Testing vacuum tubes?

[petermetzger]

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...

[edit] I pulled the tube and it looks fine visually - not burned up or anything.

[jimustanguitar]

There's a "flash" material to absorb O2 inside of tubes. It's the silver mirror-like finish on the inside of the tube. If it's a leaker, you'll see spots and oxidation in this material.

You may want to look at your filter caps if it's a constant 60hz hum.

Luckily 12AX7's aren't exotic or hard to come by. It's cheap enough to just replace them to test your theory. I've got a few sitting around somewhere if I go digging.

[petermetzger]

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).

[BeeAmaker]

Sometimes amps will hum when there is no input. Does it hum with an input as well? Have you tried a ground loop Isolator?

[petermetzger]

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.

[BeeAmaker]

Aug 1, 2014 12:56:12 GMT -5 petermetzger said:

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.

Check for bugs in the electronics - Literally.
Certain Beatles (roaches) and "Ball bugs" like the smell of electronics.

[petermetzger]

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?

[BeeAmaker]

Dec 2, 2014 23:04:53 GMT -5 petermetzger said:

Would it be possible to run a wire across the path that was previously printed on the board?

Yep, have done that many times.

It would be best to solder it at to points of contact, i.e. at the pad of a component. Be sure to branch off to other components if need be.

[jimustanguitar]

Which requires the soldering iron that you just gave back to me

[petermetzger]

Dec 3, 2014 7:57:27 GMT -5 jimustanguitar said:

Which requires the soldering iron that you just gave back to me

That, or I'll just have to get off my butt and come out to The Maker Hive again...