Champion 600 (basically an AA764 circuit) cuts out

[khelstrom]

I have one of these new little amps and recently it has started cutting out when I hit the strings on my guitar hard. The only thing I have changed in the amp so far are the tubes. I replaced the 6V6 with a NOS RCA blackplate and the 12AX7 with a JJ. The amp cuts out completely when I am plugged into the high gain input with the volume at 3:00 and I hit the strings hard. Ive also noticed that I can get the sound to come back by tapping the 12AX7 with a pencil. Now Ive tried swapping in other 12AX7 tubes which In know to be good but the symptoms persist. I then put those 12AX7s in another amp I have and they were fine. So at this point I believe I have ruled out tube failure but I'm not sure where to go now. If someone could help me with this I would appreciate it greatly.

Thanks,
Karl

P.S. I have a PDF of the schematic if that would be helpful.

[Steve Conner]

If you go back to the original tubes it came with, is the fault still there?

[Enzo]

Remember also that tubes don't operate in a vacuum - pun intended - they sit in sockets. Sockets can fail too.

[khelstrom]

Yes it still happens with the original tubes. Ive also checked all the ground points and the plate resistors and all are fine. I guess Ill check the tube socket next.

[khelstrom]

the socket checks out fine. Ive also noticed now that when it goes out I can get it to come back by tapping basically anywhere on the PCB or the chassis. Seems like a bad connection some where but where? And why would it cut out when I hit the strings hard? Ugh!

[Steve Conner]

Sounds like a bad connection for sure, then. I'd try resoldering all the pins where the socket fixes into the PCB.

Hitting the strings hard makes the speaker flap hard and vibrate everything.

[Jag]

Did you retension the pin connections in the socket. If all the solder connections are fine, you could have a loose connector.

[Enzo]

For that matter, resolder the controls and jacks. Just look the solder over real close under the board.

[khelstrom]

Problem solved!!!

I narrowed it down to the input jacks. They seemed faulty so I ripped out them and the PCB they were attached to along with the volume control. I then noticed how the chassis was painted and the place where the jacks were to make ground contact must haver been taped up when painted and when the tape was removed a residue was left behind possibly causing the bad grounding issue. Any-how, I cleaned off the residue with acetone and rebuilt the inputs with real jacks. I guess this is what I should have expected for an amp made in China!