amp that starts to hum and slowly gets louder and louder

[toner]

ive got a new build supro with a pair of 6973 tubes. amp works fine, but after a few minutes of being "on" a hum slowly starts creeping up and gets louder the longer its on. after about 15 minutes the hum is loud enough to tell something isn't right so i turn it off before something bad happens. i thought maybe it was the 250ohm cathode bias resistor on the power tubes since it was getting hot, and i replaced that with a 20w resistor and that clearly wasn't the problem. then i thought maybe one of the PI caps was leaky so i pulled the 6973 power tubes and left it on for 15 minutes. set my meter to 10vDC and couldn't get any reading on the 6973 side of the caps so i guess that rules out leaking caps? i haven't replaced the bias cap in parallel with the 250ohm 20w resistor, just figured it was new. i did take the amp to a gig and it blew a fuse. i checked it out and one of the power tubes bases cracked and it de-gassed which blew the fuse. anyone got any other ideas how i can troubleshoot this one?
thanks!

layout attached

[pdf64]

Confirm that your power tube control grids have the expected ground return resistance.
Confirm the power tubes cathode resistance to ground.
Power up and monitor the cathode and B+ voltages.
Pete.