I measured the screen current of the 4-65A regulator tube. At 540V and zero load it was 15 ma which is within the maximum 10W screen dissipation rating, but unnecessarily high. A 22K dropping resistor reduced it to only 4 ma and had no noticeable effect on regulation.
I also found the curves for 4-65A and 4-125A online. Even for running class AB
1 it looks like 540V on the screens might be too high for lowest distortion. So I jumpered one of the 100V zeners in the regulator string and now have 450V. (It doesn't change exactly 100V since the screen voltage is also being reduced and so the tube doesn't conduct as hard for a given grid bias).
Now to find $129 for the last vacuum cap and I can put this beast on the air :evil:
TopBottom