What would cause a Blues Jr to sound terrible after 30 mins?

[Oldmactech]

Friend has a Blues Jr. I recapped it with F&T,s last year and he just replaced all the tubes.
Jamming the other day and about 30 mins in sounded distorted and “honky”. Reverb cut in and out a few times. He did have a couple of pedals in circuit but on/off still sounded terrible. The next day I played my guitar without pedals through it for 20 mins with no problems. Reverb worked.

Anything it could be?

[mozz]

There are resistors for the -15 +15v supplies that often have to be resoldered to the board due to heat.

[nosaj]

Ribbon cables?
nosaj

[g1]

There are resistors for the -15 +15v supplies that often have to be resoldered to the board due to heat.

Same goes for board mounted tube sockets.

[Chuck H]

He did have a couple of pedals in circuit but on/off still sounded terrible. The next day I played my guitar without pedals through it for 20 mins with no problems. Reverb worked.

Well the reverb cutting in and out probably eliminates the pedals anyway. So now the trouble is getting the problem to manifest so it can be diagnosed. Since the problem is intermittent I'm inclined to think it's a contact rather than a component failure. So fire the amp up again and smack it up some just to see if you can piss it off. If that induces a problem it confirms an intermittent contact. But if the amp behaves after getting slapped around it doesn't exclude an intermittent contact. I've had intermittent contacts that were frequency sensitive. To the point where smacking the amp caused no symptoms but the amp would misbehave any time certain notes were played through it loud. I had a hell of a time diagnosing that one. Actually I think it was Enzo that ultimately diagnosed that one. I was almost resigned to the idea that an evil specter was living in my amp.

[Mick Bailey]

Get the board really warmed up with a hairdryer. I'm also thinking the +/-15v supplies are suspect with the reverb cutting out.