Constant buzz when warm

[bluesguitar]

I have a '74 bassman that is fine for the first half hour of playing, then it starts to get a staticy breakup, especially in the lower frequencies. Soon it becomes a constant buzz. Was wondering if this symptom rings a bell with anyone, and if you could give me a lead on where the problem may be? I replaced the filter and cathode caps with sprague atoms about 6 months ago, and the amp sounded great until recently.

[Gibsonman63]

I would check the plate resistors, especially the 470 ohm resistors mounted on the power tube sockets. I suspect that they act up a bit after they get cooked by the power tube for a while. Common problem in the older Fenders.

[Structo]

Ditto and try new power tubes

[Jana]

I am constantly warm when I'm buzzed.