I got the idea for my homebrew rig after watching Brian's video on the TMC T-368 exciter. IIRC, he ran it into a 6146 buffer/amp, then onto the final. I promptly found a T-368 exciter & plan to use it & a 6146 to drive PP 250TH. ( Haven't worked out all the details yet ) Over on the TMC list, there is a discussion about running a TMC SSB rig in "AM" mode.
( which, their particular model wasn't really designed to do.)
Someone posted the following to the TMC mail list recently;
"... An old buddy of mine devised still another solution many years ago. He added a small plate/screen modulator to the 6146 output stage of an SBE-3 exciter. A small modulation transformer which he improvised was mounted inside on the rear apron of the exciter, and provided an 8 ohm input impedance for a small external audio amplifier... In retrospect, he considers the mod a debauchery, but it worked great."
Modulating the exciter seems to me to be counter-intuitive. Inserting audio, at that point, with the 6146 used as a buffer, may be no problem. Reading up on different modulating schemes today, starting with the ones from Brian's old site, I see that low level grid modulation is possible. But, there's a trade off... efficiency and / or the ability to reach 100% modulation.
I have an old audio amp capable of +/- 400 watts, my best guess, depending on how I run it... ( 8 X 211A PPP ) and a NOS, RCA modulation xfmr fit for the job of high level plate modulation. So the question is, modulate at a low level or high level? Pros & cons?
BTW: I have separate B+ supplies for the audio and RF sections.