If your tube amp starts developing issues such as noise or no signal, the first things to suspect are the tubes. While tubes provide a unique quality that is hard to emulate with solid-state technology, the trade off is that tubes don’t last forever. Their life expectancy depends on the amplifier, how hard you’re pushing them, and the type of tube. Most tubes have a 90-day warranty. Troubleshooting your tube amp before you take it in for repair may save you time and bench fees. Here are some tips from Sweetwater’s repair department.
1) Turn all gain, channel volume, reverb level and master level controls all the way off. If the amp still makes noise, the problem is very likely with the output tubes. Replace the existing tubes with a full set of matched output tubes. If you use the same brand and part number there should be no need for re-biasing. On models with rectifier tubes, if the new output tubes don’t cure the problem, try replacing the rectifier tube.
2) If the noise goes away when one particular volume or gain control is turned off, then the noise is coming from the preamp tube for that part of the circuit. Service bench techs can look up which tubes are in that particular area. One technique for finding the problem tube is to get one known-good preamp tube and substitute it for each preamp tube until the bad tube is located. If you’ve had your amp for a long time and used it heavily, it would probably be a good idea to replace all your preamp tubes.
Amp doesn’t power on:
1) First, check the fuse. Replace it if its blown and try again. If a replacement fuse fixes the issue, you may have experienced a power surge or some other event.
2) If the fuse keeps blowing, the output and rectifier tubes are the probable suspects. Preamp tubes don’t often blow fuses when they fail. Start with replacing the rectifier tube(s) if your amp has any. If replacing rectifier tubes doesn’t help, the output tubes are the next suspects.
3) If new output tubes or rectifier tubes don’t fix the issue, it’s time to get your amp into a shop for service. Save the old tubes because they may still be good. They’ll eventually need to be replaced, and you’ll always have spares.