MT15 issues: very low volume, distorted sound, 0 mV bias. Help?


New Member
Dec 15, 2023
My MT15 served me well for the past several year, but the other night while playing, the amp suddenly lost like 90% of its volume. both channels sound unhealthy/distorted in a bad way. I swapped the power tubes with known working spares; same issue. tested the bias, 0 mV. couple days later tried it out again, same issue persists. anyone have any advice? thanks!
What Else Happened (If Anything) When The Tubes Went Bad? Any Fuses Blown? Red Plating? Did You Smell Any Of The Burnt Electric Smell? Any Discoloration In The Original Tubes (Smokey Fog Looking)? Any Other Unusual Noises Or Popping? Anything Else You Can Tell Me?
Nothing. No smell, noise, or visual (that I know of). It was in middle of band practice, suddenly volume was 90% gone. From what I can tell the tubes did not go bad. All tubes still glow. Swapped the power tubes with known working ones, same issue.
Moved the preamp tubes around to try to get the issue to at least change, but it stayed the same. Opened up the amp to check the circuit board, checked fuses with a meter, they did not blow, and can’t see any obvious points of failure visually. Guess I’ll bring it into an amp tech nearby.
Sorry to hear that it went silencioso ... just a few basic questions to start with.
- what did you mean with "tested the bias, 0mV"? Was there 0mV difference between tubes or 0mV when measuring?
- did you try swapping preamp tubes with known good preamp tubes? Not just mixing the same ones around. It happened to me and one of the preamp tubes was kaput, had to be replaced.
- did you read Rob Robbinettes tube amp troubleshooting bible ?

Thanks for the response, I’ll read that troubleshooting bible for the future. Good stuff to know. I took it into a shop. The screen resistors for the power tubes failed. They replaced both of the resistors and now it works.
Over 5 years, it was the original run of them from 2018. Yeah I’ve consistently used it for band practice with a loud drummer, so moderately cranked regular use. tho this thing for some reason is as loud as a 100w head haha.
Yep, it is loud and setting the master volume knob is sometimes a pain in the butt ... does your amp also go from 0 to 75% volume in a 5 degree turn?
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