377V 187mA

According to Ohms Law, R = V/I, which gives us an equivalent resistance of

377V / 187mA = 2kΩ

The amount of heat produced is

P = VI = (377V)(187mA) = 70W

So if we were to replace the Bassman's preamps, phase inverter, and push-pull power stage by a big 2kΩ resistor capable of handling 70 watts, then the 5F6-A power supply would sag to 377V.