another rarely discussed advantage of the remote cut off tube is the effect on signal to noise improvement ratio.

Thank you for the explanations. Last night I was listening to a busy CW contest on 40m. Conditions seemed ideal: very low noise and good propagation (they are still rather good this morning, at 5 a.m.). The contest was deafening. I turned the RF gain all the way to the bottom (bias = -5.2V) and even there, the strongest signals were "pulling" the detector somewhat (artificial chirp in the CW). I think it's likely that the tube was operating beyond cutoff, but that enough signal was "leaking through" anyway. At that gain setting, the variable capacitor on the tuned circuit had no effect at all (no "peaking" characteristic); it was acting as a broadband amplifier (or rather, I think, as a net attenuator). Am I making any sense here? Anyway, that's the kind of thing that makes one think about a remote-cutoff tube, although from your explanation, I'm not sure it would have made much difference.

Listening to the contest also prompted thoughts about selectivity. There were two, three, four signals on top of each other, with almost the same audio tone. Now, I've never owned a "highly selective" receiver, but I'm presuming these are just classic pileups of people on the same frequency, and not a failure of selectivity in my receiver. Also, the fact that I can hear them simultaneously does not mean that they are hearing each other: that depends entirely on the propagation gods. In other words they are not necessarily QRM'ing each other; and even if they are, that's par for the course, in a contest. The kind of thing that no audio filter (or IF filter in a superhet, or DSP in a modern set) would help much with.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on May 30, 2015, 03:00:09 AM
Martin,

It's WPX CW Contest this weekend, so you will get a lot of signals on top of each other. The 80/40/20/15 and 10 metre bands will be VERY busy - and lots of QRO signals, too.

Hearing signals with the RF gain turned way back, you are probably getting leakage around circuits, as you do have long leads and very little shielding.

You probably have a lot of self capacitance on those chokes, and depending on how your LCR meter works (they don't all work the same way by any means), that could explain the apparent anomaly in inductance.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 30, 2015, 03:17:57 AM
You probably have a lot of self capacitance on those chokes, and depending on how your LCR meter works (they don't all work the same way by any means), that could explain the apparent anomaly in inductance.

Thanks.

The specifications for my LCR meter are here: http://tinyurl.com/q6zuynm (http://tinyurl.com/q6zuynm). My model is the 879B.

For what it's worth, I just tried testing in Parallel mode (rather than Series mode) and the inductance readings were almost exactly the same.

Come to think of it, if those chokes have a lot of self-capacitance, that becomes part of these parallel LC circuits anyway, and would affect the resonant frequency. So perhaps, in that case, using the measured inductance in calculating C might be the right way to go even if the L reading is wrong....

Either way, as I said, the curve gets shifted a bit left or a bit right depending on which figures are correct but it would still be reasonably appropriate for a CW filter. Building is under way ... so we'll soon find out whether we have a useful filter, or just a collection of large, expensive ($36 total for the 3 chokes) and inappropriate inductors. If they don't work in the filter, they can always go in the junkbox for future use in low-current B+ supplies.

Hearing signals with the RF gain turned way back, you are probably getting leakage around circuits, as you do have long leads and very little shielding.

Make that: no shielding at all! (Apart from the fact that the tubes themselves all have a painted-on, grounded shield.)

When I construct the "well-built" version of this radio later, I will probably adopt a "hybrid" solution concerning shielding. My thinking is to keep the detector coil completely away from any metal, and to house the radio inside a wooden chassis/cabinet. However the other stages will include more metal, possibly being built on alumin(i)um baseplates (or in completely enclosed Miniboxes) and with vertical shield plates in between stages. So it would be halfway between the old timers' all-wooden construction and the metal cabinets that came in during the late 1920s and 1930s.

Kitchin writes that the wooden construction improves sensitivity and that a metal cabinet soaks up signal. The only way to find out for sure would be to build two versions, one with a metal cabinet and one without.... or, of course, I could add alumin(i)m plates to the inside surfaces of the wooden cabinet, in effect turning it into a metal one. On a totally non-technical note I'm looking forward to the woodwork involved in building a nice wooden cabinet; and I think it will also improve the audio when I add a speaker (I will be winding a coil or two for SWL listening).

For what it's worth, on this radio, hum is minimal or nonexistent (presumably helped by using battery power). This was not the case with my 3-transistor regen, which was built on an alumin(i)um chassis. With that radio, I didn't get rid of hum until I built an alumin(i)um cabinet placed around the chassis.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on May 30, 2015, 05:53:39 AM
Quote
The specifications for my LCR meter are here: http://tinyurl.com/q6zuynm. My model is the 879B.

Doesn't say how it works, though, so it's impossible to say if self capacity affects the reading.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1GMX on May 30, 2015, 07:11:49 AM
If you have an issue with capacity (a multilayer choke would) then find the self resonant
frequency and calculate the equivalent LCR.

R is DC resistance you can measure.
C is the interwinding capacitance 
L is the likely specified L of the inductor.

From that you can build a model of the effective series or parallel form.

Or you can try and resonate it with the expected value based on L and
then calculate the effective C and subtract that from the actual C to get
the distributed C(of the coil).  Then you can recalculate the needed C
to get resonance at the desired frequency.

Filter hints.

Use caps that have a series reactance such that they couple poorly at
some lower frequency like half the desired.  The idea is why
build a amplifier that is flat from 20 to 20,000 when 300-700 is all that's
desired.  So if you use small coupling caps from plate to grid then you
loose low frequency response (high pass).

Another trick is capacitor from grid to plate.  It will be a small value and it
introduces negative feedback that increases with increasing frequency.
This will make the amplifier roll off at higher frequencies (low pass).

Now if you add a single inductor parallel tuned to the desired frequency
as the plate load or substitute for the grid resistor you will get a peaked
response.

Listen to that rather than model it.  Your ear will tell if its right.  In essence
 you are trying to get an enhanced response in a narrow band with less in
outside bands.  The enhancement only has to be small to be heard rather
than trying for high stopband filter that cost in power and introduced losses.

One hint the audio system for the receiver even for broadcast AM need not
be good to 10khz as likely what they are broadcasting never gets to 5-6khz.
same for low frequencies especially below 100hz.  A little crafting of the
response can lower heard hiss (especially high frequency noise) will be
less fatiguing.

One comment the receiving system is everything starting at the antenna
itself to the ear and all of that can be tweaked.

Allison


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 30, 2015, 08:36:00 PM
So, I built the CW audio filter and ... it works!

Tonight was an excellent opportunity for testing, with a major CW contest on 40m. I found that if I tuned a particular signal to be a "normal" CW tone, not too high, not too low, and then switched in the filter, the signal "pops out" and other nearby signals with a different tone are attenuated or even vanish altogether. If you are listening to this "boosted" signal and rotate the tuning knob slightly, making the tone higher or lower, there is sharp attenuation on either side of a center "tone."

There are however several "buts." Namely: (1) When the filter is switched in, it sounds like you are inside a tin can; it is quite fatiguing to listen to over a long stretch, thus it should only be used when truly necessary. (2) Tuning around the band isn't feasible with the filter "in"; instead, it's much better to tune up and down until you find a signal, and then switch in the filter if you want to boost that signal. (3) With the filter switched in, dynamic range is much lower; careful adjustment of RF and AF gain are needed to ensure that the signal is neither too weak on the one hand, nor overloaded/distorted on the other.

There is still some optimization to be done. When I first built the filter, it was with a pair of triode-strapped 1AD4 pentodes. This provided nothing like enough gain. So I rewired the tubes as pentodes; but now, the gain was much too high. Increasing the value of the screen dropper resistors had almost no effect on gain. So I added voltage dividers on the screen circuits of both tubes (there are two voltage dividers; I really should combine them into one, to reduce drain on the batteries). After adding the voltage dividers, gain is audibly the same with and without the filter switched in.

Even with the voltage dividers, I get the impression that unnecessary distortion is being created in the filter stage. When I get the time, I may try grid-biasing the tubes with the bias battery to see whether that helps. On the other hand, I suppose it's possible that the distortion I'm hearing is a filter artefact.

Concerning the values of the components in the LC circuit, in the end I decided to trust the Hammond Corp. rather than my LCR meter.

I am quite pleased with this filter and feel that it is a worthwhile addition to the set, although it is an unwieldy accessory: as built, it weights more than 2 pounds (nearly 1 kilo) and takes up quite a lot of room, which is ironic considering that the tubes are subminiature.

Here is a picture, and the updated schematic.

(https://lh3.googleusercontent.com/-fnXzripflJE/VWp7th5Z4pI/AAAAAAAAFBk/oE7oqSTMZRE/s912/Filters_25_Built.jpg?gl=US)

(https://lh3.googleusercontent.com/-AuzLsQTjF6U/VWqEBykxaxI/AAAAAAAAFB0/WrRCdWCIqtI/s912/Filters_26_Three_Pole_10K_Full_Schematic_Built.jpg?gl=US)

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 01, 2015, 05:25:49 AM
For the future "portable, stripped-down" version of this radio I've just ordered some type 6418 subminiature tubes (that is the correct number; I transposed two digits in my original post). Data sheet here: http://tinyurl.com/p2bn65m (http://tinyurl.com/p2bn65m) -- this link was originally broken and is now fixed.

These have a tiny filament current of 10mA (!) and a rated B+ in normal operation of between 15V and 22.5V. So even in a 3-tube version (RF, Detector, AF) it should run for ages on a single AA-sized 3100 mA/H NiMH rechargeable battery for filament, and a couple of small 2x9V or smaller 23A/2x12V for B+. If I need a "C-battery" bias voltage I can try G3RZP's trick of taking it from a resistor at the bottom of the B+ supply.

The challenge will be getting reasonable performance. It is not designed as an RF tube. Transconductance is only between 190 and 300 µmos (depending on plate voltage). Others have built regens with this tube (schematic here: http://tinyurl.com/pataebr (http://tinyurl.com/pataebr)) but these were for AM-BC use. The biggest performance boost with the current "high performance" set was achieved by adding the 60H choke as the detector plate load, but that inductor is heavy and bulky for a truly portable set.

This will probably wait until the desktop, "full featured" set is finished but still worth thinking about in advance.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: N3QE on June 01, 2015, 05:39:29 AM
For the future "portable, stripped-down" version of this radio I've just ordered some type 6148 subminiature tubes (data sheet here: http://tinyurl.com/pataebr (http://tinyurl.com/pataebr)).

These have a tiny filament current of 10mA (!) and a rated B+ in normal operation of between 15V and 22.5V.

Your tinyurl link does not work for me.

I see many references to a 6148 that is a miniature tube, but not with super low-volt/low-current specs that you describe. It's more like a submini 6AK5. Submini 6AK5 will have a lot of uses. http://www.radiomuseum.org/tubes/tube_5702wa.html


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 01, 2015, 06:05:34 AM
Your tinyurl link does not work for me.

Thank you for pointing this out. I made two mistakes in the original post:
--The tube type that I ordered is 6418 (and not 6148); I had transposed two digits by mistake.
--The tinyurl link for the 6418 data sheet was garbled; I have reposted it as http://tinyurl.com/p2bn65m (http://tinyurl.com/p2bn65m).

The original post has now been corrected in both places.

This idea is speculative. I think the tube is really a hearing-aid gadget intended as an AF power amp, not a voltage amplifier for RF.

Allison: thank you also for your post about filtering. Much appreciated -- all those little touches that can be included to improve audio for communications-receiver use.

I am trying to figure out how to record the audio from my filter. I'll add a small output transformer for 8-ohm output and make mp3. Then I will try to attach it to a Picasa slideshow as "background music" or try to find some other place that will host audio for free (I'd rather not bother with Youtube). Then I will post a link to it here. Could take a while to figure out.

The filter's effect is extreme, but useful in certain circumstances where it actually makes it possible to copy a signal that was otherwise buried in QRN or QRM. The filter does introduce a strong quotient of its own noise, like a sort of bathtub echo; but it boosts the CW tone by even more than the introduced noise, so by turning down the AF gain the overall S/N ratio is noticeably improved, on difficult signals. I don't think it's technically "ringing" because the "bathtub" sound is not added to the signal itself, but to the background.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: N3QE on June 01, 2015, 08:07:48 AM
--The tube type that I ordered is 6418 (and not 6148); I had transposed two digits by mistake.
--The tinyurl link for the 6418 data sheet was garbled; I have reposted it as http://tinyurl.com/p2bn65m (http://tinyurl.com/p2bn65m).

The original post has now been corrected in both places.

This idea is speculative. I think the tube is really a hearing-aid gadget intended as an AF power amp, not a voltage amplifier for RF.

The one place where I know the 6418 was used, was in crypto gear, where it was like a low-voltage logic tube. A lot of related submini tube (long details on margining for use in logic circuits) and crypto lore: http://jproc.ca/crypto/kwr37.html


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 01, 2015, 08:33:00 AM
The one place where I know the 6418 was used, was in crypto gear, where it was like a low-voltage logic tube. A lot of related submini tube (long details on margining for use in logic circuits) and crypto lore: http://jproc.ca/crypto/kwr37.html

I see what you mean! Looks like it was used at the very tail end of tube-based computer gear. Here are some 6088s and 6418s on a logic card from the link you provided:

(http://jproc.ca/crypto/kwr37_vv_card.jpg)

(Photo by Klaus Kopacz DC8HL, taken from the above-referenced link.)

The very low consumption/dissipation would make sense given that they could use hundreds or thousands of these tubes in a single computer.

It is a "computer-rated" tube introduced in 1957, the year of my birth. I'm dubious whether it will work usefully at RF, although as I said, others have built regens with them. These tubes are still available in lots of 5, or 50, or 100....

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 01, 2015, 09:28:31 PM
A recording of the CW filter.

To hear the two-tube CW filter in action, a Youtube video can be played here: http://youtu.be/87vIi8_A6Wc (http://youtu.be/87vIi8_A6Wc).

I tried to find a way to post just a sound file, but strangely enough it turned out to be much easier to make a movie (the visual part consists of a single photograph; it's the sound that counts).

Tonight was a good time for testing because there was heavy QRN on 40m. In this 20-second clip, a weak CW signal is first heard without the filter. The filter is switched in at the 8-second mark.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KI6LZ on June 01, 2015, 09:43:01 PM
Big noticeable difference for the good. Have to comment on CW OP. Sends tmp (abbreviated) then sends about instead of abt.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 02, 2015, 03:17:40 AM
Big noticeable difference for the good. Have to comment on CW OP. Sends tmp (abbreviated) then sends about instead of abt.

Thank you. In fairness to the CW OP, I should point out that I deleted 4 seconds of audio immediately after the filter was switched in. That's because when the filter was turned on, the AF and RF gain needed to be adjusted to take account of the different load and gain, during which the signal was largely inaudible -- so I don't know what the OP sent during that gap. The 4 seconds were cut in order to provide a clearer A/B comparison.

The filter indeed makes a big difference but only in certain cases/conditions. In most cases the human ear is a much better filter than this electronic one, mainly because of the filter's introduced noise and distortion, causing operator fatigue. I've never owned or used a "top-notch" modern receiver so I don't know whether that's a general truth, or only the case because of my primitive radio equipment. I'm still working on optimizing the filter.

I think it's also a matter of individual preference. Way back in the days of audio cassette tapes, I actually disliked Dolby noise reduction because of its effect (to my ear) in cutting the high frequencies; so whenever making my own recordings, I switched Dolby off during record and playback. Technically speaking, I was completely wrong: the Dolby equalization is supposed to render a "properly balanced" version of the original and my non-Dolby recordings were inferior because of the extra hiss.

For those who haven't seen the post (two posts back), the recording of my CW filter in action is here:  http://youtu.be/87vIi8_A6Wc (http://youtu.be/87vIi8_A6Wc).

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 03, 2015, 04:32:28 AM
Moving on, I'd like to add a final AF power stage to the receiver, so that a speaker can be added. The set will be used for SWL listening some of the time; and a speaker is also good when "showing off" to non-hams. The 1AD4 has a rated maximum plate dissipation of 0.3 watts and presumably a useful output somewhat lower than that. Presumably that means two tubes will be needed in the output stage.

The most efficient would be Class B push-pull, but as far as I can tell that involves quite a lot of extra circuitry. I already have a tube push-pull output transformer in the junkbox. But it looks like you need some kind of phase splitter to feed the two final tubes; and that involves either extra tube(s), or a center-tapped interstage transformer, or both.

Despite the complexity, push-pull may well be the way to go; after all this is supposed to be a "high performance" set. But meanwhile I was thinking of experimenting with *paralleled* tubes instead, in Class A. I've read in various places that to do this you must use "matched tubes" otherwise one of the two tubes will end up doing most of the work, but we're not looking for "high fidelity" here, just decent volume (a few tenths of a watt) and non-outrageous amounts of THD. (The advice to "match the tubes" came from people on tube audio forums, whose priorities are not necessarily the same as hams'.)

If I parallel the tubes, apparently each grid should be fed with a separate resistor and those resistors could be different in value in order to account for the difference in individual tube specs? If I really need to do that, what parameter would I be measuring, when matching the loads?

(I can't help feeling that push-pull would be a more elegant solution, but it could be worth testing a parallel configuration just to see how well it works -- and to learn more stuff about amplifiers.)

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 03, 2015, 05:10:32 AM
Back on the subject of audio filters, there's a discussion going on over on the HBR-receiver mailing list that prompted me to research the "proximity fuze" used by the Allies in WWII shells/bombs.

There's a full account here: https://en.wikipedia.org/wiki/Proximity_fuze (https://en.wikipedia.org/wiki/Proximity_fuze).

To summarize, one of the WWII circuits used four subminiature tubes:
--An oscillator that transmitted a signal and autodyne detector that received any reflected signal (not clear whether this was one tube, or two).
--Because of the high relative speed between the shell/bomb and the target, the reflected signal would move rapidly in and out of phase with the transmitted signal when the target was in proximity.
--These rapid phase shifts created an audio tone of 200Hz-800Hz.
--The audio was sent through a bandpass filter tuned to this tone.
--If the tone was present, it triggered a final thyratron tube and, "boom."

I bet their bandpass circuit was a lot better than my improvisation. Here's one that I found on the 'net (the VT rocket fuze). I think that second tube must be the filter, given the maze of RC circuits that surrounds it. No inductors!

(http://www.operatorchan.org/stem/src/138955718944.jpg)

It's amazing how large the proximity-fuze industry was: something like $1 billion worth purchased during the war. The price fell from $732 each, to $18 by the end of the war.

Martin


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: N3QE on June 03, 2015, 05:38:58 AM
Moving on, I'd like to add a final AF power stage to the receiver, so that a speaker can be added. The set will be used for SWL listening some of the time; and a speaker is also good when "showing off" to non-hams.

In miniature battery-powered tubes, 3V4 makes a very nice and loud speaker driver. Is there a subminiature version of that? It may be louder than necessary.

I see several regen designs on the web using a Russian 1J29B subminiature as a speaker driver, I don't know if there's a US equivalent.

Tim.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 03, 2015, 06:28:01 AM
In miniature battery-powered tubes, 3V4 makes a very nice and loud speaker driver. Is there a subminiature version of that? It may be louder than necessary.

I see several regen designs on the web using a Russian 1J29B subminiature as a speaker driver, I don't know if there's a US equivalent.

Tim.

You're right, the 3V4 looks good; I can't find a subminiature equivalent, they are all much lower power.

If you stick to directly heated types (1.25 V) battery types, there are quite a few "power amplifier" type subminiature tubes (such as the 1AC5) but they top out at around 50mW -- which I suppose might be enough for quiet listening on a small, efficient speaker. I couldn't find data on the Russian tube but will keep looking.

Because I have a pile of 1AD4s I will probably start by experimenting with those, since they have that relatively high maximum plate dissipation. I tried feeding a small 8-ohm speaker from the output of my existing single 1AD4 AF stage, through an audio output transformer. The volume is low but just it's intelligible enough for listening in a quiet room. That's what makes me think that adding another 1AD4 stage (one tube, or two tubes in parallel or push-pull) should do the trick.

My receiver is already weird enough, perhaps there's novelty value in keeping every single tube as a 1AD4....

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on June 03, 2015, 08:50:10 AM
A lot of German WW2 equipment used mostly the same tubes wherever possible - a common one was the RV12P2000, which was a straight pentode with a 12.6 volt 75mA heater. Used as RF amplifier, mixer, oscillator, IF amplifiers, detector, BFO and AF amplifier driving headphones....

The idea of using Class B (or Quiescent Push-Pull - QPP - as it was sometimes known) was that the tubes were biased almost to cut off and so drew very little current when there was no or little signal. Because of this, they had to be in push-pull, and if they were operating Class B2 (with grid current) to get more output, they needed transformer coupling. All this was to improve the life of the B battery. If you don't use a transformer, another tube is needed to be a phase splitter, which is more drain, even if you use an anode follower which will work with a filament tube.



Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 03, 2015, 10:45:46 AM
... if they were operating Class B2 (with grid current) to get more output, they needed transformer coupling. All this was to improve the life of the B battery. If you don't use a transformer, another tube is needed to be a phase splitter, which is more drain, even if you use an anode follower which will work with a filament tube.

So if you use transformer coupling to get phase inversion, it looks something like this?

(https://lh4.googleusercontent.com/-3LrMCLT87IE/VW834nqoP_I/AAAAAAAAFC8/0F01IRSBEK0/w1342-h752-no/Submini_regen_amp_push_pull_01.jpg)

I'm wondering whether that Hammond 124A is appropriate -- it's designed as an interstage transformer, with the impedances noted on the schematic (spec sheet here: http://tinyurl.com/nan6u3v (http://tinyurl.com/nan6u3v)). For the final output transformer, I'm assuming something like 10K CT? There are numerous small-ish Hammond models available with a whole range of primary impedances; there's also a "universal" push-pull model (with numerous taps for adjustable impedance) but it's expensive.

Not clear whether grid-bias resistors are needed; does the transformer winding provide a measure of self-bias, as it were? I've seen schematics both with and without.

(On a side note, this time I managed to reduce the size of the schematic image so that it's not huge when displayed in eham. This doesn't always work; not sure why not.)

(Sure enough, 10 minutes later, the picture has become "huge" again. Sorry about that. It really does seem to be uncontrollable. Advice welcome.)

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on June 03, 2015, 02:30:30 PM
The plate to plate load that the output transformer needs to present depends on the desired output power, the plate voltage swing and the tube capability in terms of current and plate voltage. The bias depends where on the tube characteristic curves the quiescent plate current needs to sit. This is where you really need to carefully read Langford-Smith........or possibly 'Foundations of Wireless', but you need the earlier version authored by Sowerby, rather than the later one by Scroggie. The details are a bit much to go into here......


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 04, 2015, 07:17:13 AM
The plate to plate load that the output transformer needs to present depends on the desired output power, the plate voltage swing and the tube capability in terms of current and plate voltage. The bias depends where on the tube characteristic curves the quiescent plate current needs to sit. This is where you really need to carefully read Langford-Smith........or possibly 'Foundations of Wireless', but you need the earlier version authored by Sowerby, rather than the later one by Scroggie. The details are a bit much to go into here......

Thank you Peter. Langford-Smith indeed has a detailed guide to designing a Class B2 push-pull (one in which grid current flows during part of the cycle), which I am wading through, very slowly. The details are indeed "a bit much to go into here"!

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 04, 2015, 07:13:28 PM
Tonight I changed the main tuning dial mechanism. Until now I'd been using a National Velvet Vernier 5:1 drive, which is very nice but much too "fast" -- over the 180-degree rotation of the capacitor, that's actually only a 2.5:1 reduction. Tricky to get the right CW tone and fiddly to tune in SSB precisely.

I've replaced it with a British Jackson Bros "two-speed" reduction mechanism. The "fast" speed is 6:1 and the "slow" speed is 36:1. This is a bit difficult to describe if you haven't used one before, but basically, if you move the dial in one direction is will be 6:1 and then if you stop and go backwards, it will be 36:1 over the range of about one knob rotation (backwards and forwards). It takes some getting used to and I haven't yet decided whether I like it (as opposed to just having 36:1 all the time). However it's much better than the "too fast" National Velvet Vernier.

As delivered from Jackson in the UK (or rather, its successor company Mainline Electronics), the dial had an inappropriate knob: too small and with a painted white indicator line on it, which made little sense. I have temporarily replaced this with a larger grungy National knob from the junkbox, until I can find something less beat-up (I like the main tuning knob to be at least 2.25 inches diameter). Eventually I will be fitting a longer "needle indicator" (allowing for a longer scale length because of the greater radius) and designing my own three-band scale (80m/40m/20m), then building a wood-and-glass escutcheon. (That's for the "finished" version of the set, not the current "breadboard" version.)

(https://lh5.googleusercontent.com/-Xtr7X6iyofs/VXECStYMc1I/AAAAAAAAFDo/a95TO0RMGjw/w1057-h547-no/Submini_regen_Jackson_dial.JPG)

(The subject of the "optimal tuning dial" is vast and I'd better not get going on that one!)

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 05, 2015, 08:53:37 AM
I'm getting ready to add new bands (20m and 80m) using plug-in coil forms.

However, it looks like the detector coil will need seven pins: RF coupling coil, tickler, detector coil and bandspread capacitor. For 40m, it's a 47pF capacitor in series with the tuning capacitor, creating a 2pF spread from one end of the dial to the other, but for other bands it will need to be a different value to create the desired spread -- basically, the bottom (CW) portions of the bands.

(https://lh3.googleusercontent.com/-VCFcsixv4oo/VXHCJUNL_VI/AAAAAAAAFEI/-KuQW3-gCvM/w427-h417-no/Submini_regen_large_layout_schematic_coil_form_11.jpg)

In the schematic, the red-circled points are where the pins would be. I had originally planned to use 6-pin forms for this but now realize this is one pin too few (unless someone can prove me wrong).

I can think of several ways to get enough "pins":
--Use 6-pin forms, then use a wire with a banana plug for the "7th pin." I believe that National used such a trick in one of their SW-series receivers.
--Use an octal tube base to make my own coil forms. The bases can either be made by smashing tubes, or by buying tubeless bases from Antique Electronic Supply (about $3 each) and gluing polythene or polycarbonate pipe onto them. This would also preserve my supply of NOS 6-pin Amphenol forms for a future project.
(https://images.campyent.com/sites/default/files/styles/uc_product_list/public/uc_products/p-sp8-476.png?itok=DdjkY0Os)
--Or I could obtain Amphenol octal plugs intended for power supply cables. These have a "base" with the pins, and a "shell" that snaps onto the back. Discarding the shell, I could glue the form onto those (probably a better solution than an actual tube base because it's lower-profile).
--Or, use these ready-made octal coil forms from National RF: http://tinyurl.com/pcgor9d (http://tinyurl.com/pcgor9d); but I'm pretty sure they are too short for my purposes.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on June 05, 2015, 02:13:40 PM
Martin,

using tube bases was done in the 1920s.....

A tube base and a piece of fibre glass tube (probably better than plastics other than polystyrene) can be a very cheap approach, and much cheaper than any other way.

For the LF bands, you could get probably get away with varnished cardboard tube.....

Looking at your photo....That's a long lead from the panel to the capacitor. Could you not shorten it by running the lead from the nearest point on the panel to the front of the capacitor.

Do you need another Jackson slow motion drive?


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: WB6BYU on June 05, 2015, 03:02:25 PM
Quote from: KB1WSY

You're right, the 3V4 looks good; I can't find a subminiature equivalent, they are all much lower power.

If you stick to directly heated types (1.25 V) battery types...



The 3V4 has a center-tapped filament, so it can be run off either 2.5V (series) or 1.25V (parallel).


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 05, 2015, 03:08:24 PM
A tube base and a piece of fibre glass tube (probably better than plastics other than polystyrene) can be a very cheap approach, and much cheaper than any other way.

I will bear that in mind!

Looking at your photo....That's a long lead from the panel to the capacitor. Could you not shorten it by running the lead from the nearest point on the panel to the front of the capacitor.

Totally true. There is a second screw fastening the cap's armature to the wooden base much closer to the front panel, which I should have used for grounding. However remember this is a a "breadboard wonder" that won't resemble the final product in any of the fine details. In the "final build," wiring distances will be much shorter and a lot more shielding will be used. (Probably each stage will be in its own alumin(i)um box apart from the detector, which will be left "out in the open" on the wooden base.)

(If you want evidence that I'm capable of building things "right," remember my ARRL builds. This current project is only meant to be a quick "prototyping" with maximum flexibility and ability to change things experimentally.)

For what it's worth, that over-long piece of grounding wire does enable the total elimination of hand-capacitance effects. The only way to "trigger" such effects now is to put my hand "inside" the radio (i.e. behind the erstatz "front panel") and wave my hand over the detector coil, giving a fine imitation of a Theremin.

Do you need another Jackson slow motion drive?

Are you offering one? The remnants of the Jackson company are still in existence (their eBay store is here: http://tinyurl.com/o2vqdr3 (http://tinyurl.com/o2vqdr3)). I have corresponded with them a few times in the past three years, and purchased some items. They are helpful, and still seem to be building stuff, although it is somewhat difficult to tell whether they will or won't replenish stock of a particular "classic" item once it's sold-out.

Dials in general are a whole side-issue of great interest. I discovered that fact when I stocked up on several supposedly highly sought National items and was very disappointed (not much reduction ratio, surprisingly cheap construction of the escutcheon, needle pointer, plastic -- not glass --  fascia, and dial itself; and the mechanisms themselves are stiff). I also stocked up on Eddystone (one 598 and one 898) which I am saving for future, high-end projects. I have a couple of Millen dials and find them too stiff. At N2EY's urging, I recently bought a capacitor and its reduction drive from a U.S. miltary LM (or BC-221) counter; now that is quite something!

But for this "little" regen that's the subject of the current thread, the 6:1/36:1 Jackson looks like "just the ticket." I have been playing with it for the past 24 hours. Only odd thing so far: It has "visible" backlash (when you let go of the knob in 1:36 mode, it springs back slightly) but there is no "audible" backlash (the CW tone doesn't change). The golden component is the reduction drive itself; the "dial" (the "needle" and "scale") is just a cosmetic construction from Mainline Electronics; I will be replacing it with my own much larger version, fastened to the original Jackson reduction drive.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: N3QE on June 05, 2015, 03:13:22 PM
--Use an octal tube base to make my own coil forms. The bases can either be made by smashing tubes, or by buying tubeless bases from Antique Electronic Supply (about $3 each) and gluing polythene or polycarbonate pipe onto them. This would also preserve my supply of NOS 6-pin Amphenol forms for a future project.

I think octal tube bases are a great idea, and would prefer them over the other octal plug possibilities. I love how they look, especially if they have a "classic" tube brand logo on the base, and I hate the "coil form polystyrene" bases (the polystyrene always melted whenever I tried to solder the pins).

I was somehow able to use 5-pin tube bases with my solid-state regen coilsets (I had to be clever in how I used common winding pins, that may be how I got by with fewer pins than you). Got them from bad 807's and love how they turned out.

For 7 pins, bad 1625's could be the donor base. Not sure if you have a bunch of old 1625's lying around. There were also smaller receiving-tube non-mini 7-pin bases.

Below is picture of my solid state regen with 5-pin base and socket:
(http://www.trailing-edge.com/WP_20150605_001.jpg)


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 05, 2015, 03:20:32 PM
I think octal tube bases are a great idea, and would prefer them over the other octal plug possibilities. I love how they look, especially if they have a "classic" tube brand logo on the base, and I hate the "coil form polystyrene" bases (the polystyrene always melted whenever I tried to solder the pins).

It's tricky. I found that the problem can be minimized by protecting the pins with one (or two) large alligator clips, in lieu of heat sinks. I have dozens of Amphenol and Millen coil forms and a good vocabulary of cuss-words emitted during soldering. The key, as with all soldering (at least on tube-era components) is to use high temperatures, but for a short time -- just long enough to get the pin and the wire to the same temperature and have solder flow freely over them.

I built these coils for a future transmitter:

(https://lh4.googleusercontent.com/-2CYC9Oo4Nc8/VXIopUBpCEI/AAAAAAAAFEc/1cL3z_AbpsA/w730-h356-no/CoilRack_10.jpg)

Below is picture of my solid state regen with 5-pin base and socket:

Nice!!!

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on June 06, 2015, 01:48:23 AM
Take a potato, cut a slice about 1/4 thick. Push the pins of the coil form through it, and solder quickly.

Remove potato before using coil! Because there will be flux spattered on it, don't attempt frying and eating.....

A fresh slice for each coil is advised, and make sure the potato doesn't dry out.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 06, 2015, 05:19:03 AM
We are getting somewhere. It looks like I can build something like this, with a lot of help from coil-form Elmer WB5OFD:

(https://lh3.googleusercontent.com/-bopoa0I204E/VXLiPumwuiI/AAAAAAAAFEw/cFen524gqUE/w300-h400-no/Submini_regen_coil_form_01.JPG)

That's an Amphenol octal plug insert. I'm going to try doing it with a 7-pin Amphenol plug instead, because the pins are a bit wider spaced and that will make it easier to include the capacitors inside the form.

It will take a little while to obtain the parts and figure out the best way to "mate" the pipe with the plug and glue them  together securely.

Take a potato, cut a slice about 1/4 thick. Push the pins of the coil form through it, and solder quickly.

A couple of days ago I was reading an old radio book. In the elementary "electricity" section, it advised using a potato to check polarity. Apparently the positive wire of a current will make a dark spot when you push it into a 'tater! I didn't know that, must try it out.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 07, 2015, 04:36:23 PM
A 10-Milliamp Tube Radio

I recently received a batch of 6418 subminiature tubes. These have ultra-low filament current (10mA). I wondered whether any useful radio could be designed with them.

Today I built a one-tube 6418 regen for the AM/BC band. The main tuning coil is a ferrite rod antenna pulled from an old transistor radio (inductance = about 380 µH). The tickler winding is 20 turns of thin magnet wire over the bottom of the ferrite inductor. The capacitor is a small variable from the same junked transistor radio, range 25pF-225pF. The filaments run off a single AA NiMH battery, the B+ is from two 23A batteries in series (24V).

It works! Well, sort of. It actually only pulls in two stations, with the strongest being the Boston-area sports station that carries the Red Sox games.

(https://lh6.googleusercontent.com/-QgE3_hxfOPk/VXTSu1sL02I/AAAAAAAAFFM/C8wvRKzBvj0/w730-h547-no/Submini_regen_6418_01.JPG)

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 08, 2015, 04:22:28 PM
The 10-milliamp AM/BC regen was greatly improved today by reducing the number of turns on the tickler (now about 6 turns, reduced from 20) and increasing the value of the (fixed-value) "throttle" capacitor. The radio can now pick up more than a dozen broadcast stations -- and a lot more if you connect it to an external antenna instead of just using the ferrite rod. The volume is modest, into either high-impedance 'phones or low-impedance 'phones through a transformer, but adequate. Adding a second 6418 AF stage would probably enable reception of some additional stations that are currently just too quiet, at the cost of doubling the current draw to an outrageous 20mA.

This is quite interesting, given the extremely modest specs of the 6418 tube. There is a design on the web for a 6418 "super-regenerative" shortwave BC radio and I might give it a try, modified for the ham bands (details here: http://home.comcast.net/~phils_radio_designs/QRPTubeRadio.pdf (http://home.comcast.net/~phils_radio_designs/QRPTubeRadio.pdf)).

Of course these radios are not "10mA" since I left out the plate current; but it's the right order of magnitude.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on June 09, 2015, 01:10:24 AM
A superegen is good only for AM, WBFM and MCW. It won't do SSB or CW.....


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 09, 2015, 07:46:49 AM
A superegen is good only for AM, WBFM and MCW. It won't do SSB or CW.....

I should have remembered that. So it would need a separate BFO, huh?

Alternatively I can just build a standard regen for a ham band, based on the 6418, and see whether it has any usable performance. It's an intriguing challenge, but if it works it would be eminently suitable for the "portable/hiking" receiver.

OTOH this is beginning to turn into a red herring, given that I haven't completely "finished" the 1AD4 experimental desktop version yet. Am still somewhat buried in the mathematics (and curve-drawing) for a push-pull AF output stage. Langford-Smith is just too dense for me on this subject, but a couple of tube audio enthusiasts have written "designing a push-pull amplifier" screeds on the Internet that are slightly simpler or at least, give a fuller explanation of the steps required.

See for instance: http://www.guitarstudio.tv/documents/Designing-V-T-Amplifiers.pdf (http://www.guitarstudio.tv/documents/Designing-V-T-Amplifiers.pdf).

Once I've designed and built that final AF stage, and added other coils for a couple of other ham bands, the set will be ready for a "complete, final build" using a proper cabinet and good layout/wiring practices. Designing and building the "Octal base" coil forms for the detector is in progress.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on June 09, 2015, 09:10:21 AM
The superegen won't work with a BFO, because the BFO will 'capture' the superegenerative detector.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 09, 2015, 12:47:36 PM
With the success of the very-low-current 6418 AM/BC regen, I'm miniaturizing it into a shirt-pocket Hammond plastic enclosure to take on summer vacation at the end of this month.

The ferrite-rod antenna will be only 2 inches long, so I'm increasing the diameter to 0.5" (the current rod is 3/8" diameter and 6 inches long).

I've been researching ferrite. Seems like for antenna use (rather than RFI suppressor use) at lower HF and MW frequencies, would be either "Type 33 material" or "Type 61 material."

The Type 33 is described as usable as an MnZn (edited) inductor at frequencies up to 3MHz and "more economical" (indeed the prices are lower). The Type 61 is MiZn (edited) and has a higher maximum usable frequency of 25MHz.

Is the Type 61 worth the extra expense, for my purposes? Would the antenna pull in more stations and/or make regeneration easier? (The tickler is wound on the rod, below the main coil.)

(I'll be using Litz-wire windings; still trying to figure out the best wire gauge and specs.)

The portable version of this set will be ultra-simple, AM/BC only. I may try to add a switchable capacitor across the main tuning capacitor to see if I can pull in the European Long-Wave band too -- I will be in France and England, where the band is still use by major stations.

(Yes, I know this is getting a bit far from Ham Radio but it's going to prove useful when I build a portable ham receiver later.)

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: JAHAM2BE on June 09, 2015, 05:03:56 PM
The superegen won't work with a BFO, because the BFO will 'capture' the superegenerative detector.

This is a topic I've wondered about a very long time. I've only built two superregens (self-quenching transistor types for VHF) so my experience is quite limited.

Shouldn't it be possible to use a BFO with the superregen if you keep the injection level very low, of the same order as the incoming signals?

The bigger problem as I see it is the required quench frequency for shortwave, which is going to have to be somewhat low, perhaps even audible.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on June 10, 2015, 12:48:36 AM
Quote
Shouldn't it be possible to use a BFO with the superregen if you keep the injection level very low, of the same order as the incoming signals?

The bigger problem as I see it is the required quench frequency for shortwave, which is going to have to be somewhat low, perhaps even audible.

The problem is that the injection has to be very low if there is going to be gain. Plus the fact that the quench frequency has to be low compared with the modulation, (since the superegen effectively produces a PWM signal at the quench frequency) to allow enough time for the signal to build up at full gain. For SSB and CW, the usual regenerative circuit is about as simple as you can get.

For a very good explanation of the working of the superegenerative receiver, see 'Second Thoughts on Radio Theory' by ' Cathode Ray' (aka M. G. Scroggie), Iliffe, London, 1955 or 2nd edn, 1956, p 250. Originally published in 'Wireless World' in the June 1946 edition - pages unknown.

Quote
The Type 33 is described as usable as an MnZn (edited) inductor at frequencies up to 3MHz and "more economical" (indeed the prices are lower). The Type 61 is MiZn (edited) and has a higher maximum usable frequency of 25MHz.

Is the Type 61 worth the extra expense, for my purposes? Would the antenna pull in more stations and/or make regeneration easier? (The tickler is wound on the rod, below the main coil.)

I suspect the type 61 will give a higher Q. The length will be a bit short: many radios of that sort used a slab antenna rather than a rod, presumably to save space. A 3 inch rod will be better than a 2 inch.

Strictly speaking, Litz (Litzendraht) wire has strands in powers of three - so there's 3, 9, 27, 81  etc strands, and each strand goes in a consistent pattern from the inside of the plaited cable to the outside and back to the inside repetitively over a certain length of the wire. This is expensive to manufacture, and far more usual is to use what is technically called 'bunched conductors' where there are a number of enamelled insulated wires laid up together and served - originally with either single or double silk or, for cheaper cables, single or double cotton. These days, the wires are self-fluxing and the serving is a polyester so you can just solder the wires without all the problems we had years ago. These days, (and indeed, probably for the last 70 or so years if not more) bunched conductors are usually (but strictly speaking, inaccurately)  referred to as Litz wire.........For most applications, the somewhat lower Q attainable doesn't matter. You can make your own bunched conductors from 44 gauge enamelled wire; figure out the length, add half as much again, and twist the wires - a hand drill is useful. As you twist them tighter, the length reduces, so don't twist too tightly. You'll probably find that for your application, three conductors will work fine: more conductors just gets more and more unwieldy and the improvement isn't that much.

I did learn something during my apprenticeship at the Marconi Company back in the 1960s.....


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 10, 2015, 04:33:13 AM
I suspect the type 61 will give a higher Q. The length will be a bit short: many radios of that sort used a slab antenna rather than a rod, presumably to save space. A 3 inch rod will be better than a 2 inch.

Strictly speaking, Litz (Litzendraht) wire has strands in powers of three - so there's 3, 9, 27, 81  etc strands, and each strand goes in a consistent pattern from the inside of the plaited cable to the outside and back to the inside repetitively over a certain length of the wire. This is expensive to manufacture, and far more usual is to use what is technically called 'bunched conductors' where there are a number of enamelled insulated wires laid up together and served - originally with either single or double silk or, for cheaper cables, single or double cotton.

The 2" rod I've ordered is short because the shirt-pocket plastic case is small; 2" is the largest size that will fit, considering that the batteries and components also have to go in there -- including a miniature, transistor-radio-type 340pF variable capacitor. I suppose it could be a varactor instead, but I'm trying to avoid any semiconductors. I couldn't find the Type-61 in 2" so I ordered Type-33. If it doesn't work I'll go for Type-61 instead and maybe (reluctantly) go for a slightly bigger case with a longer rod. (It is apparently a bit tricky to cut ferrite rod down to size, which is why I bought the short rod rather than a longer rod that could be shortened.)

I am assuming that it is possible to scramble-wind in two layers to get a high enough L, although that's probably another thing that will lower the Q. The other issue is keeping the rod at the edge of the case and away from the batteries -- trying to avoid damping effects of metal. On the whole however, at MF the regeneration is very stable and hand-capacity/damping effects are minor. I assume this is partly because of the properties of ferrite in concentrating the field.

I remember seeing "slab-type" ferrites in very small transistor radios; they had a roughly rectangular cross-section with rounded corners.

I've ordered a spool of "150/46 Served Litz, 0.024 inch bundle, nylon served" and we'll see how it goes. Thank you for your explanation of Litz; but what does "served" mean? Any analogies with tennis?...

Yesterday I experimented some more with the tickler, reducing it to 3 turns and moving it to the other end of the rod (near the grounded end of the tank coil). This has increased the regeneration and brings in more stations, at the cost of a rather long wire run between the coil and the tube plate -- because the ferrite is 6" long. The screen regen control is set to just below audible oscillation, in order to demodulate the AM stations.

Yet another experiment that I had never contemplated originally ... I'm hopeless! One of these days I will actually finish something and build it in "final" form.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on June 10, 2015, 08:12:00 AM
 
Quote
but what does "served" mean?

A 'served' cable is one that has some form of outer wrapping. In old radios, you may well find wiring with rubber insulation and a cotton braid cover: that cover is the 'serving'. In the case of your wire, I expect that it means that there are 150 strands of 46AWG wire, wrapped with very fine nylon cord. Presumably self-fluxing - otherwise, it's a so-and-so of a job to get the enamel off each strand for soldering, and any broken or disconnected strands lower the Q.

The old telephone cables were served in lead......thus the necessity for making 'wiped joints'.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 10, 2015, 08:29:25 AM
Presumably self-fluxing....

All of the magnet wire I've bought recently has been strippable just with the heat of the soldering iron (the insulation melts off) ... much better than the old days when you had to scrape it off.

The Litz-lookalike on the 1950s-era ferrite antenna I'm using in the current 6418 AM/BC set was also strippable with a soldering iron. That was a big surprise; I had no idea that this easy-tinning existed, back then. Actually I think it's not real Litz at all: when I melted off the string-like insulation, there was only a very thin, solid wire underneath.

I have ordered that new reel of 150/46 Litz from a website that specializes in supplies for crystal radios: https://www.midnightscience.com/ (https://www.midnightscience.com/).

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1GMX on June 10, 2015, 04:26:03 PM
Your 10ma AM/BC regen is not a super-regen. Its author is in error.
 Its a conventional one like the 1ad4 version.

Superregen has a quench circuit either self-quenched or by an external device and what
quench frequency would you use for a AM radio that would not self interfere?  Most
Super-regens are built and used at upper hf and up.  BFO is not needed just go to
self oscillation mode just like the 1ad4.

Its cute though.

Allison


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 10, 2015, 05:44:56 PM
Your 10ma AM/BC regen is not a super-regen. Its author is in error.
 Its a conventional one like the 1ad4 version.
Allison

Hi Allison,

The AM/BC set that I built is a pure regen -- almost identical to my 1AD4 set, except for the values in the LC circuit and changes to various other components to account for the lower frequency. It is now working really well, with various incremental adjustments, although it will never win a prize in the audio loudness stakes! I can't wait to build the miniature, shirt-pocket version.

It was the schematic that I linked to that purports to be a super-regen (http://home.comcast.net/~phils_radio_designs/QRPTubeRadio.pdf (http://home.comcast.net/~phils_radio_designs/QRPTubeRadio.pdf)). I take it that's the one you're saying isn't a super. I have not tried building it.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on June 10, 2015, 11:50:23 PM
Interestingly, Armstrong's original super-regenerative receiver was apparently a BC band one - there not being anything else at the time. (Except for those damn amateurs - give 'em 200meters and down - they'll never get out of their backyards with that!)

The details of exactly what he had aren't that specific, but there are three designs in the Harmsworth's Wireless Encyclopedia of 1924 for BC band super-regenerative receivers. One is wired in 1/16 inch SQUARE wire - i.e. sides 1/16 by 1/16 inches. Square wire was quite the thing in the those days...The instructions include the dimensions and illustration of cutting the wood for the cabinet....


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1GMX on June 11, 2015, 05:58:31 PM
>>>It was the schematic that I linked to that purports to be a super-regen (http://home.comcast.net/~phils_radio_designs/QRPTubeRadio.pdf). I take it that's the one you're saying isn't a super. I have not tried building it.<<<

Correct.

Your approach will yield a better radio.  You have made all the errors already.  ;)

I have to get a few of them as that is a fun sounding tube at a sick power level.
one of tose as detector and two 5678s and its a radio.
it also proves you don't need a lot of gain in the device to be successful, and I bet
that one goes in and out of regeneration smoothly.

FYI one thing I tried with 5678s (or 1ad4s) is grounding grid 1 and using grid 2 as a
control grid as it makes a low mu result.  Regenerates well and less twitchy.

Allison


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 11, 2015, 07:21:14 PM
You have made all the errors already.  ;)

Yesterday my hand slipped and a screwdriver shorted the positive filament lead to something. Flash of orange light, dead tube (open filament, on the VOM). Unsoldered the dead tube and replaced it.

At the time this happened, the set's power switch was off, and I was in the process of making some alterations. The episode made me think twice about the "simplification" I had made of the power supply circuit. I had the positive terminals of the batteries "in circuit" all the time. I connected the negative terminals of the A and B batteries together, and put a single SPST switch between that point and ground, as a single power switch, instead of having separate switches for filament and B+. It works, but it's a bad idea, because you can close a live circuit (a series current running through both sets of batteries) while altering the set; unless you remember to remove at least one of the batteries from its holder first, to break the chain.

I have to get a few of them as that is a fun sounding tube at a sick power level ... it also proves you don't need a lot of gain in the device to be successful, and I bet that one goes in and out of regeneration smoothly.

Regeneration is smooth, although in a very low power AM/BC radio that may not be too hard? I'll be trying it at HF soon. The little 6418 performs way better than I ever expected. Unlike the 1AD4 or the 2E32, it's not even billed as an RF tube and transconductance tops out at 300µmos. Even though the glass envelope is clear and you can clearly see the filament and grids, I cannot see any glow, even in total darkness, and there is no detectable heat if you put your finger on the tube (the 1AD4s have a very slight warmth when powered). The 6418 is also noticeably smaller in size, compared to the envelope size of the 1AD4 and similar.

Today I added a 6418 AF stage, making it a two-tuber and doubling the filament consumption to an outrageous 20mA. It improved the audio gain, which was useful for some of the weaker stations; but the original one-tuber is more fun because of its minimalism. Plus, and I'm sorry to say it, the AM bands are pretty much a cultural desert in New England nowadays, even with an external antenna supplementing the ferrite (with ferrite alone, I get about six stations; with the long-wire antenna, quite a lot more). Even the Red Sox games have moved to FM most of the time ... I'm not a baseball fan but XYL is. Maybe I should build a VHF regen for the Red Sox; I hear that a regen can demodulate FM, in its own manner....

FYI one thing I tried with 5678s (or 1ad4s) is grounding grid 1 and using grid 2 as a control grid as it makes a low mu result.  Regenerates well and less twitchy.

Interesting, will bear that in mind.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on June 11, 2015, 11:54:14 PM
Quote
I hear that a regen can demodulate FM, in its own manner....

Don't see that, but a super-regenerative detector will do it nicely.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 12, 2015, 04:47:48 AM
Quote
I hear that a regen can demodulate FM, in its own manner....

Don't see that, but a super-regenerative detector will do it nicely.

Indeed, I misspoke ... sounds like what we want is a super-regen using "slope detection."

This morning feels like a time warp. I have the baby 6418 receiver tuned to "the Memory Station, 740 AM" connected to some amplified speakers, pumping out 1940s big-band favorites, with that warm and not-so-subtly distorted tube sound.

Need to get back to HF and ham radio, sooner or later.

Meanwhile however, I've found a pre-built new-old-stock Philmore component set from the 1960s. It's a 1.8" ferrite rod, with pre-wound coil on the ferrite, and a matching miniature variable capacitor, optimized as a matched set for AM/BC miniature transistor radios. That will be the first choice for the "shirt pocket" 6418 radio even if it's slightly lazy not to wind the Litz myself -- in any case I'll need to wind the small tickler myself. I'll also try winding my own with the already-ordered virgin ferrite and Litz, to see which one has the best performance.

Now trying to figure out how to tune a significant portion of the LW band, so that I can switch that in, while traveling in Europe.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 12, 2015, 02:27:38 PM
The Litz wire ordered from the Xtal Set Society (http://www.midnightscience.com/index.html (http://www.midnightscience.com/index.html)) arrived today. In the box was a detailed three-page description of Litz wire, its history, specifications and usage. In an earlier post G3RZP pointed out that it is much better if the wire is self-fluxing. Well, the document that came with the wire explained the tinning procedure:

(1) Remove the serving (braid).
(2) Fan out the wires a bit (like a brush).
(3) Remove the enamel on the strands (drag them through hot solder).
(4) Twist the strands back together.
(5) Tin them.

Apparently those who do a lot of this kind of thing use a tool called a "solder pot" (a pot of molten solder) to do step (3). In my experience with magnet wire, all you need to do for "occasional" enamel-removing is to melt a large blob of solder onto the tip of the iron, and then drag the wire slowly through it -- the enamel burns off in a nice smelly manner.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 12, 2015, 04:53:06 PM
... a super-regenerative detector will do it nicely.

There's a U.S. patent filed in 1952 that describes a superregenerative pocket FM broadcast radio using two subminiature tubes (CK522AX detector and CK533AX AF amp). This radio was built by a Boston-based radio manufacturer. The patent is here, including a schematic: http://tinyurl.com/nksudwc (http://tinyurl.com/nksudwc).

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on June 13, 2015, 01:04:37 AM
As an apprentice, we used a solder pot and just dipped the end of the wire in it for about 30 seconds: a little nasty smelling smoke and the job was done. These days, you would need a fume cupboard, goggles, protective hat and clothing......


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 13, 2015, 04:19:56 AM
As an apprentice, we used a solder pot and just dipped the end of the wire in it for about 30 seconds: a little nasty smelling smoke and the job was done. These days, you would need a fume cupboard, goggles, protective hat and clothing......

Actually the instructions from one of today's solder-pot manufacturers (ESICO) are quite mild, by modern standards:

"Normal use of this product is likely to expose the user to lead, a chemical known in the State of California to cause cancer and birth defects (or other reproductive harm), or exposure to other Proposition 65 chemicals. This entire device becomes HOT in use. DO NOT TOUCH the metal surfaces or contents.
"ALWAYS wear eye protection. Guard against accidental spills or splatter.
"MOLTEN SOLDER can cause serious burns."

That's it! Yes, you too could be in the vicinity of an open container of 1 kilo of molten lead, wearing nothing more than eye protection....

(http://www.esicotriton.com/images/pt_set1.jpg)

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on June 13, 2015, 05:38:12 AM
Martin,

An apt quotation comes to mind:

"I know it's something humorous, but lingering, with either boiling oil or molten lead."


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 13, 2015, 05:48:33 AM
"I know it's something humorous, but lingering, with either boiling oil or molten lead."

My object all sublime
I shall achieve in time—
To let the punishment fit the crime—
The punishment fit the crime;
And make each prisoner pent
Unwillingly represent
A source of innocent merriment,
Of innocent merriment!


O childhood memories, of sitting in a stuffy town hall and hearing that sung from the lips of my school maths teacher and his amateur troupe!

Meanwhile: I turned my 20-milliamp, two-tube 6418 radio on yesterday morning and 24 hours later, it is still going strong, with the filaments supplied with nothing more than a single AA rechargeable NiMH cell. The battery has a 2500mA/H capacity and is about 10 years old -- it was used with a Canon camera briefly, then left discharged for many years until I recharged it recently.

Theoretically the battery should last for 125 hours (about five days) but it will presumably be rather less than that, even if the battery were new. The main question is who will run out first, the radio, or me -- I can only take a certain number of hours of sickly hits from "the Memory Station, 740AM, Always on the Side of the People." The set is connected to some amplified PC speakers. It is getting to be "a punishment all sublime."

(The B+ is supplied by some "9V" Li-Ion cells with a 600mA/H capacity.)

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 13, 2015, 05:20:56 PM
Push-pull Audio Output Stage

So I waded through the calculations, continually simplifying them because my understanding of the issues is somewhat below 50%. In the end I looked at the maximum plate dissipation of a 1AD4 tube and conservatively decided to use 100mW per tube (one-third the rated maximum). Looking this up on the tube curves, I found that this would imply a grid bias of -2.0V. Applying the formula for final primary transformer impedance I got a very high plate-to-plate impedance of about 50K. (I suspect that none of this makes much sense, but that's what I did.)

In the real world, I had a couple of 1AD4 tubes, date-matched to August 1973. For the phase inverter, I used an interstage transformer; this is much simpler than a tube albeit bulkier and more expensive (primary 10K, secondary 90K CT). For the speaker, I had a "universal" push-pull output transformer -- which has lots of taps to adjust impedance -- and I set it to its maximum primary impedance of 22.5K CT. To adapt cathode-biased designs to my directly heated tubes, I came up with a circuit based on grounded-cathode designs, with a separately applied grid bias (from the already existing C battery).

It works! I've been listening to it for about 4 hours now and the audio volume is good -- much higher than I expected and good enough to fill a small room. I'm still messing around with the grid bias. I put a potentiometer in there, for testing purposes; the best result seems to be about halfway through the pot rotation. I suspect that this design is sub-optimal in all sorts of ways but ... so far ... it works.

Edited to add: I assume this amp is operating in Class B. No provision has been made for negative feedback; but presumably it would be a good idea. I'm reading about it....

Edited again to add: I'm getting a little bit of audio feedback (a high whistle) occasionally. I think it's because the first AF stage (not the push-pull final stage) is sloppily built (long leads, circuitous connections) and quite microphonic, so I'm not too worried about it. I think it will go away in the "final" build of this radio.

(https://lh4.googleusercontent.com/-UD-sa8MX2Dk/VXzDGL918yI/AAAAAAAAFFs/jGJ8NqOaPx4/w1003-h752-no/Submini_regen_amp_push_pull_02.jpg)

(https://lh4.googleusercontent.com/-1CzFpXUscd0/VXzDFhfNJUI/AAAAAAAAFFk/EP7ZNavDh2Q/w1045-h752-no/Submini_regen_amp_push_pull_03.jpg)

73 de Martin, KB1WSY



Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on June 14, 2015, 12:03:17 AM
What is the plate current with no signal? If it is very close to zero, that suggests Class  B operation, which is the maximum power saving condition, but likely to be the highest distortion. If it is high enough that it changes very little with signal level, that is, or is very close to, Class A. If it is about 10 to 20% of full signal current, that is Class AB.

If the direct voltage on the grid (I have problems with the term D.C. Voltage - that is watts!) doesn't change with signal, then it is Class A1, AB1 or B1. If the grid voltage does change, indicating grid current, then it is A2 (very rare - usually means distortion or sometimes a 'soft' i.e. gassy tube) AB2 or B2.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 14, 2015, 03:22:00 AM
What is the plate current with no signal?... etc.

I blew a fuse on the current ranges of my $2.99 Harbor Freight DMM some time back ... caused by my stupidity and abetted by bad design. The two ends of the plastic range dial/pointer look almost identical, so you can easily set it to the current range -- 180 degrees rotation -- instead of voltage by inattention. I have a spare fuse somewhere, just have to find it ... will then measure and report back.

Or I could put a 1Ω resistor in various places and measure voltage drops....

To give a bit more details about the push-pull math. This was taken in part from a book written by an electrical engineer who designs guitar amplifiers as a hobby (http://www.guitarstudio.tv/documents/Designing-V-T-Amplifiers.pdf (http://www.guitarstudio.tv/documents/Designing-V-T-Amplifiers.pdf)). I did try the more sophisticated Langford-Smith procedure but it was way above my head.

I conservatively figured on a plate Imax of 6mA for both tubes at the supplied B+ of about 50V. That's 150mW per tube; they are rated for a maximum plate dissipation of 300mW each.

This yields a quiescent Iq of 0.318 * 3mA per tube, or about 1mA.

Reading off the curves in the 1AD4 tube data, I got a grid bias of about -2V.

To get the plate-to-plate output primary impedance I used the formula Rpp = 4 * Eo / Imax and got the rather large figure of about 34KΩ. However I now realize that I screwed up by using Eo = 50V. The is the plate voltage; I should have used the voltage swing, not the absolute value. If I can figure out what the voltage swing is, I can re-calculate the impedance and possibly use a different tap on the universal output transformer -- it is currently set at 22.5KΩ.

Overall I believe my math and assumptions to be shaky in all sorts of ways. The fact that the built amplifier seems to work well is probably just a fluke.

For what it's worth, the output sound is pleasing to the ear, once the bias is adjusted (when the bias pot is close to the grounded end of the track, the sound is mushy and unclear; in the middle of the track, presumably around -2V to -3V, it's not bad at all).

Still getting a microphonic high-pitched feedback today. I can make it go away by grasping the first AF tube between my fingers for a few seconds. The feedback then disappears, but comes back a few minutes later. It is audible even when the audio gain and output grid bias are turned all the way around so there is no other sound.

Concerning distortion, if I get the courage I'll fire up my Eico oscilloscope and dual-trace electronic switch and see if I can get a before/after sine wave by feeding a modulated RF signal to the receiver. The scope is very dodgy however; it hasn't been restored yet. When I get to it, it will probably yield an eHam Boat Anchor thread.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: N3QE on June 14, 2015, 04:00:20 AM
To get the plate-to-plate output primary impedance I used the formula Rpp = 4 * Eo / Imax and got the rather large figure of about 34KΩ. However I now realize that I screwed up by using Eo = 50V. The is the plate voltage; I should have used the voltage swing, not the absolute value. If I can figure out what the voltage swing is, I can re-calculate the impedance and possibly use a different tap on the universal output transformer -- it is currently set at 22.5KΩ.

For pentode output stages, optimal "load resistance" is almost always lower than calculated "plate resistance" because you want maximum power transfer. But you are very much in the right ballpark for a battery-operated audio output stage. Look at e.g. the push-pull 3S4 or 3V4 designs that use 45V B+.

Using an output transformer with an impedance different than optimal by a factor of 2 or 4, is not the worst thing. It will limit your output power at a given distortion level but the amplifier will still work just fine for all but most critical purposes. If you look at detailed audio output stage designs, what they do is try several different impedances, build them, plot distortion vs power out curves, then pick the one that looks the best at desired power. In a push pull amplifier they also typically try different bias/standing currents (which can strongly influence distortion at very low power levels if done wrong).


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 14, 2015, 04:29:20 AM
Measurements of Plate Current, Grid Voltage

Ok I replaced the fuse in the DMM.

"No-signal" means the receiver's RF section was deliberately de-tuned to be totally silent.
"With-signal" means that the receiver was tuned to a strong modulated tone from my RF signal generator.

Plate currents. These were measured by inserting the DMM at the center tap of the output transformer -- if this is not a valid method please let me know.
--No-signal current: 2.69mA.
--With-signal current: 2.71mA.

Grid voltage. This was measured with the DMM, on one of the tubes only, and with the bias pot rotated about halfway.
--No-signal voltage: -2.24V.
--With-signal voltage: -2.24V.

Conclusion, based on Peter's instructions: It's running in pure class A. My understanding is that the power output could be significantly increased by running in Class B1 or B2 (indeed that's the point of using Class B). Not sure how to proceed, or whether it's worth doing so....

For what it's worth, the volume is substantially lower than what I get with a pair of generic PC amplified speakers connected to the same receiver, but there is nothing surprising about that!!! (The 1AD4 tubes top out at low dissipation levels, plus the speaker I'm using is standing on its own and not in any kind of cabinet.) Edited to add: however the volume is still perfectly adequate!

If you look at detailed audio output stage designs, what they do is try several different impedances, build them, plot distortion vs power out curves, then pick the one that looks the best at desired power.

I don't have the test instruments to measure distortion, but since I have a multi-tap universal output transformer, I could always try lowering the impedance and seeing what kind of difference it makes (volume, audible distortion). Off the top of my head I would assume the volume with a lower impedance would be lower, in line with the change in turns ratio; but perhaps I've got that concept the wrong way round, or perhaps it's making a proper match that creates the maximum volume rather than an abstract higher/lower impedance.

In a push pull amplifier they also typically try different bias/standing currents (which can strongly influence distortion at very low power levels if done wrong).

The setting of the bias pot does make quite a difference, with an audible sweet spot (distortion-wise) of around -2V.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on June 14, 2015, 04:55:43 AM
Martin,

It's possible that you don't have enough signal drive to get into Class AB1, let alone Class B If you have 2 volts of bias, the peak to peak grid voltage each grid to ground would be a bit under 4 volts. Worth measuring the actual AF voltage grid to grid with the multimeter.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 14, 2015, 05:18:49 AM
Martin,

It's possible that you don't have enough signal drive to get into Class AB1, let alone Class B If you have 2 volts of bias, the peak to peak grid voltage each grid to ground would be a bit under 4 volts. Worth measuring the actual AF voltage grid to grid with the multimeter.

The DMM doesn't have a low-AC range. So I used the VTVM, which I presume is OK to measure floating voltages (not referenced to ground) as long as you use the AC ranges. The grid-to-grid voltages are:
--No-signal (apart from loud hum caused by presence of probe): 0.1VAC.
--With-signal: 0.6VAC.

Does that make any sense? If it does, and if we really want to go for more volume or at least do a test of operating in AB1 or B, that presumably means adding a one-tube driver stage ahead of the phase-inverting transformer. We'd end up with a four-tube audio stage....

(Edited to add: Or I could find a strong external audio source and feed it in directly at the input of the push-pull until I get the 4V that you mentioned; just in the interest of seeing what happens if the stage is driven into AB or B.)

Here's another elementary question. If we're sticking to Class A, is push-pull any more efficient than just running the two tubes in parallel and dropping all this fancy phase-inverting business? (I seldom see parallel operation in audio amplifiers; not sure why.)

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 14, 2015, 05:35:21 AM
Ok. What I did was take the AF tone (about 1KHz) from a signal generator and feed it directly at the grid of the first AF stage. When I turned up the AF gain control, the tone was very loud and I could get the grid-to-grid AC voltage on the push-pull stage to 4, nay, 5 volts with only about a one-third rotation of the AF gain dial. Any louder, and I was a bit afraid of waking up the neighbors on this lazy Sunday morning.

Edited to add: at 5V grid-to-grid, there must be grid current, right? (The bias is about -2V.)

There is no obvious distortion, but that's just a 1KHz tone and a totally subjective opinion.

What next? Maybe this indicates the need for more gain anyway (regardless of the push-pull output stage considerations)? Until now I've been happy with the gain, heard on high-Z headphones. The plate load in the first AF stage is a 60H choke and the stage seems to have plenty of useful gain for headphone use.

Separately: I experimented with the taps on the "universal" push-pull output transformer. Halving the primary impedance (from 22K to 11K) led to a noticeable reduction in the volume of the signal generator tone (I used alligator clips so that the changeover was fast enough for short-term human "aural memory"). Since there doesn't seem to be an obvious problem with distortion on the 22K tap, I will leave it on 22K for the time being.

P.S. I think it's time to fire up the 'scope, gingerly, and with nostrils primed for melting wax capacitors and burning transformers. This is all great fun and very educational; once again, many thanks for your patience in guiding a neophyte through all these design issues.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: N3QE on June 14, 2015, 06:07:08 AM
Here's another elementary question. If we're sticking to Class A, is push-pull any more efficient than just running the two tubes in parallel and dropping all this fancy phase-inverting business? (I seldom see parallel operation in audio amplifiers; not sure why.)

I don't know of any miniature battery tube radios that were push-pull for speaker drive - I've always thought a single 3V4 was more than loud enough.

I might think that since battery life was an important parameter, that the higher efficiency of a push-pull stage would win, but it seems to be a wash after you figure in the additional circuitry and filament (and besides the second tube does not fit well into the filament dropping chain with any balance.)

There are modern designs for 3S4/3V4 push pull amps on the web but these are not battery-operated radios with series filament chains.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 14, 2015, 06:19:43 AM
I don't know of any miniature battery tube radios that were push-pull for speaker drive - I've always thought a single 3V4 was more than loud enough.

I haven't seen any push-pull miniature battery tube designs either. In my case they're subminiature tubes and I couldn't find a single-tube solution that would drive a speaker adequately, at least not at the relatively low B+ of this receiver (about 50 volts). That's the only reason I went for push-pull: I figured the extra efficiency was worth it.

Another reason to avoid push-pull in the battery sets would have been the substantially larger component count compared to a single-tube final stage. If it could be done with a single 3V4, why complicate things?

Edited to add: Push-pull stages were even relatively rare in large desktop communications receivers, I believe.

My receiver isn't a "logical" design at all. It now has far too many tubes and components to be turned into a portable set; and yet, it's battery powered and uses subminiature tubes. Makes very little sense, except to a homebrewer doing one-off stuff. What I really like about these little tubes is the great ease of experimentation and breadboarding -- they're just as easy to wire into place as semiconductors, and the voltages are low enough for messing around (within reason). In bulk, you can pick up 1AD4s for $2 each or less.

This breadboard set is now huge. It covers most of my workbench and weighs in at 16 pounds (7 kilos). It will be fun to cram it into a much smaller desktop cabinet later.

From left to right: RF stage, regenerative detector, first AF stage (in front), CW audio filter (in the back), push-pull output stage. "Power supply" runs along the back. A total of 7 tubes, with total filament consumption of 700mA when the 2-tube CW filter is powered up (the filaments are switched off when it's not in use). The radio also has 7 large iron-framed inductors (filter-style chokes or transformers).

(https://lh5.googleusercontent.com/vRyJdVgnPXwWYY16Nw7YovPhdEOD-yqVAe02YXKsxg4=w570-h219-no)

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on June 14, 2015, 08:07:17 AM
Quite a number of the old 'boat anchor' AC powered communication receivers used a push-pull output with something like a pair of 6V6s.

Double pentodes for battery powered Class B or QPP operation were more common in Europe prior to WW2. For examples, see the various QP2XXX devices at http://frank.pocnet.net/index.html.

Very rough calculations suggest that a single class A1 1AD4 with 45 volts of B+, a 12kohm AC load (i.e. transformer), -1 volt of bias and about 2 volts peak to peak of grid drive should give about 5 or 6 milliwatts. You can measure the resistance of the speaker and the volts across it with the VTVM and decide from E2/R what the power is. Or feed a signal from the AF generator to the speaker and at the desired loudness, measure the power. Then you know what you are aiming for.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 14, 2015, 09:37:28 AM
Very rough calculations suggest that a single class A1 1AD4 with 45 volts of B+, a 12kohm AC load (i.e. transformer), -1 volt of bias and about 2 volts peak to peak of grid drive should give about 5 or 6 milliwatts. You can measure the resistance of the speaker and the volts across it with the VTVM and decide from E2/R what the power is. Or feed a signal from the AF generator to the speaker and at the desired loudness, measure the power. Then you know what you are aiming for.

Thanks, that's a very neat test. Speaker's DC resistance (on VTVM, speaker disconnected from transformer) = 7Ω.

AF tone from signal generator was injected to grid of first AF stage, then various audio levels were set with the receiver's AF gain.

--Comfortable level for monitoring or QSOs yields voltage across speaker of 0.15VAC which is 3mW.
--Uncomfortably loud level yields 0.3VAC which is 13mW.
--Maximum level (gain fully rotated), loud enough to be heard by neighbors, yields 0.5V which is 36mW.

Finally, I disconnected the signal generator, then deliberately set the regenerative detector to "squeal" and the AF gain to maximum. Horrible noise and noise of neighbors' pacing around above my head. Yields voltage of 0.8VAC which is 91mW.

You know what, I've decided that this little amplifier is more than adequate despite the (on paper) puny wattage. Furthermore, it seems likely that it is operating comfortably in its "range" and I can't hear any bad stuff in the audio (which, of course, doesn't necessarily mean it isn't there).

To operate the amp in Class B (by driving it harder) would, ironically, yield too much volume unless I did something to counteract it, such as increasing the bias or finding some other volume-reducing workaround. I don't see any point in doing that.

I hereby declare this amplifier design "finished" -- it worked right off the bat, with no tweaking or alterations. I will replace the temporary bias pot with a couple of fixed resistors in a voltage divider ... and that's it. This does not preclude further experiments at a later date if, for some currently unforeseen reason, I want more power output.

I powered up the Eico 460 oscilloscope but I can't get anything usable out of it. In the couple of years since I last tried to use it, it seems to have deteriorated even further. In the picture below, the thick horizontal line in the middle is all sorts of parasitic miasma that appears even when no signal is applied at all. The very thick "sine wave" superimposed on top is what appears when I switch in the AF signal generator and adjust the vertical gain and horizontal sync as best I can.

In reality it is not that blurry; this was a "time lapse" photo and I didn't use a tripod. The reflections on either side are from venetian blinds in the shack windows.

(https://lh3.googleusercontent.com/-VYfOMDaJfe0/VX2kkxl0KTI/AAAAAAAAFGk/s_yW8qtte5g/w504-h378-no/Sumbini_regen_scope.jpg)

It will be a great "boat anchor restoration" for the future. (I also have a second one, for parts.)

Meanwhile the Little Wonder (AM/BC regen with two 6418 tubes and a single AA filament battery) is still running fine, today mainly country music and fire-and-brimstone preachers. Forty-eight hours and counting.

73 de Martin, KB1WSY



Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on June 14, 2015, 12:03:32 PM
Quote
fire-and-brimstone preachers

Are those the  'fire-and-brimstone and send me your dollars' type preachers?

The minimum for marine radios used to be 50mW! Back in the late 1950s, transistors were not considered reliable enough (!) for emergency marine radios. Marconi produced a receiver covering the 500, 2182 and 8364 kHz emergency frequencies (plus or minus a tuning range) using the 12 volt automobile tubes, but had problems getting the 50mW of audio at that voltage. So to avoid having either a vibrator or motor generator supply and yet run off 24 volts, they used two PL84 pentodes in parallel working off the supply.....The PL84 is a 15 volt 300mA heater version of the EL84, a pair of which in push pull producing 10 watts at very low distortion from 250 volts was a very popular HiFi amplifier back in those days! I suspect that the 28D7 was considered obsolete by then, although it was a 'current' type in 1953...but being 'American', they maybe considered it wasn't available enough, and in any case, it wasn't ENGLISH!

(Martin, another quotation

"Oh! Pray be Early English, ere it is too late')

The 500kHz emergency tx to go with it had a quick heat 6146 variant (QZ06-20) with a 1.6volt 3.2 amp filament instead of the usual heater/cathode.

But I doubt that even in a motor lifeboat, they really needed 50mW....


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 14, 2015, 12:18:20 PM
Are those the  'fire-and-brimstone and send me your dollars' type preachers?

Not this one; no mention of cash and nothing slick about the presentation. Mainly, he was screaming at the top of his lungs, and he sounded like he was a very vigorous 103 years old. Very strange radio station: today it was one hour of "public service" advice for Senior Citizens. Followed by one hour of screaming sermons, mainly about repentance I think; not clear whether it was aimed directly at the same Senior Citizens so they could plead forgiveness for their trespasses before "buying the farm." Then, several hours of country music. Now, we are back to Big Band. Occasionally there is a fast-forward and we get the soppiest Beatles, Elton John or Roberta Flack hits. Then, back to the '30s. I quite like it; it must be a bit like having an iPod on "random shuffle."

Edited to add: All is revealed. The station's announcer, who always seemed to be the same guy, is fund-raising for his "listener-supported" station right now. Turns out he is also the station's owner, and he just said candidly that "this station is just a hobby of mine, it's not a business." He's based in Bath, Maine (730AM) and has a repeater in Cambridge, Mass. (740AM). Good for him. More details here: https://en.wikipedia.org/wiki/WJTO (https://en.wikipedia.org/wiki/WJTO).

I am still in the process of optimizing that little radio. I increased the value of the plate/tickler RF choke from 3mH to 12mH (supposedly more appropriate for MF) and messed around with the throttle capacitor value. The result was a slight increase in regeneration, moderately better sensitivity and (subjective, this) somewhat better sound. I have ordered (or am improvising) all of the parts for the shirt-pocket version of this radio. Whether it will work properly is a question mark: the ferrite will have to be located very close to other components and there may be too much damping for decent reception (the case is 3.6"x2.6"x1.1" and there's not much room left over after accounting for the batteries, the miniature variable capacitor and miniature regen pot, and the non-antenna parts).

Back "in the day," was there a standard way to make a radio LW/MW? Was there a MW tap on the ferrite antenna for instance? I'll have to build the LW feature "blind" by using a signal generator to spot the band edges; I don't think there's much LW activity in this part of the world, but I'll be on vacation in Europe in a few weeks and it would be cool to try to get LW.

Back on the Ham Ranch, I'm still working on those 7- or 8-pin detector coil forms to enable addition of other bands on the big breadboard radio, which is currently monoband 40m. When that's been sorted out, the "high-performance regen" will be "finished" and ready for a "proper build."

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: G3RZP on June 14, 2015, 11:24:07 PM
Quote
Was there a MW tap on the ferrite antenna for instance?

From memory, the usual thing was a MW antenna coil and then a separate LW coil which was switched in series.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 15, 2015, 01:21:30 PM
I've received some of the small components for the shirt-pocket AM/BC *tube* regenerative radio.

(https://lh5.googleusercontent.com/-KmElCZvDTvY/VX8uNJe7HtI/AAAAAAAAFHE/9hgo2r6svv4/w576-h432-no/Submini_regen_pocket_AM_01.jpg)

That's a Hammond plastic case, 3.6" x 2.6" x 1.1" with a built-in battery lid on the outside of the case. To give you an idea how small it is, that black battery is AA-size (for the tube filament). The little green batteries are type 23A (12V each, for a 24V B+; they are commonly used in the "clickers" of wireless garage-door openers). The ferrite rod inside the case is Philmore NOS from the 1960s, diameter 0.32" and length 1.8", for those who wanted to build their own miniature transistor radios; it comes with that slide-on coil. The coil has 4 leads emerging from it (I will be tryng to figure our their purpose). At the left-hand edge is a 50K "thumbwheel-type" PCB-mount potentiometer (for regeneration). The variable capacitor inside the case is a tiny "sub-miniature" and that is a problem because I don't know if I'll find a flat wheel-knob that will mate with the shaft. The piece of cardboard underneath the components is there just to get an idea of how large the PCB will be (actually, it's a custom size that comes with the Hammond enclosure; I've ordered it but it hasn't arrived yet).

Outside the case on the workbench is a larger ferrite rod that I had originally hoped to use. It is bigger diameter (0.5") and 2" long, and basically a bit too large for this project -- too bad because it might have considerably better Q than the smaller Philmore one. Also outside the case is a conventional "miniature" variable capacitor. This one is easy to find thumbwheel knobs for, but may be slightly too large: to use it, I may have to cut a square hole in the PCB to get enough height inside the case. The cute 6418 subminiature tube is in the middle; it has a ridiculously low filament current of 10mA.

Note also the additional slide on-coil on the bench; I'm figuring I could use it (or a shortened version of it) by sliding it on to the end of the ferrite rod and switching it in series with the other coil to get the European longwave (LW) band too.

Switches and connectors are another issue. That regen pot doesn't have a built-in power switch; I think I'd better try to find a switched pot, to save having to find space for a power switch. As it is, there's only just enough room for a headphone socket, let alone an additional power switch.

The big unknown is whether a ferrite antenna surrounded by all those components and batteries will suffer from too much damping. In my experience, this is a major issue with any regenerative detector coil or antenna (and in this case, the two functions are combined in one). I'll just have to build it and see.... I do have a larger case available (it's the same height and width, but 0.8" longer) and that's the fallback solution if the ferrite is too "crowded" by the other components.

The prototype "breadboard" version of this radio uses two 6418 tubes (only to get a bit more gain; it worked OK with just one tube). I switched it on at around 8 a.m. on Friday morning and it is still working, 80 hours later (more than three days) running on a single AA battery for the filaments and some 9V-format batteries for B+.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 17, 2015, 03:20:53 AM
The prototype 2-tube 6418 Little Wonder, with its 20mA filament consumption, stopped running overnight after being continuously on since 8 a.m. Friday, apart from about one hour when I powered it down to improve reception by changing an RF choke and a capacitor. That's at least 108 hours of autonomy. The AA battery was a 10-year-old NiMH rechargeable with a rated capacity of 2500mA/H which theoretically would have been 125 hours. (The B+ was powered from a set of 9V-format Li-Ions with 600mA/H capacity.)

Therefore the limiting factor for the "shirt-pocket" version will be the B+ A23-battery set, which has a meager capacity of about 55mA/H. For the two-tube breadboard version, I measured the powered-up B+ current at 1.7mA. If we assume that a one-tube shirt-pocket version draws about half of that (say, 0.9mA) that would be a maximum autonomy of about 61 hours (probably more like 50 hours in real life).

As I work on the shirt-pocket version, the main challenge so far is fitting those little B+ batteries into the battery compartment. As you can see from the photo in the previous post, they fit OK if aligned longways (rather than side by side). But they don't fit once you add the thickness of the battery holder's terminals at either end (the metal terminals, and the thickness of the plastic they are mounted to). The shortfall is only about 1.5 millimeters; I can probably deal with that by carefully filing away a little bit of the plastic from the inside of the case. I tried arranging the batteries side by side, but they then became unreachable through the battery door, which would mean unscrewing one entire side of the case (four small screws) every time the B+ batteries are changed.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 19, 2015, 03:07:31 PM
Today I made major progress in building the "shirt pocket" AM/BC tube radio.

Miniaturization has a lot to do with somehow fitting in the human-operated controls: in this case, the tuning capacitor, the regeneration control, the power switch and the MW/LW bandswitch. These are all bulky and unavoidable. The 3.5mm headphone socket also takes up room.

In earlier posts I mentioned the batteries. I've decided to reduce the size of the filament battery from double-A to triple-A. There's only one 6418 tube, and its current draw is only 10mA. The AAA NiMH battery is 1000mA/H so that's 100 hours. In practice the autonomy limit is set by those green B+ batteries, which will last about 50 hours.

In the photo, the big black wheel at the bottom is the tuning dial for the 340pF miniature variable capacitor (which is hidden underneath the dial). To the right of the big dial is another dial that you can see peeking out from underneath the PCB: this is the regeneration potentiometer. The headphone socket is just above. The MW/LW band switch is underneath the tuning dial (and on the back side of the PCB); not visible in this photo.

The case is about the size of a "pack of cigarettes" for those who remember this conventional "size reference."

(https://lh3.googleusercontent.com/-lq1VzuTkzEk/VYSPb_A7uBI/AAAAAAAAFH8/EAcCsLZtY58/s576/Submini_regen_pocket_AM_02.jpg?gl=US)

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 21, 2015, 09:36:42 PM
Tonight's puzzle is how to tune the entire AM/BC band using the miniature parts for the shirt-pocket radio (pictured in the previous post).

Before wiring the parts into the PCB I've breadboarded the tuned circuit

The inductance of the ferrite rod coil is about 190μH. The variable capacitor's maximum capacitance is about 330pF. According to the vendor (see eBay listing here: http://tinyurl.com/pffjks4 (http://tinyurl.com/pffjks4)) the minimum capacitance is 10pF but I am measuring it at 60pF to 70pF -- I think this is because the capacitor actually consists of four separate capacitors, and when you put them in parallel, the residual capacitance of the poly film dielectric is quite high.

Although this tiny ferrite rod and miniature capacitor work well together, the minimum frequency of the tuned circuit is 640KHz and the maximum is 1350KHz, as measured with my RF signal generator (and confirmed by known marker BC stations). The AM band in this part of the world is 540–1610, so I'm missing 100KHz at the bottom and 260KHz at the top.

I've been scratching my head about how to fix this. Adding a series capacitor to lower the minimum capacitance will also lower the maximum capacitance, and narrow the tunable range at the bottom. Adding a shunt capacitor to raise the maximum capacitance will have similar bad side effects at the top end. Perhaps the answer to this involves both adding a series capacitor and raising the inductance (by adding more turns).

A quick and dirty way to get the whole band is to split the band in two and have a switch that inserts, or shorts, a series capacitor. I've already built a bandswitch into the case but that was supposed to be for adding the European Long-Wave band later!

The good news is that apart from the too-narrow bandspread, this tiny tuned circuit (1.8-inch-long ferrite rod, tiny poly capacitor) is working nicely! It's almost as sensitive as the much longer 6-inch ferrite and large variable capacitor that I was using until now on the breadboard, and it was not hard to get regeneration going with a handful of turns on the tickler. I must admit this is quite a surprise.

Gnyah. Time for bed.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 22, 2015, 05:32:59 AM
In the light of day, this looks like the best solution:

I rewired the miniature variable capacitor to remove two of the four gangs (these are the two smaller gangs, intended for the FM part of a dual-band radio). This should have left me with a rated maximum capacitance of 290pF but it actually seems to be more like 255pF. But the good news is that the minimum capacitance has been halved: it used to be around 65pF but is now 32pF.

If I now increase the ferrite coil inductance from its current 190μH to 300μH, by adding turns, this should provide a range of 575KHz to 1620KHz. I will be missing the bottom 35KHz of the AM/BC band, but that's still way better than what I had before.

As for adding turns to the coil: I have several of those cardboard tubes with pre-wound Litz on them, and there is enough horizontal room on the ferrite rod to add a second tube to the existing one. I can just add the second tube, and connect it in series with the first. This gives about 450μH, so I will then need to un-wind enough turns to get down to 300μH. I will probably play it carefully by staying slightly on the high side of 300μH because the true inductance may change to some extent when the ferrite is moved into the cramped quarters of the small plastic case.

Finally, I suppose it is possible that by increasing inductance by 50% and using a greater length of the ferrite for the coil, reception may improve. It's already quite good anyway. We'll see what happens.

I wanted to build a "dual-band" MW/LW radio. This will now be harder because it involves adding a substantial scramble-wound Litz winding at one end of the ferrite, The required inductance for Long Wave broadcasting would be about 4.5 milli Henries to reach the bottom of the LW band! There will now be less room available for this large coil.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 22, 2015, 09:00:42 AM
After making the changes described in the previous post, the breadboard set is now tuning from 610KHz to 1500KHz. That's much better, but not as good as the theory predicted (theoretical predictions often turn out that way!). I'm now missing 180KHz of band coverage, instead of 360KHz before.

Anyway, I've decided it is "good enough" for the time being. AM stations tend to avoid the extreme band edges anyway. I'll leave the "perfect AM radio project" for a future date and build something that works, now.

Oh, and the reception is somewhat better too; apparently it helped to have more L and less C.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on June 24, 2015, 08:47:01 AM
The past two days have been a good example of an experiment that doesn't work, but is still an excellent learning experience.

I built the "Mark I" version of the shirt-pocket radio, but it was never finished, for reasons that are explained below.

Here's a view from the top of the PCB mounted inside the case:

(https://lh3.googleusercontent.com/-vkqZIRI6uHI/VYrMxabgkfI/AAAAAAAAFIY/akBeKUixRIQ/s720/Submini_regen_pocket_AM_03.jpg?gl=US)

I didn't sit down with pencil and paper, or with circuit-design software. Instead I just plowed in and did the layout "on the fly" by trying all sorts of permutations, then finally soldering the one that was best. Here's a closeup:

(https://lh3.googleusercontent.com/-U1YLxS8EvsM/VYrMxmBFxGI/AAAAAAAAFIg/xPNjhfFEUtg/s576/Submini_regen_pocket_AM_04.jpg?gl=US)

I built the set "backwards" by starting at the audio output stage. That's the tube on the left, in the picture, and the components are below the tube. I tested the amplifier and it worked extremely well: in fact, much better than any of my breadboard mock-ups. There was a lot more gain into 8Ω headphones and no sign of hum or audible distortion. I have no idea why the performance was so good, but it's probably (a) very short signal paths and (b) the use of a new output transformer (20KΩ primary) which was demonstrably "louder." I compared it with the old 10KΩ transformer and the AC output voltage for a given signal generator tone was about 25% larger, which would imply about 50 percent more power since it's proportional to E-squared.

At this point I should have just "called it a day" while the going was good. It was 11pm and I was tired. But I was so pleased with the great progress so far ... I forged ahead and tried to build the detector stage (the tube on the right of the picture). This was much harder, in fact, because of all the "inter-weaved" signal paths in a regenerative set. I had to start adding all sorts of jumper wires on the underside of the PCB. In the end I gave up, after pretty much ruining the PCB with too many impromptu changes and only wiring about half of the detector:

(https://lh3.googleusercontent.com/-P5vdSwsOBVo/VYrMyO1v-_I/AAAAAAAAFIk/Zci9WBGn1CM/s576/Submini_regen_pocket_AM_05.jpg?gl=US)

That's an embarrassing photo! I'm going to start again, "from square one," a few weeks from now.

Miniaturization is hard! The layout of the larger components -- tubes, transformer, variable capacitor and regeneration control -- is pretty much dictated by the need to cram them into a case. Physical circuit design is tricky. I reckon the difficulty increases in proportion to the square of the number of components!

I think I've also passed the limit of what can be done with "generic" PCBs. Using such boards you can have either complex circuits, or miniaturization, but not both. It's time to learn how to design custom PCBs and etch them myself.

I'm away on vacation (and taking a perhaps-needed break from radio) for the next three weeks.

73 de Martin, KB1WSY



Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: WB6BYU on June 24, 2015, 11:31:27 AM
Often variable capacitors will contain 4 variable capacitors:  two as tuning capacitors, and two trimmers that set the
minimum capacitance.  If so, there would be a couple scewdriver adjustments on the back, and setting those for
minimum capacitance may give you more tuning range.


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: W1BR on June 25, 2015, 10:40:21 AM
Quite true any stray or parasitic capacitance will have a large influence on the upper frequency tuning range.

Pete


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on March 31, 2016, 08:34:04 PM
So nine months later I'm slowly getting back into radio, after a frantic period both professionally and personally, during which hobbies had to take a back seat.

In June of last year I had abandoned a project for a "shirt-pocket" two-tube AM radio. Today I finally finished the prototype.

It uses two subminiature 6418 directly heated pentodes (regenerative detector, and audio amplifier). The power supply is a AAA 1.25V battery (rechargeable NiMh) for the filaments, and two 12-volt type 23A batteries in series for a 24V B+ (these are small batteries often used in garage-door remote controls).

In the interim, Google has crippled Picasa (as far as new Web postings are concerned) and I haven't found any way to get an easily embeddable direct photo link from the replacement services (Google+ Photos). So to see the results of my project you'll have to click on the following links to see the photos. (After clicking on the link, you can single-click on the displayed photo to get a large version.)

An overall view of the innards:
https://goo.gl/photos/vqWFqTet8zzsm9LV6 (https://goo.gl/photos/vqWFqTet8zzsm9LV6)

A closeup of "the works": regenerative detector on the right, audio amplifier on the left, with output transformer for modern low-impedance headphones; 2-inch-long ferrite antenna at bottom, with tickler loop at the right-hand end for regeneration.
https://goo.gl/photos/wyuocMTdqxACyQxt6 (https://goo.gl/photos/wyuocMTdqxACyQxt6)

Overall view with headphones; the set fits easily into a shirt pocket (the Hammond enclosure is about 3.5" x 2.5" x 0.85" and the set weighs about 145g or 5.1oz including batteries).
https://goo.gl/photos/ieapwReXBvStpcrq7 (https://goo.gl/photos/ieapwReXBvStpcrq7)

The front panel, showing headphone socket, regeneration control at top-left, and tuning control at bottom (the empty cutout at top-right is for a future band switch for the European Long-Wave band):
https://goo.gl/photos/vzxtyHjmdcQPKzseA (https://goo.gl/photos/vzxtyHjmdcQPKzseA)

The power switch: recessed so that it cannot accidentally power up when packed in a suitcase.
https://goo.gl/photos/7HnshCtKh6JSiHZ8A (https://goo.gl/photos/7HnshCtKh6JSiHZ8A)

This is all pretty cool, even if it doesn't have much to do with ham radio at this stage. Of course the ultimate aim is to build travel-size ham rigs using mainly subminiature tubes!!

The downside, which I fully expected, is that this little radio has very low sensitivity. In the Boston area where I live, it only picks up about three mega-loud AM stations (Bloomberg radio, a Hispanic Christian station, and WBZ-news radio). I put this down mainly to the small size of the ferrite antenna. The "desktop" version of the same set pulls in dozens of stations, and the only difference is that it uses a 6-inch-long ferrite rod and a correspondingly wider and larger-diameter Litz coil (the shirt-pocket ferrite rod is only 1.75 inches long).

Late at night, a strong-signal "classics from the 1940s" station appeared on the dial; "Styles On Your Dial" at 1330kHz (WRCA). Very nice, and in period with the vintage technology!

I view this little radio mainly as a "proof of concept." (1) Amazing what you can do with tubes! (2) In due course I'll be replacing the ratty breadboard construction with a custom-designed PCB. (3) I need to play around with the tickler coil, number of turns and positioning. (4) It would theoretically be possible to use a larger-diameter ferrite rod (or perhaps a longer one if positioned along the long edge of the case, although this would mean it would be vertical when in a shirt pocket, which is not necessarily optimal). (5) Ultimately the main point is to try to do something ham-relevant, but even without the ham aspect this has been a lot of fun.

I'm off to Europe next week and will take this baby set with me. Wish me luck at the security checks!

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KK4MRN on April 01, 2016, 07:38:25 AM
So nine months later I'm slowly getting back into radio, after a frantic period both professionally and personally, during which hobbies had to take a back seat.
...
This is all pretty cool, even if it doesn't have much to do with ham radio at this stage. Of course the ultimate aim is to build travel-size ham rigs using mainly subminiature tubes!!
...

It is fine to build radios that are not for ham radio.  I have built AM broadcast radios and a FM broadcast radio.  They are not ham, but they sure were fun to build and play with!

Glad to see you're back into building radios again. 

Maybe off-topic for this forum, but does this mean you may start code practice again? 

I have run into old-timers who have vintage rigs they homebrewed themselves decades ago.  Happened on 10 meters on the AM portion of the band one afternoon.  He was looking for another ham who had homebrewed vintage gear too or a CW rookie. 


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on April 01, 2016, 07:59:26 AM
Maybe off-topic for this forum, but does this mean you may start code practice again?  

I got back into ham radio more than four years ago now, but unfortunately the past few years have been terrible in terms of finding time for hobbies. I have given up predicting when this will improve ... I will say only that I very much want to get back to code practice, putting up a proper antenna, and finally getting on the air. It will happen, in its own sweet time!

This little AM radio project was interesting. I had abandoned the thing last summer, thinking it was just too hard and I didn't have the time. When I got back to it yesterday afternoon, I quickly found three or four obvious bad wiring decisions and a whole lot of bad solder joints. Once those things were fixed, it only took about three hours to complete building the detector stage (the audio stage was already finished). Literally an afternoon's work.

Sometimes it's good to take a break and come back to something "fresh."

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on April 02, 2016, 04:06:31 PM
This afternoon I made a couple of changes to the shirt-pocket AM radio.

I replaced the ferrite antenna with a new one. The rod is a larger diameter (the maximum that will fit in the space available) and the coil is bigger (larger diameter and more turns). Result: much higher L. So I also adjusted the C by taking out some of the gangs on the small variable capacitor. I now have L = about 570µH and C variable between about 25pF and 100pF, or at least that's what my calculations showed.

The upside:
--The radio now tunes from 630kHz to 1300kHz -- that's not the whole AM band but it's a lot more than what I had before.
--Instead of pulling in only 2 or 3 stations, I can now hear more than a dozen.

The downside:
--Tuning has become more fiddly, requiring careful adjustment of the variable capacitor and the regeneration control. Also, there is now a relationship between the two controls: if you change the regeneration, the tuning shifts (this wasn't happening before). Of course this is normal for a regenerative radio, it's just something you have to learn to cope with.

This was fun to play around with!

Next: I will try to add a bandswitch for the European Long-Wave band before leaving for Europe early next week. This will be fairly crude -- basically just switching in the extra gangs on the variable capacitor. This means I will be able to tune the LW band but only a portion of it.

Edited later to add: I may need to find a way to boost the AF gain. I'm hearing a lot of stations, with good S/N ratio and good overall audio quality, but they are very quiet!

Edited again later to add: I tweaked the ferrite antenna by removing some turns from the coil, to shift upward the radio's range, which is now 700kHz to 1450kHz. This encompasses considerably more stations here in Boston (they tend to be concentrated at the top end of the AM frequencies). In the process I discovered why this particular ferrite coil has such high L compared to others I have tested. It actually uses a pair of twisted Litz wires in parallel, with a metal rod along the underside of the paper coil-form cylinder which is used as some kind of conductor. I haven't completely figured out how it's done, but it does achieve 500µH to 600µH in a width of 1.5 inches.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on April 04, 2016, 06:39:58 PM
I added a band switch for European Long-Wave. What it does is switch in extra gangs in the variable capacitor (with the LW band selected, the variable cap tops out at 280pF instead of 100pF). The switching also adds a fixed capacitor in parallel with the variable cap. My original calculations showed that the fixed capacitor should be about 2400pF but this appeared to be way off when tried in practice (the set was tuning much too high). After some trial and error I settled on a fixed capacitor of 4700pF.

According to tests using my RF signal generator, the tuning range on LW is 150kHz to 240kHz, which is just about perfect for my favorite European LW stations, France Inter on 162KHz and BBC Radio 4 on 198KHz (there is also the French service of Radio Luxembourg on 232KHz).

However I am very suspicious of this result because it is so off-kilter with my calculations (both in terms of absolute frequencies, and in terms of the width of the tuning range) -- I think I am probably hearing harmonics rather than fundamentals. The acid test will be my trip to Europe, which starts Wednesday. Either I will hear LW stations, or I will hear nothing at all!

Sadly, Radio France plans to switch off the historic Allouis LW transmitter at the end of this year, so this may be my last chance to hear France Inter in its LW incantation (unless the online petition campaign to keep it going succeeds).

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on April 15, 2016, 04:31:02 AM
Just back from a nine-day trip to Europe. I tried out my radio in Germany (central Frankfurt-am-Main, and the surrounding towns); and in France (central Paris, and the suburbs). It didn't work at all: no stations could be heard on Medium-Wave (U.S. "AM broadcast") or Long-Wave. I was convinced that a wire must have worked its way loose, but when I borrowed some tools from my brother in Paris I couldn't find any obvious problems with the set.

Surprise: when I got back home (near Boston, USA), the set worked fine! It is pulling in more than a dozen stations with relative ease. So I can only assume that American AM-BC stations are extra-powerful, at least in my area. Or alternatively that European broadcasters have really dialled down the output of their transmitters, now that almost everyone listens on FM or the Internet (and in some countries, on digital over-the-air radio).

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1GMX on April 15, 2016, 11:33:42 AM
For VLF use a much larger loopstick antenna.

Its both power and proximity to only a point, wavelength
is a huge factor.

Your basic problem is finite gain and an antenna that's
very very small for that wavelength.  For example for 1mhz
that's 300M band and for 162khz its about 1850M.  So
electrically that antenna went from minute to microscopic.

Allison


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on April 15, 2016, 02:50:28 PM
...use a much larger loopstick antenna.

Makes sense. If this is an absolute constraint ... then the "shirt-pocket" format only works in areas with very strong AM stations. This radio works great here in Boston, and it worked well in Manhattan when I was there a few weeks ago. ("1010 WINS-News, The News Never Stops" and about a half dozen other stations.)

Your basic problem is finite gain and an antenna that's very very small for that wavelength.  For example for 1mhz that's 300M band and for 162khz its about 1850M.  So electrically that antenna went from minute to microscopic.

I may muck around a bit more with the ferrite antenna: winding my own Litz coil instead of using the commercial component I have been using. Perhaps trying to squeeze a larger-diameter ferrite rod into the case. But you are right, this is probably just tilting at windmills. The upside is that my tiny two-tube radio actually works in Boston and New York!

The next step is to learn how to use PCB-layout software (probably Eagle) and build a "production" version of this radio, rather than the crude perfboard prototype I have now. I don't really care about its very low sensitivity, I'm just excited that I was able to build a 2-tube radio that actually works and fits into such a small format.

From there, to building a shirt-pocket 40-meter CW receiver, will be easy -- although in that case I will be assuming usage of an external antenna such as a straightforward dipole.

Allison: great to hear from you. One of these days I will actually get on the air. Meanwhile I'm having fun with these rather limited experiments.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1GMX on April 16, 2016, 05:14:46 PM
one of the things that can hurt regen gain is LC ratio.  When you went to VLF you upped the C
and didn't increase the L.  Hence my earlier comment you need a bigger loop.  More turns
alone may help, but more ferrite and a higher mu ferrite could both help.  Your test signals
can be NDB (non directional beacons) still in operation in the region and generally low power
and in the 200-400khz range.

Allison


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on April 17, 2016, 09:57:41 AM
one of the things that can hurt regen gain is LC ratio.  When you went to VLF you upped the C and didn't increase the L.  Hence my earlier comment you need a bigger loop.  More turns alone may help, but more ferrite and a higher mu ferrite could both help.

Good point. Even on Medium Wave (530KHz-1710KHz) I had lowered the L somewhat because the small variable capacitor didn't have a low enough minimum C to bring in the top of the band (and in fact it still tops out at around 1400KHz). Another noticeable factor is that the set seems to be much more sensitive at the top end of the band than at the bottom; at least that is how it appears when tested with my signal generator, and from the fact that the bottom of the band is "dead" even though I know there are stations there.

Today I researched the stations that come in strongly on this radio, from my location in Chestnut Hill, MA. The strongest is WXKS (Bloomberg radio, 1200KHz) which broadcasts at the legal limit of 50KW and turns out to be only a few miles from my home. The second-strongest is WRCA (1330KHz) which is also very close to my QTH and pumps out 25KW. When I was in New York City earlier this month, I got very good reception of "1010 WINS, the newswatch never stops" which transmits from near the Meadowlands at 50KW.

(For VLF) ...Your test signals can be NDB (non directional beacons) still in operation in the region and generally low power and in the 200-400khz range.
Allison

Good idea. For Long-Wave (153–279KHz) I should probably switch in an extra winding on the ferrite. I also need to figure out some tricks to get higher L on the ferrite. The store-bought Litz coil on my existing ferrite uses a trick that seems to double the L for a given coil length, apparently by using a twisted pair. But I have read that it is not a good idea to use a multilayer coil on a ferrite antenna. Anyway I will experiment both with the size of the ferrite and the quality/mu of the ferrite. The length is already maxed out; I might be able to squeeze in a larger diameter but only by shrinking the component space, which would require a carefully designed new PCB.

As I said, there is a sharp dropoff in sensitivity even at the bottom end of the MW/AM band, so it's not surprising that LW reception was dead in Europe.

Edited to add: if anything, this experience has encouraged me to try building a small superhet, although getting it into the same "cigarette packet" form factor with subminiature tubes is going to be tough -- it will probably requie using up some of the battery compartment (there is a small amount of free space above the batteries, so I could raise them onto a shelf and put components underneath).

OTOH, I am not sure that a superhet would have greater sensitivity....

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1GMX on April 17, 2016, 10:50:21 AM
Do a hybrid.  Down converter to a regen IF at ~300-400khz using one tube
maybe two for gain.  The big issue is still antenna.  A longer case can allow a
longer loop and better capture.

The problem with multilayer coils is self resonance.  If it falls net the desire
range its a Q killer and leads to reduced performance.  A longer coil with
taps would be better but only to a point (self resonance again).

Allison


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on April 17, 2016, 11:53:26 AM
Do a hybrid.  Down converter to a regen IF at ~300-400khz using one tube
maybe two for gain.  The big issue is still antenna.  A longer case can allow a
longer loop and better capture.

That sounds worth a try. Another possibility is this 3-tube reflex superhet (which admittedly uses a semiconductor diode as the detector). A Japanese gentleman posted his circuit on the internet and said it could be "used freely" so I am posting it here:

(http://www.hi-ho.ne.jp/ux-45/schmatic/GACHAPON.GIF)

This design uses relatively high-power (and relatively high B+) levels for a subminiature tube because he used 5678 tubes, apparently because that's what he had on hand. It should work with lower-power/lower-voltage tubes also, I assume.

He has several other designs for tiny radios using subminiature tubes (click on the English-language links for more details):

http://www.hi-ho.ne.jp/ux-45/index.htm (http://www.hi-ho.ne.jp/ux-45/index.htm)

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: WA2ISE on April 24, 2016, 03:53:50 PM
I did a superhet AM radio using sub mini tubes
(http://www.wa2ise.com/subminiimages/smadia.jpg)
Details at my page http://pw2.netcom.com/~wa2ise/radios/AA5submini.html (http://pw2.netcom.com/~wa2ise/radios/AA5submini.html)


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on April 26, 2016, 03:23:54 PM
I did a superhet AM radio using sub mini tubes

Thanks! That is cool ... I am going to be experimenting with various options.

Space is a major issue. The "shirt-pocket" Hammond box I am using makes it hard to use more than 2 tubes and (probably more importantly) greatly limits the size of the ferrite antenna. I have purchased some somewhat larger Hammond boxes (still in the "shirt-pocket" category but larger) which will allow more tubes and a larger antenna.

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on April 26, 2016, 03:46:21 PM
Yet another topic to add to this thread. Those among you who have visited broadcast AM recently may have found that it is a bit of a desert island, in terms of what is nowadays called "content." So within my household, I have rigged up a 60-year-old Eico 315 RF signal generator and modulated it, on the AM/MW radio band, with signals from Internet radio stations. This enables me to listen, on AM/MW radio, to any station that streams on the Internet -- by rigging the audio output of a PC to the Eico signal generator. My current favorite is London-based "Classic FM" (classical music) but my musical tastes are eclectic!

The downside is that the Eico 315's output is very low. To get "reception" on my shirt-pocket radio, I have to run a wire from the 315 to an inductive link on the shirt-pocket radio (a few coils of wire around the shirt-pocket radio).

So I would like to build a hollow-state "FCC Part 15" radio transmitter (https://transition.fcc.gov/Bureaus/Engineering_Technology/Documents/bulletins/oet63/oet63rev.pdf (https://transition.fcc.gov/Bureaus/Engineering_Technology/Documents/bulletins/oet63/oet63rev.pdf)). These are very-low-power, un-licensed, transmitters for very local use. Seems pretty straightforward to me. It just needs to be a bit higher powered than my Eico signal generator, and have the best possible audio quality in its AM modulator. I think this would make it possible to listen to any of these "streamed" stations anywhere in the house, using my hollow-state shirt-pocket radio.

Eons ago there was the Knighkit modulator but I want to do better: the best possible audio quality in an FCC Part 15 AM/BC transmitter (but hollow-state). Does not have to be with subminiature tubes!

Suggestions?

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: WA2ISE on April 27, 2016, 11:06:50 AM
For micro-AM-transmitting, I've built small transmitters to provide listenable material for my AM radios.  http://www.wa2ise.com/radios/amxmit.html (http://www.wa2ise.com/radios/amxmit.html)
(http://www.wa2ise.com/amtxmit.jpg)
(http://www.wa2ise.com/radios/amxmitch.jpg)


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on May 03, 2016, 11:51:23 AM
For micro-AM-transmitting, I've built small transmitters to provide listenable material for my AM radios. 

Thanks Robert, that's useful. Lots of fun stuff on your website....

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: THOMASHA on April 22, 2017, 03:39:37 AM
Hi,
nice thread, I'm a little late here, but I built some amplifiers with subminiature tubes too, but always used bulkier transformers for the OT.

May I ask where did you found that tiny one? And is it around 20k? 

I'm trying to build a portable radio using some russian subminiatures and a 5672 tube but can't find a small OT for it.

There is some material in a japanese site, where they adapt a sansui 50k audio transformer, but I think that this kind of transformer is only available in japan, since I couldn't find an equivalent. Please correct me if I'm wrong.

as for the radio I'm building, I'm starting from this design from a German website>
(http://www.jogis-roehrenbude.de/Bastelschule/0-V-3-KW-Audion-Bausatz/Schaltung.jpg)

Thank you!

Thomas




Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1GMX on April 22, 2017, 12:02:49 PM
There are some great ideas there....

My favorite is the Mountaineer a tube battery transceiver but the receiver alone could be easily done.

See QST September 1950 page 17.
I have a electronic copy but see ARRL website if you are member and look it up.

The basic radio was 1R5 convert from 80m to about 500khz, 1T4 regen detector, 1T4 audio.
The if was tuneable and the converter was crystal controlled in the article.  I made the
converter tunable and the If fixed and it was equally good and the regen was easily
set an forget for most of the battery life.

You can substitute 1AD5 for the 1t4 easily.  you might do a self oscillating converter
with 1AD4 as well.  With three of those the heater current at 1.5V is about 300ma
for three 1ad4s compared to 150ma at 1.5V for 1r5/1t4/1t4 mix.  The difference is size.

The Gachapon design a few postings back can have a regen IF as detector and be even
more selective though for AM BC high selectivity is not needed. The Same japnese
fellow did several others worth looking at too.

The wa2ISE pages are good fun too and have some neat ideas that worked well
when I tried them on a oddball am chassis.


Allison


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: KB1WSY on April 22, 2017, 05:08:44 PM
May I ask where did you found that tiny one? And is it around 20k?

I believe it was a Hammond 144S. Yes, it is a 20K primary (there is also a 10K model, the 144Q). Power rating 50mW. About $20 from Mouser. The relatively high price is probably because these are aimed mainly at audiophiles. There is a data sheet here: http://www.mouser.com/ds/2/177/5c0045-46-77704.pdf (http://www.mouser.com/ds/2/177/5c0045-46-77704.pdf).

73 de Martin, KB1WSY


Title: RE: Designing & Building a High-Peformance Subminiature-Tube Regenerative Receiver
Post by: THOMASHA on April 23, 2017, 01:48:01 AM
Thanks!

never saw these small hammond transformers, I was looking at the TM42 and TL42 series all the time.