1) Before building anything I built an LTspice model to explore this very interesting circuit. My final 'as built' model is below. With thanks to Rob Robinette I decided to include the following mods in my build and leave the legion of others out there to play around with later:
+470k grid stopper to eliminate blocking distortion at the PI
+470k bleed resistor across first filter cap to dump cap charge in seconds when switched off, while consuming only 300mW when on(V2b)
+100ohm heater virtual center taps installed on V3 between heater and tube cathode to minimize hum
+2x 1N4007 diodes installed on rectifier tube socket to provide a backstop should the rectifier die in a bad way
+2x 470 ohm screen resistors to limit current spikes on the power tube screens
+3 grounds - one for cord ground, one for power amp including center tap and first 2 caps, and one at input sockets for preamp, pots, etc.
2) Using the LTspice model I explored how it responded to a series of signals of increasing p-p voltage to figure out what stages started clipping in what order. I learned that with volumes at 100%, tone at 90%, and signal applied only to the Bright channel, the PI clips first, followed by the preceding gain stage. I didn't manage to get the 6v6s to clip at all. Even the 12AY7 in V1 doesn't clip at +/- 200mV p-p signal, which is a lot unless you are using a pedal. I was surprised to see that a 12AX7 in V1 didn't clip either at those levels. The key takeaway here is that all the sweet harmonics generated by this amp would most likely be coming from V2, the 12AX7, so that would seem to be the tube that matters most, and the one to invest in.
3) The model shows that the tone control is most useful in last 25% of its range.
4) With a Hammond 290BX PT I'm getting 387V B+ from 120.2V house AC. With a Hammond 1790EP OT that drops to 380/378 at the plates of the 6v6s, which results in 14w dissipation per tube. All please be aware that Hammond 1790EP OT does NOT drop in to a 5e3 chassis easily as the hole spacing is off. Hammond doesn't agree, but pretty much all the chassis vendors do. If you have no experience drilling holes in sheet stainless steel, don't even think about trying it with chromed sheet steel - it's very hard.
I still have a ways to go as my cabinet is being built at Mojotone and my speaker (Speed Shop A12Q) arrives tomorrow. Thanks again for the excellent resources.
