Thank you Enzo and g1 for inspiring the testing and OT substitution. More on results in a sec.
And my apologies to pdf64. I was "certain" that the screen overcurrent during clipping was caused by the zener. That was wrong. The screen overcurrent during clipping does happen without the zener as well. I must not have been paying attention to the right things at the right time during testing. I'm pretty embarrassed to be "that guy" already so please be gentle.
I'm here with my hat in my hand. This problem is beyond me. Thank you to those who have been kind enough to follow along. And if you're still with me I have some test results to share. I'll try to be concise and brief.
This is a schematic of how the power supply has been modified. My intent was to reduce hum. There is no hum at this time.
*NOTE: I have changed the HV rail resistor feeding the screen and preamp from 1K (as it has been in the previous discussion) to 5k which is keeping the screen voltage well below the plate.
Performance remains poor. With a 1kHz signal @ 70mV I get roughly 5W with the wave form just barely clipping the cutoff (figure a). However, compression on saturation and then an HF peaking rise steadily as the frequency is increased. With a 5kHz signal it is quite pronounced (figure b). Actual clean output with a frequency sweep is limited to about 1W.
That is problem #1. Perhaps/likely related to problem #2. Under overdrive conditions with a 1kHz signal there is a steady and dramatic increase in the spike to easily twice the cutoff or saturation levels (figure c). This is actually much worse as clipping or frequency is increased. As clipping is increased from 'a little' to 'a lot' there is an analogous decrease in current at the plate of the el34 from 50mA to 42mA and a dramatic increase in screen current from 3mA to 25mA.
The preamp signal is clean at the power tube grid to well beyond where the problems manifest. Full preamp output shows some clipping (not a lot) on the bottom of the wave form and no indication of oscillation.
Substituting the power tube for both a 6v6 and another el34 made no difference. Substituting the OT made no difference.
The amp was pitifully thin and quiet before modification but no extensive testing was done. The modifications were to upgrade the speaker and the power supply as shown. It's fuller sounding now, but not a lot louder. Not as loud as I know it should be and suffering the dangerous operating conditions under clipping.
I'm open to run any requested tests just as if I were a total noob.
I have a hair brained theory about what might be happening. I'm putting in the next post to keep this one on point and spare anyone that doesn't want to go there.