Hey Braintheory, these are beastly. Kind of a cowboyed design. Over 600-650VDC! An even a higher level of caution is needed. You're gonna need some very good KT88's or 6550's. The high voltage is really a problem. I would definitely get a 10 amp variac. Or look into a heavy duty power scaling solution.
You will also probably need a filter cap job (ARS) and other misc upgrades. Majors like to arc at the output tube sockets. Once they have, you have to change out the socket.
Don't break that impedance selector. They are irreplaceable. They go for a few hundred dollar on ebay. Mine was hard wired for 8 ohms. Majors can destroy a 4x12 full of 25 watters in a hurry. But really, those switches are shit if it doesn't fit tight. It is considered hackery, but I think swapping in a modern rotary selector in a major is justified.
Distortion at low volume is impossible without mods. Mine had a cascaded preamp / master volume mod and was awesome at any volume. Same as Michael R/T's from the clips here. Fuses wouldn't have anything to do with it, but should be checked to verify the values and type. There was a rumor that somebody was killed pulling the fuses on a major.
Rich Blackmore's tech used to post on forums. He would say that boosted preamps of any kind are not recommended. Pedals or cascades. Causes power amp failure. Mine lasted 20 years cascaded with GE 6550's.
Post some pics. Is it a clean stocker?
