Simple micro transmitter

[Murphyv310]

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.

Thanks Ken.
For such a simple circuit I find it works well.
What I'll do tomorrow sometime is to re-draw the circuit with the tank coil, the choke values and coil winding info for the tank coil. The audio needs about 1-1.5v drive, I'm using the headphone out from a minidisc player, an MP3 player or the likes does work but they can be a little low in drive. Another rod valve could be used or even a single transistor if the drive is a little low.
I can add this on if needs be.

[ken]

OK Trevor,
I'll keep watching until you perfect the circuit. If you add another valve or a transistor would you expect it to draw a lot more current.

Ken.

[Kalee20]

Looks good!

The trouble with single-stage modulated oscillators is that there's a tendency to get some FM. The envelope can look lovely on a 'scope, you can have trapezium figures with super straight sides, but if there's FM going on too, when it hits the receiver's passband you get slope detection, which combined with the intentional AM, gives weird tuning and distortion effects. Your circuit with a crystal should, hopefully, reduce this at the expense of being stuck with whatever frequency the crystal is.

The idea of an audio preamp with another Russian rod valve is really the only way to go, who'd want to drive it with a tr*ns*st*r? A third valve, configured as a detector, could demodulate the RF and allow a bit of negative feedback to reduce distortion.

I'll watch with interest!

[Geordie McBoyne]

Here's a link to the National Valve Museum: http://www.r-type.org/articles/art-094.htm

Scroll down to the bottom of the page to read abut the Cossor 210DG bi-grid valve, it was used as a frequency changer in very early superhet receivers.

Geordie McBoyne.

[Murphyv310]

Looks good!

The trouble with single-stage modulated oscillators is that there's a tendency to get some FM. The envelope can look lovely on a 'scope, you can have trapezium figures with super straight sides, but if there's FM going on too, when it hits the receiver's passband you get slope detection, which combined with the intentional AM, gives weird tuning and distortion effects. Your circuit with a crystal should, hopefully, reduce this at the expense of being stuck with whatever frequency the crystal is.

The idea of an audio preamp with another Russian rod valve is really the only way to go, who'd want to drive it with a tr*ns*st*r? A third valve, configured as a detector, could demodulate the RF and allow a bit of negative feedback to reduce distortion.

I'll watch with interest!

Thanks Kalee.
I fully agree with you regarding getting some FM modulation, one reason I use crystals. I also would think another rod pentode would be the best for an Audio preamp, current consumption certainly would never be an issue on the HT side and the use of a high capacity Ni-Mh cell for LT will mean many hours of "broadcasting". I have been aiming for an average of 1 volt for the filaments, a freshly charged battery will give 1.1v via a series resistor and this drops down to 0.9v with no change in performance, once down to 0.6 the oscillator can drop out.
The addition of an RF buffer is also a good idea and the RF can be detected, filtered and fed back to the audio pre stage, possibly to one of the G1's on its own, this would be a neat solution and would make the whole little transmitter very linear indeed.
I did NFB in the EH90 micro TX a few years ago and the improvement is well worth the effort.
I am going to visit a sick neighbour today who is in hospital so it could be tomorrow before I update the thread.

[radioman]

Hi Trevor,
I've just watched your video and am very impressed with this !
Isn't 'Manhatten' construction great for prototypes  
Certainly a much simpler circuit than my transistorised efforts...

Andy

[Murphyv310]

Thank you very much Andy.
Just like Sparks I'd like to think my little projects would spur on the novices in home brew or even simple projects.
The Russian Rod valves should help novices with valve projects as they run on low HT and in this day of OTT H&S these have to fit the bill. Most valve circuits are forgiving unlike with transistors, so unless you are really careless then it is unlikely you'll ruin a valve.

You are right Andy the component count is small and again easier for the novice.
I am considering using the same double sided board on the transmitter rebuild with the valves pushed through the board, so connections will be made on both sides, this should give the completed TX a squarer appearance but also aid screening. Might be a week before it's all done though.

[Murphyv310]

Hi.
I've been testing out a three valve version over the last few days. AF Pre stage, Modulator & RF buffer, also a two valve version with audio pre stage and modulator. I have incorporated NFB in the three valve version and the results are outstanding.
I intend to upload three separate diagrams for one, two and three valve versions. Next week I will do another couple of You-tube videos of the Two & Three valve versions.
The single valve version ran for 24 hours with no change of voltage from the PP3, the three valve version only draws around 1ma from the PP3, a fully charged Ni-Mh 2400 ma/hr AA battery should last 200+ hours on the single version or 70 hours + on the three valve version. A higher capacity C or D cell would give longer time between charges.
Watch this space.

[Kalee20]

Sounds good, Trevor! Will be most interested!

Couple of quick questions - where did you get the rod pentodes? And where did you get the crystal? I do know of a custom crystal supplier, but they're a bit pricey...

[Mike Watterson]

Yes, the Russian radios always had screening on the Rod tubes, as unlike regular types there is almost no internal screening, just two or four support rods, and possibly contribution from the four g3 rods or plates and rod carrying current to top end of filament. Bring a magnet and then a coil with RF near the glass of the rod for some fun