While on the subject, and really just for interesting discussion, for many years I have toyed with the idea of a some what dynamic audio control circuit for the screen of high level modulated tetrodes. The circuit would be capable of adjusting the DC screen voltage while also with a separate adjustment set the level of audio on the screen with reference to the plate and do this with a minimum of phase shifting. I don't have the time to experiment with this and it may be some time before I run tetrodes in the final again. But this is what I have come up with so for. It sort of resembles the clamp tube modulator, but don't confuse it with that. It can operate as a clamp tube but its main purpose is to regulate the audio by means of the plate to grid feed back. In the circuit below,
http://wa5bxo.shacknet.nu/HAMPICTURES/audio-reg-clamp.GIF the amount of audio that is fed back to the grid of the tube cause the effective plate resistance of the tube to change. That is if the grid is bypassed to ground by means of C2 where R2 is very low and R3 is high then the tube will have a very high plate resistance. And the audio developed on it's plate will be near 100% of the DC that is there. On the opposite end if R2 is made high and R# low then a lot of audio is coupled to the grid and the effective plate resistance, as for as audio voltage is concerned, is very low causing a large drop in the amount of audio that can be develop on the plate while the DC voltage stays where it was. Equivalent circuits below: http://wa5bxo.shacknet.nu/HAMPICTURES/audio-reg-clamp2.GIF This equivalent circuit works quite well on low level rigs like a 6146 final but would require much larger components for a big tube XMTR such as 4-1000 or 4-400s so I am looking for a way to do the same with some control and let the regulator tube take the blunt of the dissipation. What do you guys think and does anybody want to play with it. I'm thinking some thing like a 4-65 for the tube?? John Coleman
