A big explanation of biasing that is now on my page, maybe it might help some people out understanding what biasing is.
Tubes can only conduct current in on direction, from the cathode (-) to the anode (+). A vacuum tube is fully conducting when the input is at 0v with respect to the cathode and off when the input is more negative than the cathode, the specific voltage when it shuts off varies by tube type but is generally -4v with 12ax7s and -60v with 6L6s. In order to amplify the positive portions of the wave, the audio signal needs to be imposed with a DC offset that is half way from completely on and completely off, specifically a negative DC offset. This hold true for preamp tubes, however in a push-pull configuration the bias voltage must be so negative that under conditions where no audio signal is present, the bias voltage is so negative that it actaully stops the flow of current and only begins to conduct when a positive signal is present. All negative portions of the audio signal are essentially clipped off.
A phase inverter is the last preamp tube that's job is to split and invert the signal on one of the outputs. The result is the opposing signals go into their resepctive set of power tubes, one set of power tubes amplifies positive signals, the other negative signals. When one set of tubes is conducting the other set is off because its signal is driving the voltage more negative than the cutoff voltage, the voltage that turns the tube off. The end result is that there is a set of tubes powering the positive portion of the signal and another powering the negative portion of the signal. After is it mixed together by the output transformer it results in the original signal
The reason that this configuration is used is because it is more power effeicient, delivering more power to the speakers and wasting less power in the amp itself. This means that your tubes last longer.
There is one flaw just setting the power amps bias voltage to that which would set the tube into cutoff until the audio sigal becomes postive. It takes some time for the tube to respond and turn on when they are off and for some time remain off. This results in what is called crossover distortion which is highly undesired. To remedy this, the bias voltage is adjusted slightly more positive so that all the tubes are just beginning to conduct when no signal is present. For small signal swings, both sections are running at the same time, during the handoff from one set of tubes to the other, eliminating that small amount of time where there is disortion present.
Unfortunately the tolerances from tube to tube changes and their actual cutoff voltage varies. Becuase of this, you have to caluclate where the tube is operating by measuring its bias current, that is, the slight amount of current that is allowed to flow becuase the tube slightly turned on, or the bias voltage is just above the cutoff voltage.
The common bias current is typicaly 60% to 70% of the maximum dissipation of the tube. With a 6L6, which its maximum dissipation is 30 Watts, that puts the bias anywhere from 18-21 Watts. The bias current needed to achieve this ranges form amp to amp. The Peavey 5150 and 6505 amps are deisgned to have a plate voltage of 500 volts, but can range from 480 volts to 510 volts, depending on many variables. In order to get the correct current calculation when biasing the amp, you need to check the live plate voltage. With that you can use the equation P=IxV where P is power in watts, I is current in amps and V is voltage in volts. Since we know the power of the tube and will measure the voltage of the plate, we need to calculate the current that we are aiming for so we will use the equation I=P/V. With the 5150/6505 amps being 6L6 amps, we know power is 30W, now lets say that out plate voltage was indeed 500v, then we know that our maximum to not exceed current is 60mA. In order to get our bias current, we can choose anything from 60% to 70% to get our current. I personally recommend 70% which using the equation I=P/V means we are aiming for a bias current of 42mA. The sweet spot for most 5150/6505 users with a bias mod is anywhere from 36mA to 42mA. That is much higher that the typical stock bias that can range from 8mA to 18mA.
The higher you bias the amp, the more tube sag you will achieve, which adds a level of compression and harmonics at the expense of a tight low end. If you want a tight low end as possible but want to rid crossover disortion the best way to do this is to have a load and meaure across that load with an oscilloscope a sing wave signal that is being injected at the phase inverter and increase the bias until the sine wave looks completely clean and free of any distortions. With a 5150/6505, that will ut you in the 20mA to 30mA area which will mean you will have tighter low end and your tubes will last longer.
Anectodote: I personally like tube sag, it helps make an amp sound more aggressive and "pissed off" and am willing to go through tubes quicker for that tonality. I will note however taht the date on my tubes are from 2002, I bought my 5150 in 2009 and immediately bias modded it, it has been running at 70% for almost 4 years, it has been a regular weel;y practice amp for band practice at full volume for hours, traveled hundreds of miles for shows, been cranked at shows, in some cases I had to turn the volume up to 7+ on the post gain because some venues don't have a PA for anything other than vocalists and even with a cracked tube, a tube that was cracked when I got it, the tubes still work, so don't think that biasing your amp hotter will dramatically reduce the life of your tubes.
