Output power (watts) =
(plate voltage - cathode voltage) *
(plate current - screen current)
Each tube type is able to produce a maximum number of watts. Here are some guidelines:
6L6GC 25 W
6V6 12 W
EL34 25 W
EL84 12 W
I consider these to be maximums, and aim for about 70% of these values for class AB amps. Class A amps are biased at 100%.
Determining a bias range is most influenced by the plate voltage. The cathode voltage on fixed bias amps is often zero, since the cathodes are grounded. On cathode-biased amps it may be around 30 v. Screen currents are usually just a few milliamps. So the main variables are the plate voltage and plate current.
Plate voltage and cathode voltage can be read directly. Plate current is read using one of the bias methods. Screen current can be calculated on amps that have screen resistors by using Ohm's law. Read the voltage across the screen resistor, and divide it by the resistance.
Example:
6L6GC tube with 440 volts on the plate, fixed bias (cathode is grounded)
25W / 440v = 57mA maximum bias current
70% of 57 mA = 39.9 mA
I'd consider this a maximum current, and usually bias my 6L6GC amps between 35mA and 40 mA.
As you set the bias to increase the current in the tube, the plate voltage will drop. I've read some people using this relationship to lower the plate voltage, but that seems ass backwards to me.
Here's what Lord Valve (NBS Electronics) has to say about this--he knows tubes well.
"If you have 5881s or 6L6s, you should shoot for 18 watts or lower. (The cooler you run the tubes, the longer they'll last. If you dig the way the amp sounds when the tubes are idling at 12 watts, fine...don't worry about it.) For EL34s, go for 21 watts or less...If you find that you only like the tone when the tubes are operating near their limits, you may decide to trade some tube longevity for the tone you need. If you like the tone with the tubes running cold, you'll obtain significant extra tube life that way. It's *your* call!"
(Duncan Monro has a an article by Lord Valve that's worth reading.)