Bias and heaters are different circuits. I believe this amp provides 6.3V AC to the output tubes and rectifies the AC to DC for some or all of the small signal tubes. Based on the way the schematics are labeled, the DC may only be used by V1 in the preamp.
The schematic shows bias at approximately -45V DC.
The tubes do not require bias or high voltage for the heaters to light.
If the heaters are intermittent, it could be the tubes, or it could be an intermittent connection to include loose pins on the tube sockets. If you do not have any electronics background, the best course of action is to take the amp to a qualified tech. This is for your safety and the amp's safety.
If you are going to poke around in the amp, you need to know there are potentially lethal voltages present, so you need to be careful.
Remove jewelry and only use one hand. The reason to use only one hand is to prevent any potential shock from travel through your heart. Young techs are taught to hold the unused hand behind there back.
Check the pins on the tube sockets for heater voltage. For the 6L6 tubes the heater pins are 7 and 2.
For the small signal tubes the heater pins are 4 to 9 for the A side of the tube and 5 to 9 for the B side of the tube.
You may need to tighten up the pins in the tube sockets. This is done with a tool that looks like a dental pick. Before attempting this procedure, the power supply capacitors must be discharged, as they can hold high voltage DC after the amp is turned off.
I don't consider it ideal to simply short the capacitors. I real tech would have some sort of jig to bleed away the charge. I use some insulate test leads with a fairly large resistor. One end of the test lead is clipped to the chassis, and the other holds the resistor. The resistor is touched to all of the capacitor terminals and slows the rate of discharge. check the capacitors with a multi-meter rather than assuming the procedure worked.
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