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A VFD Receiver (Triodes in Disguise)

Des. Kostryca (M0AYF) on January 22, 2009
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A VFD (Voltage Fluorescent Device) Regenerative Receiver built in "retro" 1930's style.

Thumbnail of VFD lit-up.Thumbnail of finished receiver.Thumbnail of finished receiver.Thumbnail of FVD un-lit.

Background to this project.
Valves/Vacuum Tubes and "Triodes in Disguise".
VFD's used as Valves/Vacuum Tubes.
Receiver Description.
The R.F. and Detector stage schematics.
A.F. Stage Schematics.
Receiver Construction and Pictures.
Test Results.
MP3 Audio Clips.
Final Comments.
References and Links.
A link to my web-page with more images and detail about this project.

Thumbnail of a VFD lit-up.

Background to this project.
This little project arose from a desire to experiment with some of the more "traditional" methods of radioconstruction which enjoyed popularity some 70 years ago. I wanted to find out for myself what kind of performance could be achieved using simple circuits incorporating Valves/Vacuum Tubes and low budget design techniques popular in the 1930's. In particular I was curious to try out "Spider" coils and a Regenerative Detector using a low-mu (low gain) triode as the active device.

There is a great deal of "folklore" surrounding regenerative receivers with claims of exceptionally good performance from these simple designs. For the most part these claims are well justified. Its true to say that a well constructed regen receiver of good design will give pleasing performance. Over the yearsa number of regen receivers have been constructed here at M0AYF for both broadcast and ham radio reception usingvacuum tubes and/or solid state devices. Icanconfirm that they do indeed perform as advertised. But most of the receiversbuilt herehad taken advantage of using modern components and construction techniques so it was interesting to build the "retro" receiver using more traditional construction techniques and then to compare the performance to that of newer designs.

In order to remain as faithful as possible to the traditional methods of construction and technology available in the 1930's it was decided that low-mu (low gain) triodes would be used for the various stages of the receiver. Though I have a small quantity of 1930's triode valves/tubes in the M0AYF junk-box the decision was made to save these for a future "retro" project. So a suitableactive device to serve as a replacement for the low-mu triodes had to be found. Physical inspection of the internal construction of a VFD (Voltage Fluorescent Device) revealed that it possessed all of the key electrodes found in a triode valve/tube.

Valves/Vacuum Tubes and "Triodes in Disguise"
A basic triode valve/tube consists of three electrodes, a heated cathode, a control grid and an anode/plate. The cathodeelectrode in a typical directly heated triode takes the form of a specially coated filament. This coated filament is arranged such that when an electric current is passed through it the filament will heat up and "glow" causing electrons to be freely emitted. This is generally termeda "directly heated" or "self heated" cathode. Below is a cut-away diagram of a triode valve/tube which is of the directly heated type andtypical of those used in the 1930's for domestic broadcast receivers.

Cut-away diagram of an indirectly heated triode.

A Voltage Fluorescent Device (VFD) is a display device used extensively in domestic electrical goods such as VCR's, bedside clocks and microwave ovens to name but a few. Though it may not be immediately obvious the internal construction of a VFD is very similar to that of a simple directly heated triode valve/tube, a sort of "Triode in Disguise". The main difference is that the VFD isof a planar construction which is in contrast to the coaxial construction of the typical valve/tube shown in the diagram above. The VFD's used in this project contain a single 7-segment display within a glass envelope and physically resemble a small valve/vacuum tube. The VFD has a directly heated cathode (filament wires), a control grid and at least one anode/plate coated with a phosphor which emits light when struck by electrons. Most VFD's have more than one phosphor coated anode/plate each with its own electrical connection to provide a multi segmented display. For example a "numbers" display would typically have at least seven phosphor coated anodes/plates and possibly a few more to indicate the sign and decimal place.

Two example images of the VFD's used in the retro receiver are shown below.

Example VFD un-lit.Example VFD lit-up.

VFD anatomy.In the image (above right) of the VFD tube "lit-up"both the grid mesh and the two filament wires are clearly visible.

Theinternal electrodes of the the VFD device are shown in the diagram on the right. The phosphor coated anodes/plates have separate connections so that individual segments can be switched on or off to produce different numbers.

In the example VFD shown above the filament supply Voltage is around 1.5 to 2 Volts and when used as a number display the anode/plate Voltage is about +40 Volts. In multi-VFD's (those able to display more than one digit at a time) the filament Voltage is generally higher at 4 to 6 Volts though the anode/plate Voltages are still of the order of +40 Volts or so. The component parts of the seven segment VFD display shown above are enclosed in a transparent glass envelope from which all the gas has been removed. i.e. A "hard" vacuum just like that of the triode valve example mentioned earlier.

All the VFD displays I have encountered have only one set of filaments for the whole display regardless of the number of digits or characters it may have. In normal display applications the filament will be run from an AC supply. However, for use in radio work it is necessary to use a DC supply for the filaments. This is because an AC supply would cause an unwanted signal to be introduced into the circuit. Using a DC supply for the filaments does not pose a major problem when using single digit/character displays but with multi-digit displays within a single glass envelope (such as those found in a VCR) the effect of the DC filament supply is to produce a "potential gradient" across the display. The effect of this potential gradient is to cause the brilliance of the display to be uneven across the total display area. The reason this happens is because the current passing through the filament wires causes a progressive Voltage drop along the filament wires which acts as a progressively increasing bias to the individual sections of the display.

For example, in a VFD display with say five separate sections enclosed in a single glass envelope side-by-side (such as a clock display for a VCR) you would expect to see a significant difference in brightness from one one end of the display to the other due to the potential gradient effect. If the filament supply was say +6 Volts then the section of the display nearest the 0 Volt end of the filament wires would be the brightest while the section at the +6 Volt end of the filament wires would be darker. Its the same as putting an increasing positive bias on the cathode of a valve/vacuum tube, the greater the positive bias on the cathode the more the current in the valve/tube reduces. A picture of a five section VFD display using a DC filament supply showing the effect of potential gradient can be seen below.

Potential gradient effect image.

One of the reasons that VFD's have enjoyed widespread use for so long is that they produce a bright display with a wide viewing angle. The spectrum from the display is also fairly wide for any given colour giving a softer light which is pleasing to the eye. In recent years the VFD has been increasingly replaced by L.E.D. and/or L.C.D. displays which continue to develop and offer higher efficiency, longer life and are physically more rugged than the VFD.

For more information on V.F.D.'s see references 1,2,3 and 6 in the "References and Links" section at the bottom of this page.

VFD's used as Valves/Vacuum Tubes.
The receiver described here follows mostly tried and trusted methods of construction with one major exception, VFD's have been used in place of valves/vacuum tubes in order to simulate low-mu triodes and to satisfy my own curiosity regarding the use of VFD's for non display applications.

Single VFD tubes (sometimes described as "first generation" VFD's) salvaged from an old desk-top calculator have been used because they seemed well suited for constructing the individual stages of the receiver on a one tube/VFD per stage basis. In each case all the anodes/plates (seven segments plus decimal point) of the tubes/VFD's have been strapped together to form a single anode/plate connection. In tests this arrangement seemed to give the best performance. Though there is some evidence of the potential gradient effect mentioned earlier it caused no problems in practice aside from a very slight non uniformity of brilliance over the display area. Indeed, if you look at this "VFD Lit-Up" image it is hard to see any significant difference in brightness between the top and bottom of the display. Using single tubes/VFD's also permits physical separation of the individual stages of the receiver which makes it easier to achieve good stability.

First tests using VFD's in place of triode valves/tubes did not look very promising. In normal display operation a VFD may have only +40 Volts on its anodes/plates and in order to secure sufficient current flow and display brightness they often resort to some positive Voltage bias applied to the control grid which in turn permits some control grid current to flow. This mode of operation is quite satisfactory for display purposes but is not well suited for small signal amplification.

For the initial tests I used a +40 Volt DC supply for the anodes/plates since this is the Voltage which had been used in the desk-top calculator from which the VFD's had been salvaged. I joined all the anodes/plates together to form a single anode/plate connection. A resistor in the anode was chosen to limit the current to a safe value of around 100 micro amps for testing. This was about the same value of current used for each individual segment of the display when used in the calculator and provided a bright display.

With +40 Volts on the anodes I could not get the VFD's to work correctly as a small signal amplifier. Indeed, with the control grid connected to zero Volts via a one mega-Ohm resistor little or no current would flow from the filament to the anodes/plates. Increasing the anode/plate Voltage to +120 Volts improved matters and I could now see some signs of current flow (glowing anodes/plates) and signs of amplification. Finally I increased the HT supply to +200 Volts (and also increased the value of the anode load resistor) at which point the VFD device began behaving much like a valve/vacuum tube.

Receiver Description.
The receiver is a "regenerative" typeor (if you prefer) "straight" set with some controlled positive feedback applied to enhance the selectivity and sensitivity. This type of receiver was very popular in the 1930's and still finds favour today among radio enthusiasts. Since the VFD's are in a triode format it means that all the stages of the receiver would have to use circuitry best suited to the triode configuration. Though the gain of the individual tubes is quite low they are electrically very "quiet" resulting in a good signal-to-noise ratio. This is also true of "real" triode valves/tubes.

The configuration chosen for the receiver is a 1-V-2 arrangement, that is, one stage of RF amplification followed by a regenerative detector stage and two stages of audio frequency amplification. The set-up is as follows...

R.F. stage = single neutralized triode/VFD.
Detector stage = single triode/VFD.
First A.F. (preamplifier) stage = single triode/VFD.
Second A.F. stage = two triodes/VFD's wired in parallel.

The R.F. stage.
The use of an R.F. stage in a regenerative receiver is often misunderstood. When correctly implemented the regenerative detector is exceptionally sensitive and has no need of additional R.F. amplification below frequencies in the upper H.F. range. However, an R.F. stage is often added in front of a regenerative detector stage even at L.F./M.F. This is nothing to do with "boosting" the R.F. signal but more to do with isolating or decoupling the antenna from the detector stage. A number of problems can arise when the antenna is tightly coupled to the detectors tuned circuit, these problems include, detector radiation, "dead spots" within the tuning range, overloading of the detector and mis-tuning of the detector tuned circuit by the antenna. The cure for all these effectsis the addition of a low or unity gain R.F. stage. This can be of the tuned or un-tuned variety but I chose to use a tuned R.F. stage because the extra tuned circuit serves to improve selectivity and provides increased performance when tuning in weak signals which are very close in frequency to stronger (local) radio signals.

In rough tests on the bench it was found that a VFDR.F. stage would operate at medium wave frequencies without any neutralization and remained stable. This is perhaps due to the low-mu/low gain of the VFD's. However, for "peace of mind" it was decided that a neutralized triode circuit would be used loosely based on the "
Hazeltine-Neutrodyne"designs which date back to the 1920's. Neutralization is often required in order to counteract the effects of grid to anode/plate capacitance which exists in all valves/tubes to some extent. This grid to anode/plate capacitance can result in some of the amplified anode/plate signal being returned to the control grid. Depending on the amplitude/phase relationship of the feedback it can result in instability or oscillation of the stage. The schematic of the RF and Detector stages of the receiver appear below.

VFD Regen R.F. and Detector stage schematic.

The neutralization circuit works by feeding back a signal of the same amplitude but 180 degrees out-of-phase with that of the signal due to grid/plate capacitance. The amplitude of the out-of-phase signal is adjusted using the "neutralizing trimmer" shown in the schematic above. When correctly set the unwanted feedback caused by the anode/plate capacitance is canceled out by the out-of-phase feedback signal at which point the stage is said to be "neutralized". I chose this particular method of neutralization because it proved to be easy to implement and set-up.

The R.F. stage also has a negative bias Voltage of 1.5 Volts applied to the control grid. In tests the stage worked perfectly well with or without the bias but with the negative bias applied the VFD's standingcurrent was reduced which in turn reduced the battery consumption. I also felt that a little bit of negative bias would ensure better linearity of the R.F. stage in the presence of strong signals.

Here at M0AYF we have a very strong local signal at the higher end of the MW band from a transmitter located just 17 miles away so in order to avoid overloading of the R.F. stage a variable inductive couping was implemented. The fixed spider coil which forms part of the tuned circuit in the R.F. stage is coupled to a smaller rotating spider coil. This second smaller coil is able to rotate through 90 degrees and is adjusted via a front panel control. When the two coils are at right angles to each other the coupling is at a minimum. A picture of the coupler is shown below.

Antenna-coupler thumbnail.

Regen Detector Stage.
The regenerative detector stage is fairly conventional and is of the "grid leak detector" variety incorporating positive feedback from the anode/plate via inductive coupling using a "tickler" coil. Because the receiver tunes over the entire MW band (plus a little extra) it was decided that both a "coarse" and "fine" regen (feedback) control would be fitted. When tuning over a wide range of frequencies using a regen circuit it is often found that the regen (positive feedback) tends to fall-off or reduce as the frequency gets lower. While it is possible to make a regen control with sufficient "scope" to work correctly over the full band it is often found that the control becomes very "coarse" in operation making the receiver difficult to use. Smoother control of the regen results if the operation of the control is s-p-r-e-a-d out. This can be done using two controls for regen, one coarse and one fine.

The coarse regen is implemented using another variable inductance coupling, unlike the R.F. stage antenna coupler the coarse regen coupler has both the detector stage tuned coil and the feedback (tickler) coil in-line with each other. Control of the tickler coupling is achieved using a rotating axle (plastic knitting needle) upon which the tickler coil is mounted. The axle has 90 degrees of rotation and is coupled to a front panel control. The rotating axle also passes through the detector coil but since the detector coil is not fixed to the axle it does notmove. Fine control of the regen is implemented using a variable capacitor connected from one end of the tickler coil to ground as a "throttle" control arrangement. Some pictures of the mechanical arrangements can be seen below.

Regen tickler coupler image.Image of front panel regen controls.

AF Stages.
The A.F. stages of the receiver need little explanation except to say that three tubes have been used in order to give "rock crushing"volume with reasonably low distortion into a pair of low impedance personal stereo headphones. The use of two tubes in parallel for the headphone output stage was to give added "peace of mind" as it was thought that a single tube might be "stressed" a little. Theoretically the two tubes (when connected in parallel) should be closely matched but in practice I selected two tubes at random and found the current spit between the two tubes was pretty evenly distributed. The same 1.5 Volt negative bias supply used for the R.F. stage is also used for the output stage in order to move the operating point of the VFD's to secure a more linear (lower distortion) region and thus reduce distortion on loud signals. A schematic of the A.F. stages appears here.

Power Supply.
The receiver requires three separate power supplies, a 1.5 Volt filament supply at around 300 mA, a 1.5 Volt negative grid bias bias supply and a H.T. supply of between 150 and 200 Volts at around 1 mA. All three supplies are derived from dry batteries. Though less convenient than a mains powered unit they offer the advantage of a truly "silent" background when tuning between stations, no "hum" or "buzz" from power supply transformer magnetic fields or power supply harmonics etc.

Both the negative grid bias supply and the filament supplyuse a 1.5 Volt alkaline cell. Alkaline cells have a very long shelf life which makes them ideal for the grid bias supply. The grid bias supply draws virtually zero current regardless of the radio being switched on or off so the cell will last for perhaps years. I used a "run down" cell for the negative bias supply since the internal resistance of the cell makes little difference in this application and because it helps to recycle an old cell.

An alkaline cell is also used for the filament supply, all five VFD tubes have the filaments connected together in parallel so a single cell runs all five tubes. Wires have been soldered to the terminals of the cell such that it could be mounted in a cardboard box with external screw terminals fitted. This is a re-usable box which permits a new cell to be fitted when the old one is exhausted.

The H.T. supply (up to 200 Volts) is derived from a total of twenty 9 Volt transistor radio batteries. Two H.T. battery boxes house the batteries (ten in each box) and are connected in series to give a nominal +180 Volts. In practice the Voltage is greater than +200 Volts when the batteries are new. This is potentially dangerous so please exercise caution if you choose to replicate any of the experiments outlined on this page. Because of the relatively low consumption of the receiver (about one milliamp) and only intermittent use the batteries will last for a very long time. A couple of pictures of the battery boxes appear below.

Thumbnail of filament cell boxed.Thumbnail of H.T. battery box.

Schematic of regen A.F. stages.

Receiver Construction and Pictures.
The construction techniques used are intended to reflect some of the methods used for home constructed receivers back in the 1930's. These are generally low budget methods with many of the components home made if possible. I stopped short of making my own resistors and capacitors, while this is possible it would have also taken much longer to build the receiver. I was also very impatient to see how the VFD's and "spider" coils performed in the finished radio. Some more information and templates for making spider coils can be found in references 11, 12, 13 and 14 in the "References and Links" section at the bottom of this page.
A close-up picture of one of the spider coils appears below.

Thumbnail of MW spider coil.

The materials used include a varnished wood baseboard to serve as a chassis and an enclosure made from scrap hardboard which was painted mat black. The individual stages of the receiver areconstructed on tin lids salvaged from food cans. They made effective supports for the wire-ended VFD devices and also provided a local "ground plane" area. Interconnection between the different stages is via insulated wires. The "tin lid" construction can be seen in the images below.

Thumbnail of R.F. stage.Thumbnail of detector stage.Thumbnail of A.F. preamplifier stage.

Picture of regen under construction.Picture of regen under construction.
VFD tube in-action.

Test Results.
The "Retro-Regen" was tested using a home-brewWide Bandwidth Active Loop Receiving Antenna and performed much better than expected. It easily matches or (in some cases) exceeds the performance of an old Cossor model 393 regen receiver which was manufactured in the late 1930's and used here as areference with which to compare the retro regen's performance.

While the retro regen is not capable of driving a loudspeaker directly it more than makes up for this in other respects. The receiver is free of "reaction chasing" (or regen chasing), this is an annoying problem which sometimes plagues regen receivers. The symptoms of reaction/regen chasing are a difficulty in securing the optimum tuning point as the regen control is advanced (more positive feedback) such that as the operator advances the regen the main tuning also has to be re-adjusted. This in turn causes the optimum regen point to shift so it to must be re-adjusted. This interaction or "chasing" between the two controls continues and gets worse as the regen is advanced. The cause of reaction chasing is often due to over coupling of the "tickler" coil to the detectors tuned circuit such that changing reactance in the feedback circuit is coupled to the detector tuned circuit and "pulls" the tuning a little. Thankfully, the retro regen behaves well with no evidence of reaction pulling. Some MP3 clips of the retro-regen in action can be found
in the "MP3 Audio Clips" section of this web page.

The receivers sensitivity is very good and weaker stations can be pulled from the forest of stronger EU stations quite easily. There is some loss of sensitivity at the L.F. end of the MW band due I suspect to the reduced coupling between the R.F. and Detector stages as the frequency gets lower. Within the limitations of the regenerative detector the receiver copes fairly well with strong signals, in the event of a strong local signal s-p-r-e-a-d-i-n-g over the band the antenna coupling can be reduced which usually cures the problem. One unexpected result of the receivers high sensitivity is its ability to receive signals with no antenna connected, at first this was thought to be due to pick-up in the wiring but further tests revealed that the home made spider coils behaved as small frame antennas. Had the receiver been housed in a steel box this would not happen.

Though the gain of the individual tubes is quite low they are electrically very "quiet" resulting in a good signal-to-noise ratio. This is also true of "real" triode valves to. In common with regular battery valves (1T4 and similar) the VFD's are "microphonic" to some extent so if they are tapped with a pencil while powered-up a "ping" can be heard in the headphones. Thankfully the wire ended connections of the VFD's act as shock absorbers to some extent and reduce the effects of vibration from the case and its surroundings. This microphonic effect is mainly due to the filament and internal electrode structure of the VFD mechanically vibrating. The effect becomes more problematic with an increasing number of cascaded audio stages. The Retro-Regen has three cascaded audio stages, the detector (which also amplifies at A.F.) followed by an A.F. pre-amplifier stage and finally the A.F. headphone output stage. Since the detector is the first of these cascaded A.F. stages it is also the most microphonic. You can hear the "ping" sound made by the detector stage VFD tube being "tapped"by downloading and playing the MP3 audio clip (number 8) in the "MP3 Audio Clips" section of this web page. Another VFD experimenter has made full use of this microphonic effect in VFD tubes by building a pseudo ring modulator, see reference 8 inthe "References and Links" section at the bottom of this page.

One amusing side-effect of using VFD's for the receiver is the "visual feedback" they provide. When the VFD's are in the "linear region" or driven with very small signals then the "glow" is continuous. However, if the VFD's are over-driven into the non linear region then they can be seen to "flicker", this effect is most noticeable on the A.F. output stage VFD's on strong/loud signals. Similar visual feedback occurs with the detector stage, as the detector is tuned through a strong signal the brilliance of the glow can be seen to reduce slightly. This is caused by the grid-leak detector action, as the negative Voltage builds up on the control grid due to the strong signal it biases the VFD tube so as to reduce the quiescent current. As the signal strength gets higher so to does the negative bias and hence a greater reduction in brilliance of the VFD. This reduction in brilliance of the VFD detector tube can also be seen as the detector is taken into oscillation.

Final Comments.
This receiver turned out to be a very rewarding and educational project, I discovered several new thingsabout regen circuits and VFD's which have inspired several new projects. These include a VFD transmitter, a VFD 40 Mtr amateur band crystal controlled converter and severalother VFD regen designs. These additional projects are (at the time of writing) still undergoing tests on the work bench but may (in time) find themselves on a web page. Of those projects listed above the frequency converterhas given very encouraging results. When connected in front of the retro-regen it permits reception of the 40 Mtr amateur band. Reception of AM/CW and SSB are all possible. MP3 audio clips 1 and 2 in the "MP3 Audio Clips" section of this web page demonstrate SSB reception achieved using the 40 Mtr VFD converter ahead of the retro-regen.

For anyone considering building a project using VFD's salvaged from VCR's or calculators etc then please make sure that the VFD's are fully operational before you start. Old VCR displays in particular are sometimes lacking in brilliance. This lack of brilliance may be the result of long hours of service resulting in damaged or burnt phosphors or the result of failing emission from the cathode/filament. If the lack of brilliance is due to worn phosphors then the emission may still be good enough for VFD radio experiments but if the loss of brilliance is due to failing emission then this may pose a problem for prospective VFD radio use. So, look for a surplus or scrap item which still has a working VFD with a bright display.

A number of other radio hams and experimenters have also used VFD's in other applications, see references 4, 5, 6, 7, 8 and 9in the "References and Links" section at the bottom of this page.

Finally, if after reading this you decide to build any projects using VFD's or if you have already built a radio project using VFD's then please drop me a line, I would be interested to hear about your experiments.

Well, thats about it, thank you for reading this and please send any questions, comments or "heckles" etc to the e-mail address linked below.

Des (M0AYF)

Member Comments:
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A VFD Receiver (Triodes in Disguise)  
by N4CQR on January 22, 2009 Mail this to a friend!
Outstanding piece
A VFD Receiver (Triodes in Disguise)  
by AC7CW on January 22, 2009 Mail this to a friend!
I cut my teeth on a regen receiver, a US Navy RAL-5.. I had no idea that SSB could be pulled in!! That looks like a very enjoyable project to build and you get visual feedback from the tubes, too cool :)
A VFD Receiver (Triodes in Disguise)  
by M0AFJ on January 22, 2009 Mail this to a friend!
Well done Des, an outstanding piece of work, best I've seen on here for a long time
A VFD Receiver (Triodes in Disguise)  
by K8QV on January 22, 2009 Mail this to a friend!
That was an outstanding article. Thanks.
RE: A VFD Receiver (Triodes in Disguise)  
by K0BG on January 22, 2009 Mail this to a friend!
Some people have way too much time on their hands!

The next project you should tackle is to use a bunch of the displays to build a QRP transmitter. Just imagine the jaw dropping when you tell folks your equipment is salvaged from old calculators!

Alan, KBG
RE: A VFD Receiver (Triodes in Disguise)  
by ARRLBOOSTER on January 22, 2009 Mail this to a friend!
Fantastic article...I am glad you have time and imagination and knowledge "on your hands". Poor Alan has only time on his hands...time and cynicism that is....
RE: A VFD Receiver (Triodes in Disguise)  
by K6AER on January 22, 2009 Mail this to a friend!
Great artical. It;s to bad QST doesn't have articals like this. Can't wait to see their next installment on how to build a "J" pole.
A VFD Receiver (Triodes in Disguise)  
by K1CJS on January 22, 2009 Mail this to a friend!
Just one question--

Would this be considered a 'light' receiver?

Sorry--couldn't resist that one! ;-)

Seriously though, nice construction article. How about some more details on this project.
RE: A VFD Receiver (Triodes in Disguise)  
by KT8K on January 22, 2009 Mail this to a friend!
This is great - in some ways similar to AA1TJ's creation of a working 40m transmitter using the parts from inside a compact flourescent light bulb.
Nice job.
Thanks & 73 de kt8k - Tim
RE: A VFD Receiver (Triodes in Disguise)  
by WI7B on January 22, 2009 Mail this to a friend!

Great insight, innovative thinking, and no doubt a lot of sweat.


---* Ken
RE: A VFD Receiver (Triodes in Disguise)  
by K8QV on January 22, 2009 Mail this to a friend!
"Some people have way too much time on their hands!

The next project you should tackle is to use a bunch of the displays to build a QRP transmitter. Just imagine the jaw dropping when you tell folks your equipment is salvaged from old calculators!

Alan, KBG"

Experimenting, learning, doing something in the hobby. That's what it's all about.

Of course, when one already knows everything, there's not much left to do but pontificate.
RE: A VFD Receiver (Triodes in Disguise)  
by WMCO on January 22, 2009 Mail this to a friend!
"Some people have way too much time on their hands!

The next project you should tackle is to use a bunch of the displays to build a QRP transmitter. Just imagine the jaw dropping when you tell folks your equipment is salvaged from old calculators!

Alan, KBG"

Experimenting, learning, doing something in the hobby. That's what it's all about.

Of course, when one already....THINKS he.... knows everything, there's not much left to do but pontificate.

There I fixed it for ye.

RE: A VFD Receiver (Triodes in Disguise)  
by K0BG on January 22, 2009 Mail this to a friend!
I think a few folks took what I wrote as destructive criticism, and that isn't (wasn't) the case at all. If you have the time for such projects, you certainly do learn from them. The point being, it does take a lot of time to go through the iterations.

If there is any criticism to be handed out, it is toward those who don't attempt to do any bench work, even simple, basic repair. They use the excuse that they DON'T have time on their hands. That's not me, and apparently not the author either.

Alan, KBG

RE: A VFD Receiver (Triodes in Disguise)  
by GW0DIV on January 22, 2009 Mail this to a friend!
I understood what you meant Allan - OMG you must be aquiring a British sense of humour!! It'll be all Monty python, John Cleese and Ricky Gervaise before you know it!!!

RE: A VFD Receiver (Triodes in Disguise)  
by K7LRB on January 22, 2009 Mail this to a friend!
"Some people have way too much time on their hands!"

I too understood exactly what you meant Alan and did not find it offensive AT ALL.

Man, some of you people really need to loosen up!

Have fun with ham radio and 73,

de Larry
A VFD Receiver (Triodes in Disguise)  
by KL7AJ on January 22, 2009 Mail this to a friend!
Very Very nice. I have a 6E5 magic eye tube wondering what it should do with itself. :)

By the way..the SPIDER coils are very cool. Lotsa folk don't know the whys and wherefores for them. The spider winding is to reduce interwinding capacitance. If constructed well, the wires are never parallel to each other, but cross at nearly right angles. It seems like a pain in the posterior to do it that way, but this really truly and genuinely did huge things for the overall performance and Q.

A VFD Receiver (Triodes in Disguise)  
by W8AAZ on January 22, 2009 Mail this to a friend!
I have just recently finished a pretty faithful replica of a "Twinplex" shortwave regen using all old parts as much as possible. This design uses the early number 19 dual triode. Two direct heated triodes in an ST glass envelope and a 6 pin base. The regen. control needs constant attention while tuning but the performance is quite stunning to somene who has been using superhets exclusively for SW reception. I have plenty of regen. antique BC band sets. The 19 dual triode is quite a bargain at around 5-5.50$ as NOS. The competing configuration at the time(ca. 1934) was two number 30 triodes. Those are quite alot more expensive for some reason. This set using plug-in band coils, gives quite amazing performance for a single envelope dual triode. Detector and audio. Strong SW stations are quite loud in any sort of hi impedance antique phones, and on loud stations it will sometimes drive my 1925 horn loudspeaker to good audible levels. I too string 9 volt batteries in series to obtain the 90 Volts required, although current drain with the set is less than 2 mA. from it. Care should be taken. I once clipped 9 volt batteries in series on a wooden workbench at work for fun. Had quite a few and the potential was probably great, how many of those batteries can be fitted onto a 5 foot bench in series? Clipped a lead to one end and was able to draw some rather impressive big bright arcs off the other end of the stack!
RE: A VFD Receiver (Triodes in Disguise)  
by K4RAF on January 22, 2009 Mail this to a friend!
"a total of twenty 9 Volt transistor radio batteries..."


RE: A VFD Receiver (Triodes in Disguise)  
by NB3O on January 22, 2009 Mail this to a friend!
Twenty 9 volt batteries are easier to find than a pair of functioning "B" batteries.
RE: A VFD Receiver (Triodes in Disguise)  
by AC7CW on January 22, 2009 Mail this to a friend!
Amateur radio is a HOBBY!! [excuse the yelling please :)]. Desultory comments here and there ruin it, momentarily at least, for those who HAVE FUN AT THEIR HOBBY [again, apologies, hope everybody's ears are still ok !(--)!].

Again, I really enjoyed the article and kudos on the basic design. Putting the RF stage in there to decouple the antenna from the mixer is sweet, paralleling the audio elements.. well it's good info you can do that and it doesn't get unbalanced in the current draw... I think maybe the author gets a lot of enjoyment out of just doing something right maybe.

Doing something really well is efficiency, doing the right things is effectiveness.... but, it's a hobby, who cares about effective? Effective is for guys that buy a box they can barely operate, let alone design and build... go on, buy your boxes, work a zillion countries... show us all how effective you are.. I'll put my endorphin level up against yours anytime!!

A VFD Receiver (Triodes in Disguise)  
by WB6DGN on January 22, 2009 Mail this to a friend!
At the risk of being called a nit picker (I've been called a lot worse) I thought VFD referred to "vacuum fluorescent display". I've never heard the term "voltage fluorescent display". Can someone please clarify?
RE: A VFD Receiver (Triodes in Disguise)  
by WB6DGN on January 22, 2009 Mail this to a friend!
"Twenty 9 volt batteries are easier to find than a pair of functioning "B" batteries."

I know! I've got a nice portable radio from when I was a kid...say, you've just given me a great idea! Now, if I can just come up with a container about the right size and shape! And, it won't leak like those old carbon/zinc things did!
RE: A VFD Receiver (Triodes in Disguise)  
by M0AYF on January 23, 2009 Mail this to a friend!
Hi there,

Tom (WB6DGN) wrote:
I thought VFD referred to "vacuum fluorescent display". I've never heard the term "voltage fluorescent display". Can someone please clarify?

Thankyou, yes thats one "massive" typo on my part, worse still its in the title :-(

I spent ages going through the text and still managed to miss that one. It should of course read "Vacuum Fluorescent Display" and not "voltage fluorescent display", sorry for the confussion.

I will correct the error on the RSGB web-page in due course, thanks again for pointing it out.


Des (M0AYF the author of "Triodes in Disguise")
RE: A VFD Receiver (Triodes in Disguise)  
by M0AYF on January 23, 2009 Mail this to a friend!
Hi again,

Just a few lines to say "thanks" for all the positive comment and support received so far relating to the "Triodes in Disguise" article.

Many thanks es 73,s

Des (M0AYF author of "Triodes in Disguise")
A VFD Receiver (Triodes in Disguise)  
by WR9H on January 23, 2009 Mail this to a friend!
You sir, are an artist!! My goal is to be half the designer and builder that you are. Thanks for the very cool post!!
RE: A VFD Receiver (Triodes in Disguise)  
by K5END on January 23, 2009 Mail this to a friend!
Outstanding article! It's perhaps the most original and well-penned electronics piece I've seen, in any venue.

This article is award-level material.


I took K0BG's comment at face value and had already started planning my 160 m QRP/Ti-Calc rig.

Coincidentally I had to rescue the box of appointed calculators from the trash ("rubbish," for you folks across the pond) this morning. XYL rev. 2.0 had been doing some house cleaning and had sentenced these innocent gadgets to the landfill. That is just so, so wrong, on so many levels.


A VFD Receiver (Triodes in Disguise)  
by K4IQT on January 23, 2009 Mail this to a friend!
Thanks for the idea, Des. I've got an old VFD calculator laying around, still working but not worth a dollar at any flea market, and too good to throw away.

I used to build my TX's and RX's and everything else from salvaged TV's, so it makes perfect sense to recycle this calculator too.

Again, thanks for the informative and innovative writeup!
A VFD Receiver (Triodes in Disguise)  
by N9CYS on January 23, 2009 Mail this to a friend!
Terrific article!!

Reminds me of the circuits I first studied as a Novice a looooooong time ago. If you look closely at these schematics, you can understand CORE receiver functions and understand how primary electronic components function.


RE: A VFD Receiver (Triodes in Disguise)  
by W8ZNX on January 25, 2009 Mail this to a friend!
but sry om but im not going to play with triode autodynes

there are tons of nos

screen grid tubes to use

nice try
keep up the good work
using the wrong tool
for a job
just because its handy and glows in the dark

does not quite get it

dit dit
Lemony good  
by K1IO on February 9, 2009 Mail this to a friend!
This is absolutely insane. You have to be off your rocker to do this.

I love it!

When I see the term "VFD", I'm reminded of Lemony Snicket's "A Series of Unfortunate Events" books, which feature the search for a shadowy organization known only by its initials "VFD". Sort of fitting.
A VFD Receiver (Triodes in Disguise)  
by XE1GXG on February 11, 2009 Mail this to a friend!
Great article, well written and highly engaging..even if I don't build the RCVR.
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