I'm asked about biasing tube amps quite frequently. And often my dilemma is how much information do I give and just how to package it? I have always like how Aspen Pittman, founder of Groove Tubes explains biasing. Here are a few excerpts from Groove Tubes Reference Guide, published circa 1983.
The bias control on an amp is much like the idle control on an engine. That is to say, there is an optimum point of bias for an amplifier that allows for good sound and maximum tube life. This optimum bias point will be different with a new set of tubes no matter which tubes you buy so the bias should always be checked when changing power tubes. (pre-amp tubes are self-biasing)
"Under Biased Amp"
Amp is "idling to high" so tubes are running too very hot causing them to burn out fast and possibly to short-out. The tubes plates (large gray metal housing) will glow red from heat and amp will lack punch and might hum. The orange GROOVE TUBES paint will turn brown rapidly if amp is under biased.
"Over Biased Amp"
Amp is "idling too low" so tubes are running cool. Amp will sound dirty at any level and will sound low on power. This type of distortion is called "cross-over" distortion. Cross-over distortion is a non-musical type of distortion and isn't as pleasing to hear as "harmonic distortion".
"Correctly Biased Amp"
Amp will sound clean and tight at low to medium levels. When pushed to to maximum , amp will produce an even harmonic distortion---musical distortion if you will.
Biasing an Amp W/Variable Bias Control
The proper method of biasing a tube amplifier requires a signal generator, an oscilloscope, a volt meter and preferably a dummy load resistor. First, remove power tubes and measure the bias voltage at the grid (usually pin 5). Adjust the voltage to the largest negative voltage and install power tubes, the tubes are now at the over-biased position and are running cool with lots of cross-over distortion. Now apply a 2000 cycle signal to the input of the amp and connect the proper impedance load to the output. Turn volume of the amp up to 70% output and get a picture of the signal on the scope, notice the notch indicating crossover distortion in the signal. Adjust the bias control gradually until this notch just disappears. The amp is now properly biased.
The method described above is called "Cross-over Notch biasing". It is my main method for biasing both vacuum tube and solid state amplifiers. Yes, that is correct your solid state amp has a bias too. The great thing about the cross-over notch method is, it works every time with anything. The drawl back is, you need knowledge and experience of what class amp you are working on (A, AB or B) and just how that amp should run.
Amongst the inexperienced the "Current Drawl" method, made popular by Gerald Weber of Kendrick amplifiers has become widely popular. There is nothing wrong with this method if you are using the same model amplifier and same exact tubes. Even different brand 6L6GC tubes will bias a bit, well different. The problem with the current drawl method is that it employs the use of an "arbitrary specification". Amplifier and tube designs very widely. I do some times use this method, but only because I have the knowledge and experience to devise my own arbitrary specifications for common amplifiers that are often seen in for repair.
I hope this answers some questions. With tone there is no Voodoo, only science.
For more on Gerald Weber, Groove tubes and some good reference material, check out A Desktop Reference of Hip Vintage Guitar Amps, by Gerald Weber.
DO NOT attempt to bias your amplifier unless you are qualified to work on electronic equipment. There are LETHAL voltages inside of your amp that can kill you, kill you dead!