WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
I have been having difficulty with int. low output on one CH.
it is not the speakers . When I swap output tubes the problem will
correct , but i do not think it is the tube because the problem will reoccur after a while, could it be the socket ?
It could be a bad socket or a bad solder joint on the socket or somewhere else in line.
When the joint heats up contact may be partially lost. Shutting down and switching tubes may allow it to cool sufficiently to make good contact again. Then as it heats back up the problem recurrs. The same thing could happen if the socket itself is somehow bad, heating the elements may force them to bend away from each other creating a poor connection.
Bad solder joints have sometimes occurred in my amp, probably because I use point-to-point wiring, I'm always making changes and the thing has become a bit of a rat's nest over time. Sometimes, what seemed a perfectly sound joint when I tested it can end up parting company and becoming just a contact, giving the sort of problem you mention. However, whenever that happens I can usually detect it by thumping the chassis, resulting in a crackle or worse, depending on which part of the circuit is affected.
ray_moth has a good point. Don't limit your search for bad solder joints to just the tube socket (though that is where I would start since I suspect heating is causing the joint fail). Look through the entire circuit.
Also if you have snap-in type caps I think I'd clean the contacts and reseat them.