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Author Topic: My First Superhet Build -- Planning  (Read 1024 times)

G3EDM

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Re: My First Superhet Build -- Planning
« Reply #15 on: June 30, 2021, 05:19:07 AM »

If you want a simple, authentic valve/tube CW receiver, well described, you could do worse than look at this one:

https://www.frostburg.edu/personal/latta/ee/6x2rcvr/6x2mechanicalconstruction/6x2mechanicalconstruction.html

[snip]

73, Peter.

After trawling through the ARRL books from that period, I have found pretty much all of the bits and pieces that make up the above-mentioned set. The builder does mention that he pieced it together from ARRL designs. The good news is that I've been stocking up parts for several of those designs, the simplest of which uses just three tubes: the so-called "Simplex" superhet. The key component that I do not yet have is the 1700KHz crystal for the single-crystal filter so I will be on the watch-out for one of those (three crystals within 10KHz of that frequency recently sold on eBay).

I quite like the idea of starting out with something like the Simplex, then progressing through the various designs.

73 de Martin, KB1WSY (soon to be G3EDM)
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KB1GMX

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Re: My First Superhet Build -- Planning
« Reply #16 on: June 30, 2021, 07:08:06 AM »

Kb1WSY,

Generally adapting modern filters to older designs is not a bad
way to go.  Selectivity if often the most desired feature and
coils and caps (tuned circuits) unless abundantly used with
many stages cannot easily deliver that.

The filter would perform well for CW but compared to the simple filter
it will have much higher insertion loss and also the circuit details to
match it to the Mizers Dream will result in some loss of RX gain as
well.  FYI been there did that using Heath SSB filter 3395khz and
they want about 4000ohms in  and out.  Once done and working I
found a RF prelector added made it a much better.

Generally,7360 is a great tube... if you can find one they are usually
extremely expensive, or not useable (used tired).  They are also
sensitive to magnetic fields so you need a shielded socket with
matching tube shield.  Also placed well away from power and
audio output transformers.  It is possible to use a 6AR8 (similar
but not identical beam deflection tube) with same caveats and
performance.

The was another circuit proposed the 6x2... easier to get parts and
same filter issue and lack of RF amp.  That is a common design with
marginally different variants all about same.  That being Mixer at the
input with some amount of tuned circuits before it.  One stage of IF rarely
two with preceeding single or half lattice filter. Detector is a BFO and
gridleak triode as a poor mans product detector (also easily overloaded)
and two stages of audio gain.  The LO is rarely buffered so tuning the
mixer input pulls the VFO.  so total tube cunt is low and its basic
superhet but generally a lot of work and parts hunting for a soso
receiver.

The crystal filter frequency is not critical, usually the author of the
circuit/article used what was on hand or available.  I found over
the last 50 years plus I could rarely find crystals called out or
had to pay over an hours pay (at that moment in time) for
surplus or commercial ground crystals.  Changing the IF means
only changing the VFO frequency and dial calibration is always
done after finished.

Radio designs that call for "Command set" crystals usually in the
400-500khz range with channel numbers like 44 and 45 are
impossible to find and those I've found would not even oscillate.
They were the abundant surplus item over 60 years ago but
now forget them.

FYI using microprocessor crystals you can make a very good
ladder filter with typically 4 to 8 of them matched to within
100HZ (SSB) or 50HZ (CW).  The design is easily found
on the internet.  Note using lower frequency crystals like
3.575 mhz is good for CW bandwidth and higher 1000 to
4000 ohms termination impedances.  For lower impedance
and SSB band width use 9mhz.  While tis is common there
are people doing that with tubes as well using transformers
at the ends of the filter to match to the tubes,

Popular IFs:
The common 455khz is good for anything under about 5mhz
unless great efforts are taken to avoid images (tuned RF tracked
with VFO).  If you can find a mechanical filter for that range first
test it, I had a few all bad. Ther eare ceramic filters most are
AM band width, CFU455J if found and good is SSB.  The next
one is in the 1600 to 1750 khz range and that may have issues
with broadcast AM getting into the IF.  Also at that range a crystal
 filter is a must for adequate SSB/CW selectivity.  Then the HF
IFs 3395, 5500khz and most famous 9mhz.  HIgher IF with
crystal filter offer good selectivity and freedom from Images.
The usual cost for the higher IFs is need for two stages of IF
gain but that also allows for AGC.

Upside of 9mhz is it was common to do dual band 80 and 20 M
with the same 5mhz VFO.  MCoy made filters for ham use
at 9mhz but not cheap.  It was also done for 40 and 80 using
1750khz IF and crystal filters but good luck finding suitable
crystals or someone that can supply custom ground rocks.

Generally tube radios without tuned RF amp before the mixer will
be only marginal performers for overload and images. The tube
is not so much for gain but to add RF selectivity before the mixer
and can have AGC applied.  Also any design without AGC is
going to hurt your ears.  If they have only one IF stage they are
going to be marginally sensitive.

As a matter of commentary and experience to add one or more
tubes to the total count is not as big a deal as transistors and can
mean adding the RF stage and some form of dec ent AGC.  The
general rule in the tube days is the quality and capability of the
receiver was related to the tube count (if multisection tubes then
the total number of tube sections used) and a count of less than
8 was considered to be a weak minimalist design.  FYI when you
get to about 15 it was usually a very high performing radio.
Those numbers do not include the power rectifier.

There was the HBR series, versions of that went from 8 to
over 15 tubes.  There are other designs on the net most
Handbook/QST designs and generally they fall to rehash of
similar design with maybe different tubes and less often a
good design using a modest number of tubes and relying
on mechanical or crystal filters.

Generally the area of greatest effort will be the VFO, stability
both mechanical and thermal will take effort.  Use a very good
dual bearing capacitor for that of the highest mechanical quality
you can find.  If old/recycled insure the bearings are good and
properly lubed and clean.

Second to that is mechanical layout for all the other stages as
packing them close creates problems with heat and also
unwanted coupling.

Then power supply and transformer.  Please no line powered
hot chassis radios.  Allied is a possible source.  Also designs
that only need 120 to 160V are less painful if you get across
them.  Believe it or not 120V DC is more than enough for a
good tube design (see Drake 2b and 2C).

If you want to experience marginal performance at 40M find a
hallicrafters SX120 or the older SX38.  At 5 tubes its not going
to dazzle.

Also step drills are really handy for building tube gear as they can
drill a clean hole large enough for most sockets. Those step drills are
hardware and home depot items and available from many sizes
starting from about 1/8" up to over 1"   They are also cheaper
than the traditional Greenlee punches.


Allison
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G3EDM

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Re: My First Superhet Build -- Planning
« Reply #17 on: June 30, 2021, 11:45:17 AM »

Kb1WSY,

Generally adapting modern filters to older designs is not a bad
way to go (etc.)

Allison

Hi Allison, thank you so much for the detailed advice. Will take me a while to digest!

I have just bought a NATO surplus CW filter.

Seller's description (edited down) is: "CW ... quartz filters of the English company Cathodeon.... The filters are intended for use in professional radiocommunication equipment.... They are made in the McCoy arrangement.... The center frequency of the filters is 1.75MHz.... The frequency response of the CW filter is 400Hz."

Compared to a lot of other offers on the watery auction site, this was reasonably priced and perhaps promising. It should enable me to experiment with the various ARRL designs that are based on a 1700KHz IF, as well as the one mentioned earlier by VK6HP. It only requires a modest shift in IF, and small changes in the winding of the inductors or their associated capacitors.

Trouble is though, 1.75MHz is a perfect divisor of the bottom of each of the original ham HF bands. Is that going to cause a problem by definition? Figured that out a couple of minutes after clicking "buy"....

73 de Martin, G3EDM (UK licence now issued)
« Last Edit: June 30, 2021, 11:55:45 AM by KB1WSY »
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KM1H

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Re: My First Superhet Build -- Planning
« Reply #18 on: June 30, 2021, 02:08:24 PM »

Quote
Then power supply and transformer.  Please no line powered
hot chassis radios.  Allied is a possible source.  Also designs
that only need 120 to 160V are less painful if you get across
them.  Believe it or not 120V DC is more than enough for a
good tube design (see Drake 2b and 2C).

Excellent post Allison and a few short comments follow.

James Millen, W1HRX, of National Radio fame wrote about running the HRO on a battery supply in a 1930's era QST. It only provided 130V but he discovered the radios sensitivity was improved. His suggestion was if you didnt need a speaker use headphones and 130V.  Ill let the sites resident troll to come up with the details.

The AC-DC radios such as the NC-33 were known for good sensitivity and the NC-300 ran at a much reduced B+ than most others and is still considered one of the best ever tube radios. I use mine mostly on 10M AM with a one owner Collins 32V2 and it blows away the matching one owner 75A3 receiver that came with it....even after a thorough overhaul.

Quote
Generally,7360 is a great tube... if you can find one they are usually
extremely expensive, or not useable (used tired).  They are also
sensitive to magnetic fields so you need a shielded socket with
matching tube shield.  Also placed well away from power and
audio output transformers.  It is possible to use a 6AR8 (similar
but not identical beam deflection tube) with same caveats and
performance.

The 6AR8 had a short sales life due to instability and microphonics and as a result the market is still loaded with them. The pin compatible 6JH8 is the excellent substitute that quickly replaced it.
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N2EY

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Re: My First Superhet Build -- Planning
« Reply #19 on: June 30, 2021, 03:27:51 PM »


I have just bought a NATO surplus CW filter.

Seller's description (edited down) is: "CW ... quartz filters of the English company Cathodeon.... The filters are intended for use in professional radiocommunication equipment.... They are made in the McCoy arrangement.... The center frequency of the filters is 1.75MHz.... The frequency response of the CW filter is 400Hz."

Compared to a lot of other offers on the watery auction site, this was reasonably priced and perhaps promising. It should enable me to experiment with the various ARRL designs that are based on a 1700KHz IF, as well as the one mentioned earlier by VK6HP. It only requires a modest shift in IF, and small changes in the winding of the inductors or their associated capacitors.

Trouble is though, 1.75MHz is a perfect divisor of the bottom of each of the original ham HF bands. Is that going to cause a problem by definition? Figured that out a couple of minutes after clicking "buy"....


Martin,

I don't think you will have a problem. In fact I want one of those filters!

That said, here are some things to consider:

1) Having the filter at 1750 kHz means the BFO will be at about that frequency, and you WILL hear the harmonics. This isn't necessarily a problem because you probably won't be pushing the lower band edge that closely.

The trick is to set the BFO slightly BELOW the IF - say, 1749.250 kHz. That puts the second harmonic at 3498.5 kHz (1500 Hz below the low end of 80) and the fourth harmonic at 6997.0 kHz (3000 Hz below the low end of 40).

I know this from experience. The Southgate Type 7 uses an IF of 1400 kHz, and the BFO is at 1397.250 kHz. The fifth harmonic of the BFO is at 6996.250, and causes no problem - but it IS there on 40 meters. The tenth harmonic can be heard very faintly on 20 meters but is so far away that it's not an issue.

2) To use the Cathodeon filter (or any such) to best advantage, you need to know its characteristics - input/output impedance and loss. Small errors in impedance matching don't matter much, but big errors result in odd filter responses and excessive loss.

In my homebrew rigs using packaged filters, I've been lucky to have the filter characteristics, so I could design the circuit to match what the filter wanted. The matching circuit I've had great success with is a simple parallel-tuned circuit using a powdered-iron toroid, with a link winding to transform the high impedance of the tuned winding to the low impedance that the filter wants. A trimmer capacitor permits tuning for a match.

3) The 1750 kHz IF is perfect for a band-imaging 80/40 receiver - you've probably seen several designs for such receivers. With a 1750 kHz IF and a local oscillator tuning 5250 to 5400 kHz, the receiver will tune 3500 to 3650 kHz and 7000 to 7150 kHz, which should be more than adequate for CW operation.

However, there's a catch.....

The weakness of band-imaging receivers is that the only thing that prevents you from hearing signals from the "wrong" band is the front-end selectivity. The tuned circuits before the mixer must pass the band you want and reject the band you don't, otherwise you'll hear strong 80 meter signals when tuning 40 and strong 40 meter signals when tuning 80. This is why many of the band-imaging designs have a double-tuned input circuit.

4) There were several ARRL receiver designs of the 1960s that used the 7360. They work well, but the 7360 has become rather scarce and high priced. A suitable replacement is the 6JH8, which shares none of the problems of the 6AR8. There's also the 6ME8, which is somewhat less common. Note that these tubes have different pinouts and are not directly interchangeable.

IMHO, for a first superhet, a better choice is a Pullen mixer. It is simpler to implement and can use common dual triodes. For some reason the ARRL never gave it any ink, but the RSGB Handbooks of the 1960s did. You can always start with the Pullen mixer and then try a beam-deflection mixer if you so desire.

I can send you several articles on the Pullen mixer if you are interested.

More to follow....

73 de Jim, N2EY

 
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N2EY

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Re: My First Superhet Build -- Planning
« Reply #20 on: June 30, 2021, 03:44:43 PM »


After trawling through the ARRL books from that period, I have found pretty much all of the bits and pieces that make up the above-mentioned set. The builder does mention that he pieced it together from ARRL designs. The good news is that I've been stocking up parts for several of those designs, the simplest of which uses just three tubes: the so-called "Simplex" superhet. The key component that I do not yet have is the 1700KHz crystal for the single-crystal filter so I will be on the watch-out for one of those (three crystals within 10KHz of that frequency recently sold on eBay).

I quite like the idea of starting out with something like the Simplex, then progressing through the various designs.


That design is a good starting point. A couple of comments:

1) The builder made the receiver rather compact - there isn't a lot of extra space. This looks nice, but makes experimentation difficult if not impossible. For example, suppose you want to add an IF or RF stage - there's no place to put it. Or if you decide to replace the 6EA8 oscillator-mixer setup with separate oscillator and mixer tubes - again, there's no place to go. If a larger chassis were used, with lots of extra "real estate", such changes would be easy. Extra space is also a good idea if you are using one-of-a-kind parts.

2) The builder made the entire receiver self-contained - the power supply is built-in. That's fine, but you might want to consider putting the power supply on its own chassis. This approach means a bit more work, but it has advantages:

  - the same power supply can be used for other projects
  - the heat of the power supply is removed from the receiver itself
  - if shack space is limited, the power supply can be tucked away on a high shelf, under the table, etc., rather than using up valuable space on the table.

3) Most published receiver designs use the classic separate-dial-and-variable-capacitor tuning arrangement. Usually the tuning rate - the number of kHz per turn of the knob - is very high. With receivers that aren't very selective, that's OK, but with a sharp filter it's easy to tune right past the weak ones. Also the mechanical construction requirements become rather precise.

An easy solution is to use the integrated capacitor-and-dial setups found in some WW2 military radios. I have used the capacitors from ARC-5 transmitters and BC-221 frequency meters with great success for 50 years. You may have to buy an entire transmitter or frequency meter, but the good news is that parts units that are beyond reasonable restoration can still be found. (Plus you know someone with a supply of them....)

Still more....

73 de Jim, N2EY
   
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G3RZP

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Re: My First Superhet Build -- Planning
« Reply #21 on: June 30, 2021, 04:00:40 PM »

The Pullen mixer is interesting, in that many people enthuse over it, but nobody AFAIK, has published any results on it regarding IMD, NF, gain and compression point, and the optimum voltage values for the various voltage and tube variants. If it is so good, why no published results? The usual amateur "it worked OK for me so why measure it?" approach.

Judging by the RCA data book TT5, the IMD of a 7360 is no better than can be achieved with a 12AU7 - see the 1964 edition of Pappenfus et al. Some noise figures quoted for the 7360 are obviously  assuming it's a straight amplifier - they can't be for a mixer, which would be 6dB worse.

I am reminded yet again of Phineas T.......
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N2EY

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Re: My First Superhet Build -- Planning
« Reply #22 on: June 30, 2021, 04:02:50 PM »

I've gradually been stocking up with parts to build my own 1960s beginner's superhet.

It will be loosely based on the "Miser's Dream", a capable five-tube receiver project in the ARRL Handbook for 1968.

Actually, that's the "Junior Miser's Dream". It was also in the 1967 ARRL Handbook.

The original "Miser's Dream" was in QST for May, 1965, and the schematic can be found here:

http://dpnwritings.nfshost.com/ej/misers_dream/

The article is more about concepts than something that could be directly copied. But it's a good read for the ideas presented. It's more complex than you'd want for a first superhet, though.

Quote
To speed things up and keep it simple, it will initially be single-band only (40m).

Good choice! Or, since you have a 1750 kHz filter, an 80/40 "band imaging" design.

Quote

The set has an IF of 3300kHz and in the original design, uses a two-crystal IF filter at 3300 and 3301 to get 1kHz selectivity. I will only be using the set for CW, so I'd want more like 500Hz.

I wonder if homebrewing OTs can answer the following question. For excellent filtering, I am thinking of using a YG-3395C filter unit originally fitted in Kenwood transceivers. This has a 500Hz passband at an IF of 3395kHz, so I would have to shift the Miser's Dream's IF up by 95kHz, a trivial change presumably.

My question is how to alter the original filter circuit to accommodate the Kenwood filter. I cannot find any technical description of this filter online, but the pinouts look like this:



The original circuit of the Miser's Dream looks like this:



I presume we can eliminate some of the components in the original filter circuit (not just the crystals). The Kenwood filter has just those two pins, plus a presumably grounded case. How do I make it fit in the Miser circuit?

I think I covered this in an earlier post. I'll make some sketches to give a clearer picture.

Quote
Edited to add: Am I correct that if the set is used only for CW, the entire AGC circuit is superfluous?

Consider it an option. I don't use AGC, and my ears are just fine after 54 years of CW operating. Master homebrewer W2LYH (SK - look up his QST articles if you want to see serious quality homebrewing) had no use for AGC or S-meters either, and I think he was right.

The AGC circuit used in that receiver and several others is "audio derived". Audio derived AGC works OK, but has the tendency to not be "fast attack" enough to avoid loud pops when a strong signal comes into the passband with the gain turned up.

If you use a chassis with extra space, you can build the receiver without AGC and then add it later if you desire.

Quote
It will probably be several months before I get going on this project, but would like to get my ducks lined up in advance.

That is an excellent plan. You will probably find that your design goes through several revisions and changes before you begin. That's a much better route than getting halfway through a project and finding a serious problem that requires major changes or a complete restart.

Or as we say at Southgate Radio:

"Paper is cheaper than parts".

73 de Jim, N2EY



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N2EY

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Re: My First Superhet Build -- Planning
« Reply #23 on: June 30, 2021, 04:40:02 PM »

The Pullen mixer is interesting, in that many people enthuse over it, but nobody AFAIK, has published any results on it regarding IMD, NF, gain and compression point, and the optimum voltage values for the various voltage and tube variants. If it is so good, why no published results? The usual amateur "it worked OK for me so why measure it?" approach.

Good points. Here's an hypothesis:

The Pullen mixer didn't get much publicity on this side of the pond until long after the hollow-state era was done. It does not appear in any ARRL publication I know of, and if it were, I'd almost certainly have found it. There was an article or two in CQ and possibly 73, but they were "how to improve your old receiver", where the typical application was to replace a noisy pentagrid mixer with a Pullen using a common dual triode. For example, a WW2 surplus BC-342/312 would probably be greatly improved if its 6L7 were replaced with, say, a 6SN7 Pullen.

Quote
Judging by the RCA data book TT5, the IMD of a 7360 is no better than can be achieved with a 12AU7 - see the 1964 edition of Pappenfus et al. Some noise figures quoted for the 7360 are obviously  assuming it's a straight amplifier - they can't be for a mixer, which would be 6dB worse.

The only manufactured amateur receivers I know of that used the 7360 as a mixer are the Squires-Sanders SS-1R, which has some impressive numbers for its time (but its trick input circuit makes it VERY sensitive to antennas that aren't 50 ohms, and its sensitivity on 15 and 10 meters may leave something to be desired) and the Southgate Type 7, which has a 6EH7 RF stage before the 7360 mixer.

A beam-deflection tube which may be even better than the 7360 is the 6JH8. G4OEP has done some interesting work with it:

http://g4oep.epizy.com/retro/retro.htm

http://g4oep.epizy.com/retro/retro.htm#The%20Retro%20Mk%20II

including a calculation of the conversion gain of beam-deflection mixers.

http://g4oep.epizy.com/retro/conversiongain/conversion_gain.htm

Your comments on the above would be greatly appreciated.

----

The Collins 75A-4 was a top-of-the-line receiver in its time, but for some reason Collins used the 6BA7 for both mixers. Of the pentagrids, the 6BA7 is probably the best, but that's not saying much.

A common mod to the 75A-4 was to replace the 6BA7s with 7360s. There was an article in Electric Radio (issue #47, March 1993) which describes the "W2VCZ mods". W2VCZ (later K4NV) changed out the first mixer to a 6ES8 dual triode Pullen mixer and the second to a 6EA8 pentode. The 6DC6 RF amp was also changed out to a 6GM6. The result was reportedly lower noise AND better "dynamic range".

Then there's  the receiver in QST for February 1972 - "An Experimental Receiver for 75 Meter DX Work", by W1KLK (SK).

This receiver has four gang-tuned circuits in the preselector (two between the antenna and RF amplifier, two between the RF amplifier and mixer) to maximize selectivity at the signal frequency. The RF amplifier is a 7044 dual triode (5687 is similar) run at low gain - its only purpose is to compensate for the losses in the tuned circuits. The mixer is a 7360, followed by a Collins mechanical filter from an R-390A. Since the receiver only covers 75 meters, single conversion and a 455 kc. IF works fine.

Another design which is not too well known on this side of the pond is the G2DAF Mk2, a double-conversion design using a push-pull front end (!). It is described in the RSGB Handbook, but I'm not sure which edition. I do know that it is later than the 4th edition, because the G2DAF receiver in the 4th edition (which I have) is an earlier version.

73 de Jim, N2EY
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G3EDM

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Re: My First Superhet Build -- Planning
« Reply #24 on: July 01, 2021, 12:24:39 AM »

I don't think you will have a problem. In fact I want one of those filters!

It was on the watery auction site. The seller (in Poland) is selling them as bundles of NATO filters, each set is three filters, CW, SSB, AM for about $40 per set. S/he still has nine sets available.... I was tempted to buy more than one set but it is a bit of a stab in the dark without the full specs. Will see what I can dig up, the manufacturer was British so perhaps some industrial archaelogy can be done here.

I am gratified that, as you explained, these are usable despite the ham band-edge harmonic. Strangely enough, with my UK Full Licence I have access to the whole band (like a U.S. Extra) but you are right that I probably won't be spending too much time "down there". I understand your trick of setting the VFO frequency appropriately.

These 80/40 sets where you just swing the bandset from one end to the other are very interesting to me because they eliminate all the faff with band-switching. Then, eventually, a crystal-controlled converter can go in front, for 20m.

I do have a tuning capacitor and its mechanism taken from an LM or BC frequency meter. I also have several Eddystone 898s but you did suggest back then that the LM dial is better. If I use it the only challenge would be to build my own display dial for the frequencies, which is something you've done quite inventively in your various Southgates. These solutions are probably overkill for the low-end superhet that I am planning, and I may just go for one of the old Jackson dials in the junkbox plus a well-mounted, high-quality capacitor.

I was planning to use a relatively loose physical layout, for the reasons you cite, to aid expansion and also just to make building easier. Also, although I will probably use an off the shelf chassis, I would want to stiffen it with an extra top plate plus angle brackets. May follow your suggestion to use an outboard power supply. My new shack is tiny (a single very shallow closet holding the equipment) with a largely vertical layout.

Once again folks, construction of this set is not imminent. I don't have a workshop yet, and my overriding focus is simply Getting On The Air with the simple equipment I have already.

73 de Martin, G3EDM
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K0IZ

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Re: My First Superhet Build -- Planning
« Reply #25 on: July 01, 2021, 06:02:19 AM »

Carl, you beat me to it.  I was going to suggest the 6JH8 rather than 7360.  7360 is hard to locate and expensive.  6JH8 much cheaper.  Also has a nearly perfect linear deflection transfer.  6AR8 is older GE version, I've used it but prefer the 6JH8.  6AR8 and 6JH8 have same pin outs but both are different from 7360.

Here's a link to a decent overview:   https://kj7um.wordpress.com/2020/11/27/a-new-approach-to-receiver-front-end-design-revisited/
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KM1H

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Re: My First Superhet Build -- Planning
« Reply #26 on: July 01, 2021, 09:45:01 AM »

Quote
IMHO, for a first superhet, a better choice is a Pullen mixer. It is simpler to implement and can use common dual triodes. For some reason the ARRL never gave it any ink, but the RSGB Handbooks of the 1960s did. You can always start with the Pullen mixer and then try a beam-deflection mixer if you so desire.

I would not suggest a Pullen on a receiver without a RF stage OR/AND a buffer between the variable HFO and Pullen.
I know this from experience...... having converted several boatanchors over a few decades.
Pulling of the VFO is common and the unterminated front end offers the same problems as the beam deflection tubes.

Quote
The Pullen mixer didn't get much publicity on this side of the pond until long after the hollow-state era was done. It does not appear in any ARRL publication I know of, and if it were, I'd almost certainly have found it. There was an article or two in CQ and possibly 73, but they were "how to improve your old receiver", where the typical application was to replace a noisy pentagrid mixer with a Pullen using a common dual triode. For example, a WW2 surplus BC-342/312 would probably be greatly improved if its 6L7 were replaced with, say, a 6SN7 Pullen.

73 ran a full series on the Pullen including the original publication. Also included was the performance of several suitable dual triodes from octal to both 7 and 9 pin miniatures.  I used the 6SN7 in both a SX-28 and SP-400 as I dont like to hack a radio to change sockets. The 6J6 and 12AT7 are the choices for those sockets while the 6ES8 as the only readily available variable mu dual triode shows a lot of promise as mentioned in one article but Ive no experience yet. The 75A3 is the intended target as it uses the same rather deaf front end as the 75A4.

Quote
The Collins 75A-4 was a top-of-the-line receiver in its time, but for some reason Collins used the 6BA7 for both mixers. Of the pentagrids, the 6BA7 is probably the best, but that's not saying much.

Because RCA promoted them heavily as a great tube for the FM band. National used it in the first mixer of the NC-300/303 and with a 6BZ6 RF stage the 10M sensitivity was to the point that a 6GM6 barely made an improvement on the output meter so was left alone. Running those radios at ~ 150V B+ also was a factor.

Quote
A common mod to the 75A-4 was to replace the 6BA7s with 7360s. There was an article in Electric Radio (issue #47, March 1993) which describes the "W2VCZ mods". W2VCZ (later K4NV) changed out the first mixer to a 6ES8 dual triode Pullen mixer and the second to a 6EA8 pentode. The 6DC6 RF amp was also changed out to a 6GM6. The result was reportedly lower noise AND better "dynamic range".

The 7360 mod to the 75A4 was published in 1964 and was the one I used on mine. I may be one of the first to try the 6GM6 in the same year or in 65 as National Radio R&D was experimenting with them for several new products. I converted my A4 over parts of both years and the 10M noise figure was measured at 6.5dB in later years when a HP 8970B became available at work. Ive had my own 89070A for about 20 years.

Another design which is not too well known on this side of the pond is the G2DAF Mk2, a double-conversion design using a push-pull front end (!). It is described in the RSGB Handbook, but I'm not sure which edition. I do know that it is later than the 4th edition, because the G2DAF receiver in the 4th edition (which I have) is an earlier version.
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KM1H

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Re: My First Superhet Build -- Planning
« Reply #27 on: July 01, 2021, 09:49:30 AM »

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IMHO, for a first superhet, a better choice is a Pullen mixer. It is simpler to implement and can use common dual triodes. For some reason the ARRL never gave it any ink, but the RSGB Handbooks of the 1960s did. You can always start with the Pullen mixer and then try a beam-deflection mixer if you so desire.

I would not suggest a Pullen on a receiver without a RF stage OR/AND a buffer between the variable HFO and Pullen.
I know this from experience...... having converted several boatanchors over a few decades.
Pulling of the VFO is common and the unterminated front end offers the same problems as the beam deflection tubes.

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The Pullen mixer didn't get much publicity on this side of the pond until long after the hollow-state era was done. It does not appear in any ARRL publication I know of, and if it were, I'd almost certainly have found it. There was an article or two in CQ and possibly 73, but they were "how to improve your old receiver", where the typical application was to replace a noisy pentagrid mixer with a Pullen using a common dual triode. For example, a WW2 surplus BC-342/312 would probably be greatly improved if its 6L7 were replaced with, say, a 6SN7 Pullen.

73 ran a full series on the Pullen including the original publication. Also included was the performance of several suitable dual triodes from octal to both 7 and 9 pin miniatures.  I used the 6SN7 in both a SX-28 and SP-400 as I dont like to hack a radio to change sockets. The 6J6 and 12AT7 are the choices for those sockets while the 6ES8 as the only readily available variable mu dual triode shows a lot of promise as mentioned in one article but Ive no experience yet. The 75A3 is the intended target as it uses the same rather deaf front end as the 75A4.

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The Collins 75A-4 was a top-of-the-line receiver in its time, but for some reason Collins used the 6BA7 for both mixers. Of the pentagrids, the 6BA7 is probably the best, but that's not saying much.

Because RCA promoted them heavily as a great tube for the FM band. National used it in the first mixer of the NC-300/303 and with a 6BZ6 RF stage the 10M sensitivity was to the point that a 6GM6 barely made an improvement on the output meter so was left alone. Running those radios at ~ 150V B+ also was a factor.

Quote
A common mod to the 75A-4 was to replace the 6BA7s with 7360s. There was an article in Electric Radio (issue #47, March 1993) which describes the "W2VCZ mods". W2VCZ (later K4NV) changed out the first mixer to a 6ES8 dual triode Pullen mixer and the second to a 6EA8 pentode. The 6DC6 RF amp was also changed out to a 6GM6. The result was reportedly lower noise AND better "dynamic range".

The 7360 mod to the 75A4 was published in 1964 and was the one I used on mine. I may be one of the first to try the 6GM6 in the same year or in 65 as National Radio R&D was experimenting with them for several new products. I converted my A4 over parts of both years and the 10M noise figure was measured at 6.5dB in later years when a HP 8970B became available at work. Ive had my own 89070A for about 20 years.

Another design which is not too well known on this side of the pond is the G2DAF Mk2, a double-conversion design using a push-pull front end (!). It is described in the RSGB Handbook, but I'm not sure which edition. I do know that it is later than the 4th edition, because the G2DAF receiver in the 4th edition (which I have) is an earlier version.

One of the high end European radios used the PP front end and I dont believe G2DAF was involved.

Carl
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G3EDM

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Re: My First Superhet Build -- Planning
« Reply #28 on: July 01, 2021, 02:09:09 PM »

In a stroke of luck, I just obtained a couple of 1700KHz crystals. They are CR-18/U, so there is no possibility to open one of them up and grind it to 1700.5 for a two-crystal design, but never mind for the moment.

The avalanche of advice here has been most gratifying. I have tentatively decided to start small and build something like the "Simplex Super Mark II" from the 1963 ARRL Handbook. It's a 3-tube design with a single-crystal 1700KHz filter. There are no unobtainium parts and I already have a stock of the specified Illumitronic/Miniductor coil stock (and even if I hadn't, there is enough info in the book to reproduce them).

The Simplex uses a 6U8A Mixer/oscillator, another 6UA8A detector/BFO and a 6CG7 audio stage. "Selectivity at the IF is obtained through the use of a single crystal. This, in conjunction with some regeneration provided by the detector, is sharp enough to provide a fair degree of single-signal CW reception." Faint praise, but it's only a three-tuber.

I agree with those who have said above that adding more tubes is not that big a deal. But I would prefer to start small and try to understand in depth how it all works on a component by component basis. I have no formal EE training but a love of learning and experimentation.

Yes I know the performance will be extremely limited but that in itself will be interesting when comparing with the two other receivers I have built already in the past few years, both of them regenerative designs, one of them solid-state (3-transistor ARRL design) and the other one hollow-state (my own design with considerable help from folks here such as G3RZP).

When the building gets under way I will document the project in a thread here, hopefully in the next few months if I can figure out how to set up a small workshop in my current small QTH. (It's a challenge, to put it mildly.)

Once the Simplex is up and running it will be a pleasure to move on to more sophisticated projects.

73 de Martin, KB1WSY G3EDM
« Last Edit: July 01, 2021, 02:11:11 PM by KB1WSY »
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K3UIM

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Re: My First Superhet Build -- Planning
« Reply #29 on: July 01, 2021, 02:19:00 PM »

WSY: "I have no formal EE training but a love of learning and experimentation."

I believe that goes for the bulk of us, whether at the beginning of our finding the hobby, or after years away from it and then returning. It's a special kind of love with no way of explaining it to the average person. (I guess golfers might perhaps understand.) ... sigh ... LOL

Charlie

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Charlie. K3UIM
Where you are: I was!
Where I am: You will be!
So be nice to us old fogies!!
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