[morzh]

Oh one more thing....your transformer may simply buzz. I have this in my 70. There is not much one can do about it short of disassembling, re-shimming, varnishing, clamping the shell (the latter I tried to no avail).

Could it be a failed tube? Possibly it could. Though I hadn't seen a tube acting like it and I would have to understand how a class A single tube amp can produce hum due to the tube's parameters' changing. But...all is worth checking.

One more thing: try to change the 80 tube (and check it). You have full wave rectifier with the center tap, and if one half starts going weak and another is strong that can produce hum due to increased ripple.