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I personally don't use the variac trick---have not personally noticed an improvement on electrolytics using it. If you use it you should remove the horiz. output tube as there is not enough drive signal from the oscillator at the reduced voltage, therefore not enough bias voltage is developed in the HO tube and it overheats.
The hiss is probably from insulation breakdown, one likely culprit is the HV rectifier filament leads.
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