Really sounds like a power supply problem, could be as easy as a bad connection inside the amp or as bad as a fusible link buried inside the primary winding of the transformer.
I don't know how comfortable you are working around electrical wires but I must warn you....THERE BE HIGH VOLTAGES INSIDE THIS AMP! AS MUCH AS 500V TO THE TUBES DEPENDING ON THE DESIGN, IF YOU ARE EVEN A LITTLE UNSURE ON DOING REPAIRS, TAKE IT TO A SHOP! THESE VOLTAGES CAN AND HAVE KILLED!
Also, just because an amp is unplugged doesn't mean that it is safe, there are capacitors inside that keep a voltage for a surprising amount of time.
If you are comfortable, look for 120v going to the transformer primary, the side of the trans. wired to the line side, if voltage is present, probably problems with the transformer. If no voltage to the trans. trace back to the cord step by step. This may show you a problem you can fix like a bad or burnt connection.
Me, I'm really stupid, I have actually fixed bad fusible links on transformers and such but I'm REALLY cheap and had to do it for a gig......
When I die, I want to go peacefully in my sleep like my grandfather, not screaming......
like the passengers in his car.