The bias resistor(s) might have been burned out by a tube that went cherry-red. Look for visible burn marks on resistors. Or tube that won't bias is bad; replace. Then re-bias other tubes.
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If an output tube cannot be biased within its correct operating range, remove and replace it, don't continue to run the unit with that tube in place. If the the interior power supply fuse blows (not one of the externally accesible output channel fuses), do not run the unit again until it has been professionally diagnosed and serviced, and do not reuse any of the current tubes again, replace them all. This advice is based on my experience with my MV-55 and instructions from c-j techs, but don't take the word of online forum respondents about something like this, give c-j a call!