> I cut the signal lead and attached it to the signal ground (pin 10 on the HA-100x) instead of the ground pin on the tranny and voila! No more radio!
I don't have HA-100x pinout memorized. But it is surely a floating input. Ground may not be essential if you just use it right. Maybe CJ or one of the other old-iron geeks here can help you get it really-right.
I suspect, for general use, you want to UN-ground the center-tap on the input winding. If I read that right (check with a HA-100x expert) you are wired 150Ω and grounded center-tap, you want 150Ω or maybe 600Ω floating.
> a severe lack of high-end...
Pentode connects to first triode with the usual cap and grid-resistor, plus a 2C+1R network that probably is (as Mark says) a tape/film-head playback EQ network. Cut that where it connects to V1 Plate.
This may give you WAY too much gain. Turn pot R10 to minimum resistance. Clip the wire to V2a's cathode cap Can B-1 (but be ready to put it back if it breaks into a squeal). If still too much gain, just reduce V1's plate resistor. Tack a 100K across it as a test. V2a/V2b have so much gain, you can throw-away a lot of signal on V1's plate and still not be in danger of overloading V1. (It helps that V1 is a pentode, so distortion won't rise as plate resistor is reduced.)
OR: clip one end of C1, Screen cap, and short 1K from V1 screen to V1 plate. That makes V1 into a triode. That will reduce V1 gain from about 125 to about 15, a major reduction. But 1:10 in the transformer, 15 in V1, 200 in V2, and 6:1 in the output, is gain of 5,000 or 74dB, still a LOT of gain for most microphones.
You really have too many tubes there. (It looks like they needed a LOT of gain to make-up EQ loss.) You could just skip V1, feed the input transformer to V2a's grid, and be about right for most mike-work. Or put a 100K audio-taper pot after V1 in V2a's grid circuit and call it a gain control. Or wire both sections of V2 in parallel, which would give a better match to the output transformer, and work it as pentode input parallel-triode output with feedback from triode plate to pentode cathode. Classic 2-tube mike-amp. Or re-wire V2 socket to take another 5879 as triode to drive the output. Plate dissipation would be high, or current would have to be low.... you could resistance-load the output and capacitor-couple that to the output iron, which might improve bass response. So many ways to go.
