1974 Quad Reverb Trouble

[apehead]

Hello again,

The Quad I "repaired" is acting up again! It works trouble free most of the time, but every once in a while, it will suddenly start humming very loudly (sounds like 60 Hz). Guitar can still be heard through it while this happens. If the amp is shut completely off and then re-energized, it goes away. Also, there is a very faint fuzzy type of sound intermingled with the output audio. At first we thought is was a speaker problem, but the amp makes the same noise when plugged into known good external cabinets as well. I think this small noise has been happening for as long as my friend has owned the amp, but the loud humming is new for sure. Could anyone help me diagnose these problems?

[Bruce / Mission Amps]

That is way too general of a complaint to have much luck with.

... have you opened the chassis?
Look to see if the screen resistors are burned or if they power tubes are all functioning correctly, or if the tube sockets are dirty.. etc etc etc....

[apehead]

Blast! O.K., I'll open 'er up and have a look see.

[Enzo]

And in general, speakers are passive things. They can rub and vibrate and distort and sound crappy, but they won't add sound of their own. In other words, a speaker will never be the source of a loud hum coming out of itself.

[apehead]

I meant that the soft, fuzzy sound, not the loud buzz

[Enzo]

Oh.