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Post subject: Late '80s Champ12
Posted: Tue Dec 20, 2016 1:41 am
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I've got (as the title suggests) a late 1980s Champ12 full valve amp with what appears to be an odd problem.

When I turn it on it warms up, overdrives briefly (about a second - normal) and seems to work normally. After about 3 or 5 minutes it stops working (no sound coming out). The power valve is still "on" and hot.

Has anyone else had this problem, and is/was it an easy fix?


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Post subject: Re: Late '80s Champ12
Posted: Tue Dec 20, 2016 4:58 pm
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Joined: Fri Feb 05, 2010 4:31 am
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Location: Province de Québec, Canada
Not sure what you mean ;

Schematic ;

http://el34world.com/charts/Schematics/ ... _schem.pdf


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Post subject: Re: Late '80s Champ12
Posted: Tue Dec 20, 2016 8:42 pm
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Welcome to the forum.
The glowing filament usually stays hot even though the tube has failed.
You seem to have an overheating problem.
Just as a car will operate before it gets up to full temp and only then will it overheat and blow steam, your tubes are the same situation.
So I have a 2 minute rule. If it works for a couple minutes and then craps out, its probably a tube problem.
When the amp first starts warming up, thwack the side of the tubes with the flat of your fingernail. If it sounds springy or flashes, replace them.

Its slightly possible its a socket problem.. After warmup, grab the tip of the small tube and rotate it in a 1/4" circle to clean the socket pins. If you hear crackling when you move the tube, continue the rotation until the pins clean up and get quiet.
If they won't quiet, the socket may meed to be retensioned or resoldered.
Tap the power tube the same way and wiggle it a little by its plastic base to ensure it has good quiet connections as well
.


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Post subject: Re: Late '80s Champ12
Posted: Tue Dec 20, 2016 9:03 pm
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TimsAudio wrote:
So I have a 2 minute rule. If it works for a couple minutes and then craps out, its probably a tube problem.
When the amp first starts warming up, thwack the side of the tubes with the flat of your fingernail. If it sounds springy or flashes, replace them.


+1

The Champ 12 is a class A, single-ended design. As such it's notoriously hard on its power tube due to the 100% duty cycle. Drop a fresh 6L6GC in it then observe and monitor what happens when you switch the amp on.

Arjay

_________________
"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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