Golborne Vintage Radio

[attachment=15384]
Hi.
I quite like trying different approaches to making micro transmitters for Medium Wave and feeding the signal round the shed.
I recently bought some dual control grid Russian Rod valves 1Ж42А.
Today I had a little experiment to see if it would work as a micro transmitter using one control grid for HF oscillation and the other to modulate the valve with AF.
I eventually got the valve to oscillate reliably strapping a crystal directly between G2 and the first G1, G2 was fed from HT (9v) via an RF choke, G3 is grounded and the second G1 is fed audio via a 0.047uf cap and returned to ground via a 680k
The Anode is again fed from 9v via another RF choke. The output is a very clean sine wave around 5v p-p. Using a couple of meters of wire from the anode you can feed the shed quite happily, the signal dies before reaching the house just under 10 meters away.
I improved the range to about 25 metres by making up a tank circuit.
Audio quality is OK, not hi-fi but still good, modulation is only good up to 50% but some changes could improve this, above 50% distortion creeps in.
The circuit will just work at 3v HT and anything between 4.5v & 9v it works fine above 9v the RF drops and performance is less.
With so few components it has to be one of the simplest I've made.

It's not dual grid like a dual gate fet!
One signal needs to drive both grids in phase and the other signal drive them in antiphase, so a centre-tap RF transformer feeding g1a and g1b and then audio to centre tap might work?
Though to be osc/mixer, you need an audio transformer driving g1a & g1b and the g1a g1b both capacitive coupled to osc transformer. Use a pair of 2M2 resistors for grid leaks or one to audio transformer centre tap.
RF out(or IF if RF mixer + LO, or audio out if a BFO) on the anode.

Interesting Michael.
It certainly works, possibly not the best but perfectly useable (often the theory doesn't relate to practise as we know) I'll give the RF transformer a go and see if driving the "grids" on the centre tap works better. Would need another valve for the oscillator I would think so the dual control valve would be the modulator.
Driving the G3 with audio also works after a fashion but no where near as good as using the G1's.

You should give it a go and see what you think

Hi.
I was just thinking that a centre tapped RF transformer will feed both grids in antiphase so in effect the output will be zero at RF, so I would think a centre tapped TX isn't the way to go.
Looking at the data of the valve you can see that the GM of the valve is highest with both control grids connected in parallel which makes sense but it's lower than halved when used singly.
What I did find today is the grid used for audio input, linearity could be improved by biassing it at about 0.5v negative with respect to the filament minus connection. I was able to gain a little more modulation before distortion, also running the "HT" at 6v reduced the distortion too. The original circuit is a basic building block, I'm sure I can improve on it with time.

Hi.
I've added a proper Pi-tank to the transmitter, I also have done a You-tube video of the transmitter in action.
Next is to build it into a little tobacco tin. The PP3 Battery should last for years as the circuit is only drawing 0.24 ma. #

Comments on the result not my awful production thanks https://youtu.be/v1D0D8qTHjs

youtube says it's unavailable
Am I just too quick?

Alan

Try it now Alan, forgot to publish it

Hi Trevor,
I've just downloaded and watched your video. As you will not be surprised to hear I don't know much about transmitters. However, I have thought about building one in order to be able to use my radios when MW eventually goes or even because of interference now. Most of the circuits I've looked at use transistors but it would be more in keeping to build on with a valve. Using 9V HT and 1.5 LT is a big attraction here as no PSU is required.

Regards,

Ken.

Splendid, that works now
Alan

Since doing the Video I ave added a 0.047uf coupler to the grid for the audio and returned that grid to ground via a 20meg resistor to cancel the space charge, now I have 80% modulation without distortion. #
I[attachment=15394][attachment=15395][attachment=15396][attachment=15397] include some scope images at around 70% modulation & 100%.