Ask Amp Man: Troubleshooting a Peavey Classic 50

Hello Michael,

Peavey’s Classic Series represents some of the best amps the company has produced, at least for straight-ahead rock ’n’ roll. Sorry you’re having problems with yours, but let’s see if we can make it better by following some routine troubleshooting steps. These are all pretty generic and can be applied to most guitar rigs. (I know you’re on a fixed income, but the second half of this troubleshooting exercise requires you to obtain two substitute tubes, as I’ll explain later. But first, let’s see how the first half of the procedure goes.)

You mention that you have five pedals in your setup, and that the popping occurs whether the pedals are on or off. The problem here is that if the pedals are not wired for true bypass, just because they’re off doesn’t mean they’re out of the signal path. Many pedals have a buffer circuit that the guitar signal runs through even when the effect is disengaged. One by one, remove each pedal from the signal chain and see if this alleviates the problem. If you find the culprit, great. If not, we need to try them without the AC adaptor. This is easy on the EHX units, which can be battery powered. Remove the TC units from the signal chain for now, as they don’t accept batteries. Load up the EHX units with 9V batteries (the inexpensive “heavy duty” ones work fine for this test) and see if the problem is resolved. If so, great—now we know it’s the power supply. If not, we need to move on to the amplifier.

First, make sure the reverb control is set to minimum. You mentioned that the internal reverb doesn’t work, but there’s a chance the reverb recovery circuit could be making noise, especially if its input stage is unloaded due to an open transducer in the reverb tank output.

Now we’ll rule out each tube as the noise source. On most amplifiers I’d simply have you remove one tube at a time. But for this particular amp, you need to acquire one known good 12AX7 and one EL84. That’s because Peavey wired some of the tube filaments in series in these amplifiers, so removing one tube can prevent the filament in another from lighting. (Kind of like an old-fashioned string of Christmas lights—one goes, and they all go.) To see if a particular tube is causing the noise, you need to remove it and replace it with a known good one.

One more point regarding safety: On most amps it’s okay to perform tube substitutions with the amp on standby, but on these particular amps you should turn both the standby and power switches off prior to swapping tubes. That’s because there’s always high voltage applied to the output tubes’ plates, even when the amp is on standby. (Yet another anomaly from the guys at Peavey.)

Start with the preamp tube closest to the input jack, removing it and replacing it with the known good substitute. If the amp no longer pops, you’ve found the cause. If not, replace the original and move to the next tube down the line until you’ve gone through all three preamp tubes. If you’ve had no success, move on to the output tubes. Remove them one at a time, substituting the known good EL84. Hopefully one of these will be the cause of the noise.

If these steps don’t expose the cause of the noise, I’m afraid I’ve exhausted ideas for repairing it yourself without a tech’s help. But hopefully that won’t be the case, and we’ll have removed the cacophony from your Classic.