Champ 12 Reverb Issue

[dpm309]

Picked up a 1999 Champ 12 recently in working condition with the exception of the Reverb and input jacks. The amp has a Jensen P12N which alone is worth more than I paid for the amp. Will replace the jacks and tubes but I cannot find how to adjust the reverb. These use a cheap little tanks mounted in the chassis with a trim pot on the board to make adjustments. The PCB needs to be removed in order to access the trim pot so there is no easy way to adjust while running the amp. Any suggestions? Did a search on this and other forums with no luck.
Thanks,Dan

[sluckey]

It's a pretty simple foldback reverb circuit. The tank is driven directly from the OT secondary. Q1 and V2B are the reverb recovery circuit. The output of V2B is fed back into the grid of the output tube. I doubt you need to access the trim pot. Check the tank first. Input should measure a few ohms. Output should measure maybe a couple hundred ohms. Open circuit is definitely bad. Then just trace the signal thru Q1 and V2B. Probably gonna have to get intimate with the pcb.

You can substitute a standard Fender tube amp reverb tank. Probably need some gator clips to make connections.

[dpm309]

The tank checks out fine, am getting about 700 ohms on the output and about 6 on the input. Will wait until I get the new tubes and recheck the circuit. I will set the trim pot at the middle and hope for the best. Don't know if it is worth it to install a new reverb tank. I have built a couple of reverb pedals using the Belton/Accutronics brick that sound almost as good as the Fender reverb.

[shooter]

Probably gonna have to get intimate with the pcb.

If you go this route, I have been known to bore holes the Mechanical Engineers didn't think about.  I've usually done it to the chassis since pcb's can be many layers deep

[92Volts]

I dealt with one of these but fortunately didn't have reverb trouble...

The tank is cheap looking/feeling as you've probably noticed, and I wouldn't be surprised if there was a loose wire between there and the board, or inside the cardboard box that houses the reverb. IIRC both input and output are soldered with flimsy wires, not connected with RCA jacks or anything like that.

If you didn't disconnect reverb output from the PCB, you'd get a non-open reading because you're measuring components on the PCB, and haven't confirmed the tank is good. If you did disconnect it, your reading indicates the tank is good (not open circuit) but you haven't ruled out a bad connection to/at the PCB, which I'd suspect next.

Because reverb is injected into the circuit before the location it's sourced from (the OPT), if there's too much you'll get feedback. That's why the trimpot is there. If you turn it too high it will hurt your ears but it won't break anything.

[sluckey]

You can substitute a standard Fender tube amp reverb tank. Probably need some gator clips to make connections.

My statement about subbing a standard Fender tank was meant as a way to determine if the tank is the problem. It wasn't meant as a permanent fix.

[dpm309]

Installed 2 new input jacks and new 12AX7s and now the reverb is working perfectly. I did find a solder bridge on the board where the input wires are connected. This must have been the problem. Anyway, this amps sound great especially with Jensen P12N. Looks like I got my money's worth.Dan