Strange amplifier problem

Remember the amp cools quickly during the time it takes to replace a fuse and then once you play music again it may run cooler on whichever side of the amp (+ve or -ve signal) that is tending to breakdown.

Yes, exactly. One side still runs cooler.

Transl, FWIW it is possible that one or more of the tubes in the amp when it was blowing fuses had a 'bias' point which exceded the values of the bias circuit. A personal experience (on two different amps - same tube set). One amp had 'auto bias', one tube ran hot (plate turned red). Sub'ed in another set and this same location (V3) ran hot again. Sent the amp (under warranty) back for servicing. They reset the bias - the amp has worked fine ever since, but with different tubes - I thought the initial tube that tuned red might have been the problem.

Next I tried to use that same set in an amp with fixed bias. I reduced the bias pots to minimum as normal. Three out of the four tubes had a normal (minimum reading), but the tube that had turned red in the other amp registered 50ma on my meter with the pot in minimum position. I subsequently figured out that the correct bias point on this tube should have been 75 and it was somehow mixed into a batch with a 50 bias point.

How, or if that applies to your amp, I don't know. Just don't assume that auto bias circuits and tube bias points assigned by tube matchers are foolproof. I'm very carful now when I buy matched tubes to insist of bias points of about 40 and have had 0 problems since.

Or, it might have been an short caused by an undersized pin, or loose contacts in the socket, causing a short too (although with the latter it was always noise, popping etc, and was corrected by tighting the contacts in the socket.

Lost any fuses since you went back to the original tubes. BTW, what were the tube types and brand of the tubes you were trying to roll in?

FWIW.

The stock output tubes are Golden Dragon Retro KT66 (Shuguang?). Good tubes, slightly warm.
I also tried vintage grey glass GEC KT-66 tubes. As they are very old, I admit that though they were sold to me as NOS/NIB, some of them may have lost some vacuum or might have undergone other deterioration due to old age. They sound marvellous, more airy, deep and intimate than the stock tubes.
I also have clear glass GEC KT-66 tubes. Unlike the grey glass variety, they sound very dynamic, fast and articulate, if a bit harsh and clinical. Quite unexpected for GEC tubes (unless they are faked).
The stock tubes don't have such air and depth as the grey glass GEC KT-66 tubes, but sound very deep and consistent,

The amp also uses two 5687 driver tubes (in came with NOS Raytheion "orange paint" tubes which I replaced with NOS TungSol tubes) and one 6922 tube (I use Amperex JAN 7308 "green paint").

I did not have an opportunity to listen much with the stock tubes, but so far it does not blow the fuse.