The bias is set to make sure the proper plate dissipation rating (in watts) of the output tubes is reached. The maximum plate dissipation rating for a single 6V6 tube is about 14 watts. Since it is best to operate at about 70% of maximum plate dissipation, the bias is adjusted to bring the plate dissipation down to about 10 watts (.7 * 14 = 9.8). The calculation of the proper bias setting is done like this:
The formula to calculate plate dissipation says that P = E * I, which means that Power (watts) = Volts * Current Draw (amperes). Let’s say the plate voltage is 400V. The current draw (the unknown variable we need for setting the bias) can be calculated like this:
P = E * I
10 = 400 * I
10/400 = I
.025 = I (amperes)
In this example, since .025 amperes = 25ma (milliamps), and because of the 1-ohm 1% resistor, the bias setting for a power tube will be right when the voltmeter reads 25mv. The next time I put in a set of matched power tubes, I would just set the bias to 25mv.
When I pick up my amp, Mikey will measure the actual plate voltage (it probably isn’t exactly 400) and we’ll play with the bias to find my favorite bias setting that puts each tube close to 10 watts. Since the amp uses two 6V6 tubes in class AB (push/pull), the output of the amp will be about 20 watts, which in my opinion is just about the right amount of power for most gigs in smaller rooms.