This is another one from the slightly odd repairs file. Here's the reverb pan from a Peavey Classic 30 which came to me with a complaint that the reverb was "humming and feeding back".
When the reverb knob was turned up past 3 the amp made a pulsing howl which got louder the further the reverb knob was turned up.
Suspecting some magnetic coupling I took the reverb pan out of the amp, leaving the wires connected. By orienting the tank in different ways the problem could be greatly reduced but I couldn't get it to go away completely.
Disconnecting the RCA plug from the input jack on the reverb tank itself didn't affect the howl at all. Reconnecting it and disconnecting the outplug plug stopped it completely.
The quickest next step would be to swap out the tank to see if it was bad. Below is the Accutronics reverb tank part number 4EB2C1B printed on the tank itself:
Fortunately I happened to have another on of these around the shop. I plugged in the substitute and the amp was back to normal with a perfectly working reverb.
Now here's the odd part (and the reason I'm bothering to post such a run of the mill repair). I always try recheck once a repair is finished to make sure what I've done has actually fixed the problem. So for good measure I plugged the original tank back in. Now the amp worked fine with the original tank! Even though I'd plugged and unplugged the orginal tank a number of times somehow the act of plugging in the replacement tank caused the original tank to make a good electrical connection. I've seen oxidized or poorly toleranced plugs cause reverbs to stop working or have intermittent signal, but causing this howling was a new one on me. It just goes to show, it always a good idea to clean jacks and plugs even if you don't have a reason to suspect them.
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