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An update on how this problem has been solved: Changing the 6C4 had no effect (I was pretty sure I tried that in the past). Somewhat to my surprise, increasing the value of one of the bypass capacitor on the B+ feeding the HV supply helped a little. So did increasing the B+ supply voltage to the HV module. However, none of these steps was sufficient.
Somewhat to my surprise, adding a small muffin fan inside the HV box, and raising its lid 1/4" with some spacers for better ventilation to the outside world, has proven to be a definitive fix. After doing this, I was able to lower the B+ feeding the HV box to its original value. I left the bigger bypass cap in place, not sure whether it is really needed now or not.
Adding the muffin fan was done in a reversible manner, so although it's not a very "historically correct" fix, it's not as though any permanent change has been made.
So now the HV seems to be working just fine on this set.
It sure does have a finicky RF/tuner section. Another project to figure out one of these days...
Tom
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