Just in case you didn’t, did you check the power cord for nicks? If the male and female ends are operable did yah look there too? This is 99.8% sure not to be it.
Did you switch cords? Tap tubes for microphonics? Swear a lot?
10 gauge power cord. Too much power for tube amp?
Tube amplifiers tend to be sensitive on incoming voltages. Is there a chance a bigger gauge power cord like 10 gauge may not be a good thing?
My amplifier tends to shut down occasionally upon start up. maybe moving to a higher gauge might be better. Does it matter?