There is a simple rule to observe when you bias the power tubes on your amp:
1/ Power = voltage x current
6L6GC tubes should idle somewhere between 10 to 20 Watts.
So, the bias current you set for your tubes depends on the Voltage of your plates.
Examples:
My Sovtek Mig 50 has 550 Volts DC on its plates. If I want the power tubes to sound warm and responsive, I'll set the bias current so the tubes pass about 15 Watts at idle. So 15 / 550 = .027 amps = 27 milli amps.
My blackface Bassman has 400 Volts DC on the power tube plates. If I want the tubes to sound warm and responsive, I will set the bias current so the tubes pass about 15 Watts at idle. So 15 / 400 = 37 milli amps.
If I were to modify an amp to have "starved" plate voltages, say 250 Volts DC, then to bias the amp to get 15 Watts through the tubes at idle: 15 / 250 = 60 milli Amps !
Power dissipation of a tube is the critical value, not just current. After you have measured the plate voltage of your tubes, you calculate the bias current you need to get safe but still warm power dissipation. If I were to set the bias current to 40 milli amps on an amp that uses EL34 with a plate voltage of 650 Volts the tube's power dissipation would be .040 x 650 = 26 Watts at idle which is too hot for prolonged, dependable usage.
Power tubes and preamp tubes each have their specific power dissipation ratings and they can be operated on a wide range of plate voltages.
So, you can't just look at bias current through a tube without knowing plate voltages and the tube's design specs.
Best regards... Steve Rousseau Ampli-Tubes, Montreal
