Fender Princeton Reverb starts humming loudly after 15 min.

[Redline]

This Princeton sounds great for about 15 minutes and starts humming insanely. I changed the tubes, unplugged the reverb and checked all resistors and caps. I think it's the big power supply can but I'm not sure.

[Leo_Gnardo]

I think it's the big power supply can but I'm not sure.

Was your Princeton made in 1960's -70's? Hi voltage filter cap failures are common. FWIW I replace all 4 sections with individual caps. One more - bias filter cap - replace that too.

[xtian]

Is a power tube red plating when the amp hums loudly? Has the bias been checked?