12AQ5 Transmitter update - We have a heart beat!

[@Centar1964]

45 yrs ago I made a similar one out of a 6V6, had a roller inducer and a cap output. I also had a small tube amp I used to use as a modulator for it to do AM. Plate modulation. 40 meters. Took that one apart later to make it into a am band transmitter that I did pirate radio on for a bit, ha the memories...most of the parts were from an very old military transmitter that I scrapped.

[@Radionut63]

Sounds like a fun project!

[@Novice1965]

Got caught up in the holidays and just saw your video. Congrats on getting it oscillating! For the circuit you're working with, it looks like the Paraset oscillator. They used a gimmick capacitor between grid and plate to get the required feedback. Search a Paraset schematic and you'll see it. Beyond that, you definitely want that grid resistor back. Putting in a variable is a good idea, I always do that myself. You were also right in putting the output coil and capacitor on the far end of a coupling capacitor in order to keep DC off those parts. The 10nF value is fine. Even down to 1nF should be fine. Finally, I don't remember you saying what your B+ voltage is, but the 2.2k screen resistor may be too low (assume 5mA screen current). I'd recommend that you shoot for 100V on the screen. If you're still not satisfied after all of those things, I'd suggest switching over to the Colpitts circuit. It seems to be the best for one-tube transmitters, and is pretty bulletproof for me.

[@free_electron]

Try moving the 0.01uF cap to the other side of the 2.2k resistor. Electrons are going to strike the plate and bounce off, see the screen grid small signal short to ground, and not be attracted back to the plate. I believe you’d see some improvement in output power. I’d also recommend placing a 1Meg resistor to ground in parallel with the crystal. The grid of the tube needs referenced to ground to maintain proper bias when keying. 47k was indeed loading the grid too much but floating all together is also not ideal.

[@Radionut63]

Thank you, i will try those changes and see what happens. 🙂

[@pd1jdw630]

Looking good buddy!

[@Radionut63]

Thanks 👍

[@bio-techlarry9602]

My suggestion is to build a separate oscillator stage to act as a buffer and couple it to the 12AQ5. That way your oscillator is always running and stable. Your crystal is protected from any DC voltage and you can tweak the crystal a little to get it exactly where ya want. And its output is not affected by any changes in B+ while keying the output stage. These projects are always fun.

[@Radionut63]

I’ll definitely keep that in mind for the next one!

[@TomSherwood-z5l]

I built a similar 40M transmitter from a 73 magazine when I was a kid. It actually worked. I believe it was a 6AQ5. I got gawbs of those in the tube bin but I do not recall ever seeing a 12AQ5. I thought of building one of the older versions with a 6V6. But those are more valuable to audio people. Only thing that ever worries me is what the harmonic content is. I was traumatized as a novice by a postcard from an O-O for that. Humiliation. My harmonics are showing!

[@Radionut63]

It's good to see people building radios from magazines again!

[@Novice1965]

Also, the screen bypass cap needs to be at the screen grid, as you have done it. Putting it on the other side of the screen resistor will make things worse, it might not oscillate at all.