Jim,
Thanks for your reply.
The Gonset G-76 method of modulating the 6DQ5 PA screen with a VR tube and voltage divider had escaped my attention. I
had noticed that the 6DQ6B modulators were screen driven. I guess should have looked harder its overall clever design - and I will. It's interesting.
I briefly fooled around with using a mix of modulated and unmodulated screen voltage on my test bed DX-100. The lash-up didn't work well at first and I didn't pursue tweaking the ratio at the time. (I'm in the process of trying sweep tubes in its modulator using the "crazy drive" method of proportioned drive to the screen and control grids. If it works out well I'll publish the results in this forum.)
Getting the right amount of modulation to the screen appears to be the primary issue. Perhaps there isn't much difference between, say, a 6146 and a 6DQ5 in this regard. I was wondering if there were some inherent tube characteristic that made the job easier. I couldn't find anything about this in the literature - maybe it doesn't matter all that much.
Screen bypassing and screen protection seem to be two problems that just won't go away. They need to be addressed in any good design. Screen self-modulation requires a big chunk of iron which I prefer to avoid. Resistors are cheap and plentiful. Naturally, I'm trying to find a simple solution to a complex problem, but one that works well. Easy, right?
So, I'm going to file your report as a positive one for a sweep tube final given your thumbs-up for the G-76.
I'm looking at the application of sweep tubes as a plate modulated final but there aren't that many examples in the wild to draw insight from. Given their high emission vs low plate dissipation ratio they may not be the best choice for that service. But many of them can be had for small money, so it's worth looking into. Paralleling a few to achieve moderate power levels may produce its own set of problems. I'll need to give physical layout due thought and experimentation. I plan use sweep tubes as modulators but the RF final tubes are still up in the air. Maybe it will be two or three 4D32's. Maybe half a dozen 6LW6's. I could say that it's a blank sheet of paper right now, but the truth is it's many pieces of paper scattered about the shack. Either way, it's undecided at this time. This is all planned as a follow-on project after my DX-100 test bed experiments.
Don