Soundmasterg is well versed on this. Here's what I got from him.
Determine the maximum power dissipation of your power tube type. 6L6? Looks like 24 watts per tube. Next, find the static plate to cathode voltage in your amp. Bias should be set such that the tube dissipates 60% to 70% of maximum power. That should be about 14.4 to 16.8 watts. Here's where it gets slightly tricky. To determine the power dissipation of the tube, you need the plate to cathode voltage and the current. To determine the current, you need a way to measure it. Conceivably, you could put an ammeter in series, but the way usually used is to put a 1 ohm resistor in series with the cathode. Measure the voltage across the resistor. Ohm's Law states that, for a 1 ohm resistor, one millivolt equals one milliamp. Set the bias current such that the bias current times the plate to cathode voltage gives a value between 14.4 and 16.8 watts.
You can buy a bias probe kit. Plug the bias probe into the tube socket, then the tube into the probe, and read the voltage. Or, if you're adept at such things, you can solder in a resistor in series with the tube socket and measure the voltage across the resistor.
