norton wrote:
I found that my bias voltage readings were WAY off when I had a 12ax7 in the phase inverter position... as in 47 to 62mv's set to 6v6's. Changing the tube to a 12at7 the voltages dropped way down, but I was barely able to get past 15 mv's with the pot maxed out.. with something like 6 or 7mv's being the cold side of the spectrum. Does this sound off?
The PI tube should make absolutely no difference to bias voltage. The two 47n (0.047uF) caps, C15 & C16, on the power tube grids block all DC voltages from the PI. So what you're saying suggests that either you've got a leaky cap or two there, or else something is slightly shorting across one or both of them.
norton wrote:
Also, with the 12ax7 in the phase inverter slot, the Presence knob had an audible thhhhwwwwickkk... and a sharp increase in background noise as I move it from 2 o'clock to full bore. The normal operation of the amp is very quiet, otherwise. This problem has been 90% alleviated by switching back to the 12at7... Maybe I've got something screwed up in that area?
Something' not quite right there either. I don't know if it's related to the biasing problem, since negative feedback/presence also involves the PI. So I suggest you try fixing the first problem first. I suspect you may have some parasitic oscillation problem with the higher gain of the 12AX7, in which case you may need to play around with the amp's lead dress - try moving some wires around with chopsticks.
norton wrote:
Also... Turning the amp off with let's say a chord ringing out as the amp powers down..... should I expect the sound to signal to get nastily distorted as it drifts into silence?
You should get a heavily distorted chord fading out. Not sure about "nasty" - that's a very subjective term. In any case there's at least one fairly serious problem in your amp that needs fixing.