[DaveWM]

note this set uses a voltage doubler, so if B+ is low I would start there. Also while I doubt its a problem, there is a Thermistor in the B+ circuit, just in case you may want to check the voltage drop across it after it warms up.

If the cans are orig, see if they are warm (remember on doublers one of the cans while have voltage on the case with respect to the chassis, should have a cardboard cover).

Confirm line voltage ok (I go to the easy stuff 1st).

If B+ is ok then I would start checking voltage on the 6JS6 tube mainly the control grid voltage (-60v)and the screen (135v).

other things, are those large white tube caps, think at least one is a boost filter cap, maybe leaky if not replaced already. .1 1kv could be loading down the fly.