Tube amplifiers are easy to troubleshoot, but without schematic it would get very difficult.

Tube amps was all we had way back, I even designed and built my own in the 60′s.

Telling you exactly what this could be, would be blowing hot air and that I don’t do.

The symptoms mentioned could point to a bad electrolytic capacitor, sometimes you can spot them when you open the thing up and see a deformed (they get bigger) capacitor where some corrosive juice has leaked out.
It could also be a bad tube that has a shorted grid after a while, when it get s warm. If you have the schematic you could scan and e-mail it to me, I could point out a few things where to look for.

++ Additional hint :

Bad tubes can sometimes be detected by carefully tapping a pencil against the glass, they might then fail or start to function again, also tubes can become ‘microphonic’ which means the tapping of the pencil can be heard in the speakers.

If so, replace the tube. If you need tubes and can’t find them e-mail me, I may be able to point out sources where to get them.