How to Troubleshoot a Guitar Amp. Many guitar amp problems can be solved by simple troubleshooting. Even if the amp cannot be fixed, often once the problem is diagnosed, it can be worked around for the duration of a show. The key is to troubleshoot the amplifier with a specific order of tests.
Switch out all cords to confirm it is not a problem with one of the lines. If the amp is a head, try switching speaker cabinets if practical.
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Check that all tubes are lit up and warm in a tube amplifier. If any are cracked, are cold or appear to have a whitish film inside, they need to be replaced.
Try using other channels of the amplifier. If they work, then the amp can be used on the other channels until it can be repaired. If not, continue the process.
Circumvent the preamplifiers by plugging the guitar directly into a power amp in or direct in jack. These are frequently found in the rear of the unit. If the amp works when the guitar is plugged directly in, the problem is with the preamplifiers or the jacks that bridge the preamp and amp.
Test the jacks bridging the preamp-to-amp connection by plugging a guitar into a channel and then plugging a closed dummy plug into them. Jacks to check include auxiliary in, preamp out, power amp in and the effects loop.
If the dummy plugs restore sound, the jack they were plugged into must be replaced. Otherwise, the amplifier must be examined by a qualified electronics technician.
Guitar amplifiers, even when unplugged and turned off, may still retain dangerous amounts of DC voltage. Never open an electronic device without proper training.