Fun with a biscuit tin – A Pantry Transmitter.

[Vicboduk]

I expect that many forum members collect or are given festive biscuit tins shaped as vintage equipment. I usually use mine to store parts, however this year I was given one that lent itself to another purpose! It would form the perfect case for a pantry transmitter – something I have been meaning to build for a while. As I intend to listen on valve sets it seemed only right to use valves and to construct it on a separate chassis which would just slip into the case.

Chassis;
A simple chassis was folded up from aluminium sheet. The only component needing critical placing was the tuning capacitor. I had intended the shaft of this to protrude from the centre of the front dial. Unfortunately the printed scale was in the wrong direction for the capacitor that I had. It was necessary to reverse the motion. This I did with a couple of plastic gears and an old potentiometer mounted on a piece of 4mm thick PVC. This also solved the problem of how to isolate the capacitor shaft from the user (me) as it would be at full HT potential! All the other components are mounted on two tag strips or directly to the valve bases.

Circuit;
Due to the lack of space the circuit had to be simple, two double triodes 12AT7/ECC81 are used. The first as an AF amplifier and cathode follower, the second as oscillator and modulator. To ensure that the heater-cathode voltage of the oscillator wasn’t exceeded and to keep the output power low, a HT of 90v is used. This I obtained from an old 20VA transformer which had a shorted secondary. I rewound this to provide the HT and heater supplies. I also added a centre tap to the heater winding so that it could be earthed to remove hum if needed.
The inductor I wound on an off-cut of 22mm PVC conduit and the audio input to the circuit is via two phono connectors.


In Use;
For such a simple circuit it performs quite well. The stability is better than I expected. I measured a tuning drift of +420Hz after two hours of use. Most of this occurred during the first hour, +380Hz.
The addition of the heater centre tap proved to be a good move as there was small amount of hum present which was totally removed once this was earthed.
The circuit uses cathode modulation of the oscillator – This is not good design practice as is tends to pull the oscillator frequency. This was found to be the case, with 50% modulation of an 800kHz carrier the frequency was deviating by ±850Hz. However listening tests on my sets showed this to be barely noticeable particularly with speech content. With the 310mm telescopic aerial fitted the useful range is approximately 25ft. However, connecting 8ft of wire to the aerial socket easily covered the whole of my house.


Vic

[Vicboduk]

A couple more pictures and oscillograms showing quite a linear performance.

Vic

What a splendid job! I must admit having such a "modulator" does make our vintage sets spring to life with worthwhile content.

Please put up the circuit for interest, others making one or (just in case) there may be improvements to be made (lots of everso clever people on this forum).

[paulsherwin]

Yes, a beautiful job, congratulations.

I would also be interested in seeing the circuit. Using two ECC81s allows a range of spare transformers to be used, as the heater voltage can be 6, 12 or 24 volts.

[Vicboduk]

The circuit as requested. No claims to originality here!
It was drawn in LTSpice and with models from here. The simulation was a reasonable approximation of the real thing.
As yet I haven’t drawn up the PSU but any thing that can supply approx. 90vdc and a centre tapped 6.3v 1Amp for the heaters will do. HT current is only a couple of mA.
As Paul states the heater arrangement is quite flexible but I found earthing the centre tap removed the residual hum that I had.

The inductor is wound on 22mm PVC conduit and consist of 150 turns of 36swg for the primary and 25 turns of 36swg for the feedback winding. So as not to change the inductance too much I used two rubber grommets, a Perspex disc and a Nylon nut & bolt to secure it to the chassis. The values shown were measured when not mounted.

The tuning capacitor was something I had in my bits box 50pF- 450pF ish! , the 250pf shown was the value I used for Spice simulation.

Vic

With 24V of LT a voltage trebler would make the HT no problem 24 * 2 * root 2 = 100V. Put U1 at the bottom of the heater chain to minimize hum.

[cmjones01]

That's a lovely piece of work, beautifully crafted.

Silly question time: why the cathode follower? Since the audio amplifier is only feeding the grid of the modulator, and that via a 10k resistor, surely C4 could be connected straight to the audio amplifier's anode. Or have I missed something?

Chris

[Radio Wrangler]

Very, very elegant.

I know it's only 90v HT, but if the aerial goes outside, it's geenerally considered good practice to ground it with an RF choke, to protect against failure of the 1nF DC blocking capacitor.

David

[Vicboduk]

Thank you all for your comments.

Originally Posted by cmjones01

Silly question time: why the cathode follower? Since the audio amplifier is only feeding the grid of the modulator, and that via a 10k resistor, surely C4 could be connected straight to the audio amplifier's anode. Or have I missed something?

Chris, firstly, I hate only using one half of a double triode! Secondly, I found that there was a significant amount of RF appearing on the input sockets when I did as you suggested.
Putting the low impedance follower between the two stages all but stopped it.

Thanks, David. I'll do as you suggest.


Vic

[cmjones01]

All good reasons! I understand now.

Chris

[kalee20]

Originally Posted by Vicboduk

...firstly, I hate only using one half of a double triode! Secondly, I found that there was a significant amount of RF appearing on the input sockets when I did as you suggested.
Putting the low impedance follower between the two stages all but stopped it.

My initial thoughts exactly as cmjones01! But I can see the benefits of adding the buffer as a 1-way transmission.

My recommendation (I hate leaving valve sections unused, too!!) would have been use the cathode follower to buffer the RF. That way you would isolate the load (aerial) from the tuned circuit, possibly helping frequency stability.

Good project!

[G6Tanuki]

Looks nice!

Must admit though, the idea of applying modultion directly to the oscillator seems instinctively wrong, and your measured drift supports my instinct: I'd redesign the circuit so the first valve was an audio-amp and the second used one half as the oscillator with the second half being a "mixer" to combine the audio and the RF.

[kalee20]

Must admit though, the idea of applying modultion directly to the oscillator seems instinctively wrong...

Again, I'd agree - however one big point in its favour is that modulation of amplitude is pretty linear (as Vicboduk's trapezium figures show). Amplitude of a simple oscillator is fairly accurately proportional to supply voltage and that's exactly what is wanted. Down side is that second-order effects do shift the frequency slightly throughout the modulation cycle, which can lead to funny effects in the receiver (unless the passband is wide enough - FM theory applies!)

If the oscillator is fixed and modulation happens at a subsequent stage, this would have to be non-linear for any modulation to take place.

It does strike me that many of us - myself included - are looking at the OP's design on the basis "Well, I'd have done it like this..." I'd certainly say that all our opinions are valid, but it's not to decry Vicboduk's original achievement, not only for making something that works for him, but also for giving rise to a decent discussion!

[paulsherwin]

It's a nice simple circuit which appears to give acceptable results, so it's fine by me. In fact you could probably lose U1/U2 if a high output audio source like a CD player is used, which would make it ridiculously simple.

[Vicboduk]

Constructive criticism is always welcome. It’s how we learn and progress. So with your comments in mind I set about trying different configurations within LTSPice.
I kept the AF amplifier and oscillator pretty much as they are and tried to form a modulator from the remaining two half triodes.
As Kalee predicted, not an easy task – OK, I’ll be honest I didn’t even come close!

Here are some LTSpice output oscillograms. The original configuration first (beautiful) plus some further attempts, Oh dear...

If anyone has some suggestions how this might be achieved with two my two remaining half triodes...
Else, for the time being I think I’ll leave it as it is.


Vic

Edit;

In fact you could probably lose U1/U2 if a high output audio source like a CD player is used, which would make it ridiculously simple.

Hi Paul, I you did want to lose the first valve I found that I needed approximatley 2v pk-pk for 50% modulation on the grid of U4.
So not beyond the realms of a high output source.

[kalee20]

Well, the other three oscillograms may not be quite as bad as you think. It looks to me as though there is a significant AF component in there. The top half is nearly flat; the bottom half varies massively with the modulation, therefore the average has some AF.

This is exacly what you would see, for instance, if the modulating signal was applied in series with an RF load, even if the modulation was perfect.

Can you do the 'scoping via a high-pass filter, say 10pF / 1MΩ?

[Vicboduk]

Hi Kalee, I took your advice and added a high pass filter – definitely a step in the right direction.

Paul, sorry for my typo, I missed an f. It should have read “If you did want to lose the first valve...”

Vic

[gezza123]

Originally Posted by Vicboduk

Constructive criticism is always welcome. It’s how we learn and progress. Else, for the time being I think I’ll leave it as it is.

Hi Vic.
What an (Excellent achievment), and a credit to you.
The only criticism I can see is where does all the heat go from the valves, as I dont see any ventilation, or are you going to put a grill on the back where the black square is.

Still a great piece off kit, and a great Idea for those tins that keep stacking up full off junk. WELL DONE.

Love the biscuits though.Gezza123.

[kalee20]

Hi Vic,

That's better - it is easy now to see that there is distortion. Can you put up the circuit as-is that LTSpice is simulating?

[Vicboduk]

Hi Gezza123, You’re quite right – no ventilation slots. I didn’t want those waves sneaking out the back whilst I wasn’t looking!
Actually, I’m in two minds about this – The tin is incredibly thin and deforms easily and the paint flakes off with the slightest of touches. I really don’t want to add any more holes unless it’s really necessary, for fear of damaging the whole project. I’ve had it running for hours at a time hours and the case temperature only gets very mildly warm. I can’t detect any real hot-spots the temperature seems to be quite well distributed around the tin. I hope to borrow a couple of thermometers and measure this properly. It is probably the main cause of the oscillator frequency drift that I have but I think I can live with it.

Kalee, I don’t have the file with me at this time. Plus, it’s an awful hack! I’m far too embarrassed to put it here... I’ll keep tinkering in LTSpice though.

Vic