Silent amp - where to start?

[Tdale]

A friend of mine has a Vox tube amp that's gone completely quiet. He played it one night, shut it down, brought it home, and the next day it didn't work. He didn't hear any noise/squeel when he played it.

All the tubes glow.

Where would you guys start looking for the problem? I'm thinking OT, but I don't think that's very likely, since it didn't blow...just died overnight.

Any ideas?

Tommy

[martin manning]

http://www.geofex.com/ampdbug/ampdebug.htm

[ampgeek]

Hey Tommy,
I always start by measuring voltages at every tube pin. If that doesn't flush out the issue, I then "walk" through the circuit using an amp stethoscope.

This approach has NEVER failed me in identifying where the major problem(s) lies.

A description of the stethoscope/listening amp tool can be found here at the bottom of the page:

http://www.el34world.com/Hoffman/tools.htm

It is, without a doubt, the best little tool in my kit. It isn't a substitute for an O'scope. But...perfect for the hobbyist like myself.

Good luck,
Dave O.

[Bob-I]

I'm thinking OT, but I don't think that's very likely, since it didn't blow...just died overnight.

Any ideas?

Tommy

That's like thinking you're dying of lung cancer when you cough.

Nah, more likely it's a bad tube. Even though the heaters light a tube could be bad.

[Tdale]

I'm thinking OT, but I don't think that's very likely, since it didn't blow...just died overnight.

Any ideas?

Tommy

That's like thinking you're dying of lung cancer when you cough.

Nah, more likely it's a bad tube. Even though the heaters light a tube could be bad.

Hehe I mentioned the OT, since it's so quiet, not even a small hiss... but I guess a bad tube will do that too.

Tommy

[martin manning]

...I mentioned the OT, since it's so quiet, not even a small hiss...

I'd start by making sure that the speaker is still connected to the OT. Wouldn't be the first time the wires got pulled when the amp was being transported, and you really want to have the speaker load connected if you have to look further.

[Tdale]

The amp stethoscope looks really interesting! Maybe I'll finally get to use my old Peavey Pacer...

Tommy

[Jana]

Check the fuse for the B+ if it has one.

[Cygnus X1]

On testing the output transformer...since I don't have a filament transformer I connected a portable radio to the
OT primary.
From the headphone jack.
Connected speaker to the correct jack.

It pushes the signal, although it is weak.
I guess that tells me the OT is good?
Or does this not mean anything at all?

I must emphasize that the amp was unplugged and discharged when doing this, the OT was connected and the tubes out of the amp.

[billyz]

Check the speaker. Sometimes it is the most obvious.

[Structo]

Perhaps the amp has an internal HT fuse that let go when he turned it off.
You will have to open the amp to find it.

[tubeswell]

See the chapter below for a description of troubleshooting techniques