Switch the "low power pre-amp" tubes from left to right and see if the fault follows.
10 responses Add your response
I agree with all suggestions. A tube swap out is usually the first step to see if the issue "switches" channels. If it doesn’t switch channels then something internally (not likely the tubes) is going on. Then possibly a fuse but it does sound like a thermal issue. Some of the technically oriented here will know what to do next.
Ok, I switched the input tubes but the problem remained on the same side. So it’s not the input tubes’ fault. I will now try the output tubes (although they all have the same glow and similar temperatures).
I wonder what other component may react to the gradual build-up of heat with switching off one side of the amp. Also - and please do forgive the question - is there anything else that can accumulate during use in the components of a tube amp? Something that would only trigger, say a fuse, after 20-30 minutes?
Sure there is. You could have a damaged passive component that is overheating or out of spec such that it is breaking down enough to trigger a protection circuit. You could open it up and do a visual inspection but it is unlikely you'll see anything. However, if you do not know what you are doing, it's best to have a pro look at it.