[codefox1]

I believe the bias cap across 33 and 34 should have + side to chassis, - side to transformer center tap so as to provide a source more negative than "ground." All to many times the symbols are drawn incorrecltly but looks to be correct on this schematic. Don't know if set would play at all if this one get put in backward. Easy enough to check.

I'd check the voltages on 78, if voltages are OK, pull it out and see if you still have hum. I am assuming the hum varies with volume control, and that control is known good. Then there is the possibility of a component touching the filament wire. I'd have a good look for that, maybe poke around with a chopstick.

Let us know.