Post by petermetzger on Jul 30, 2014 10:50:50 GMT -5
Anybody know of a way to test vacuum tubes? I have a guitar amp that uses a 12AX7 in the preamp and when the amp is turned on it produces a loud humming noise. I think the problem is either in the tube or the power supply...
 I pulled the tube and it looks fine visually - not burned up or anything.
Post by petermetzger on Jul 31, 2014 7:45:47 GMT -5
Yeah, the silver part of the tube looks fine, I think this one might even be one that my brother pulled out of something else (it was his amp, we co-owned a snowmobile, fan belt broke while he was riding and the engine seized, he traded me the amp for my half of the sled. cool story right?).
It is a constant hum, 60Hz is plausible. I haven't put it through a tuner or anything but when I listened to a 60Hz test tone just now it's the same pitch (to my memory, anyway).
Post by petermetzger on Aug 1, 2014 12:56:12 GMT -5
The hum is much too loud to be a normal 'nothing plugged in' kind of sound; doesn't sound like the kind of noise you get from a ground loop either. That said, I haven't tested with a ground loop isolator. I think I should be able to make it to the meeting next week so I'll bring it along and see what can be learned via dissection. If I have time this weekend I'll pull it apart and have a look as well.
It had this problem in the past, sat for a long time (months) and then didn't have the problem. Sat for a long time again (years?) and now it's back.
Post by petermetzger on Dec 2, 2014 23:04:53 GMT -5
Who doesn't like reviving an ancient thread?
I got this amp taken apart a while back and found that the circuit board was burning up - there's a wide trace on it that was flaking apart and barely making contact - in fact when I touched it the whole structure that had bubbled up just fell apart. So I bet that's where my problem is. Short of sourcing a new board is there any way to repair the amp? Would it be possible to run a wire across the path that was previously printed on the board?