just to add more issues it could also be the bias. I had an old Marshall that did this when it had been on a while. The heat made the bias drift drastically.
This is a common problem in older Marshall 50W heads (100W amps usually have a separate bias winding, although the 900 series uses a capacitor coupled supply).
It is caused by the electrolytic caps in the bias supply leaking current. Normally this wouldn't be a huge problem, but in a 50W amp the bias supply is derived from the transformer HT secondary via a large (typically 220k) resistor, so even a small amount of current leakage will cause a significant voltage drop across this resistor.
Worse electrolytic capacitor leakage increases with temperature, so if you are unlucky your amp can go into thermal runaway as the heat from the valves causes the caps to leak more current, which drops the bias voltage, which causes the valve to dissipate more heat which heats the caps further etc....
I often see amp where someone has reduced the value of the dropping resistor to compensate for the loss of bias voltage. This is of course the wrong thing to do.
The correct course of action is to replace the caps in the bias supply.
I ALWAYS do this before working on a 50W Marshall.
