It could also be a problem with the filament supply i.e. bridge rectifier, and not the high voltage supply. Hard to say without a circuit diagram.
Amp blowing fuses
I have a pair of quicksilver kt-88 monos and one is blowing 3A slo-blo fuses instantly on powerup. I have swapped all the tubes from its mate and looked at the chassis wiring which is discrete, point to point. It looks clean with no hints of overheating or trouble. I've thought about trying a 4A fuse but wanted to get opinions as maybe one of you have had a similar problem... thanks for any help, opinions etc...