Champ 12 HELP.... major problem...

[teletwanger48]

I have a Champ 12 (USA made) and it all of a sudden started sounding like a busted speaker. I was playing at a low volume and it was sounding fine until then.

Other symptoms.. no contorol over any volume, no reverb.. no channel switching.

What I have done so far: swapped all the tubes including the power tube to known good ones.. no help..
Tried a second speaker that is in a cabinet that I use.. no help..

After thinking about it, I have decided to start with the Filter Caps in the Power Supply.. I would assume the voltages at the different PS points should be about the same as a 5F1.

Does that sound like a good place to start?
Any ideas?
Anyone with experience with these?

Thanks guys..

[Enzo]

5F1? They may or may not be, but I sure would not assume that.

http://www.webphix.com/schematic%20h...p_12_schem.pdf

What do you mean "start" with filte3r caps? Please don't just throw parts at the amp. It is broken, let's find the problem systematically and fix it.


Anything is possible, but I tend to doubt your filter caps all of a sudden just failed. When they do fail, we usually hear LOTS of hum, or fuses blow. They generally will not pull the volume down, and surely have no way to make controls no-functional.

Isolate the problem

Does it sound the same way in the headphones?

Your low voltage supplies steal voltage from the power tube cathode. SO check the cathode voltage at the power tube pin 8, says 27v, even close? And verify that pins 3 and 4 BOTH have B+ on them.

That cathode circuit is tapped half way down, and provides power to the reverb return JFET stage as well as the LDRs that enable the volume controls (channel switching).


Why would the cathode be void of voltage? COuld be a shot C19. Your B+ could be bad. If the screen voltage is missing - node C - then the tube won't conducts, and no voltage. B+ is a sort of doubler, so close to 500v on the plate and 250 on the screen. I guess that could be a cap, you may be right after all.

[teletwanger48]

Thanks for the info.. just exactly what I was hoping to get. I have a good bit of electronic experience, but not with amps and especially not with tubes. Have built a couple and a third in the works.. but I figure take what I know, what I can learn and try to fix it myself..

I do appreciate you taking the time to be so specific. I planned on checking the voltages at the filter caps as a start and then at the tubes. may have to come back at you with questions.

[teletwanger48]

Finally got around to fixing the Champ 12. When I got in there, found a burned resistor in the power supply. Replaced the resistor and all worked well.. for a few minutes and then the power tube started fading and sounding trashy. I powered the amp down and replaced the 6l6 and all is well. Not sure if resistor failure caused the tube to go out or the tube caused hte resistor. Both are replaced and all is fine now... sounds great again.

[Jazz P Bass]

10:1 the tube took out the resistor.
Being an active device (the tube) it can & will make demands on other components.
The resistor is a passive device.

[teletwanger48]

that is pretty much my thinking..... be hard to proove one way or the other but best thing to do is fix it and keep pickin.....