Issue With Newly Built Crack

[ihrm]

The crack was my first real project, and I got through it with no real problems. All the voltages and resistances check out. But now that I'm using it, every so often its like the sound disappears into the back ground. Its hard to explain, but the low and mid frequencies are distorted and far in the background while I can still hear the highs are still quite audible but kind of distorted. I have no idea whats going on, any help would be appreciated!

 

[Grainger49]

Sounds like the heater circuit cuts out.  Do the tubes glow when this is happening? 

Otherwise I don't have a clue.

 

[ihrm]

As far as I know they do, I'll double check next time it happens. I usually turn it off when it happens as it sometimes fixes the issue, but I can't figure out any rhyme or reason to it.

EDIT: They do continue to glow

 

[Paul Birkeland]

This sounds potentially like a bad solder joint that is responding to heat as the amp heats up.  I'd reflow all your joints and this ought to go away.