"Fixed" bias versus "auto-biasing"

Auto biasing is more convenient than manual biasing. That's a good thing. Is there a sound difference? Each person will tell you something else. If you are going to put the amp inside a rack, you may want to put more emphasis on autobiasing but if the amp(s) is(are) open, it doesn't matter.
My experience with a BAT (auto) and both Manley(manual) and Atma-Sphere(manual, but better executed) affirmed my opinion that manually biasing an amp can be a pain in the ass but really doesn't make a difference. One byproduct of owning a Auto biasing amp was that I never wondered if the amp would sound better if I rebiased it. I trusted that it was always at its premium setting.
My current amp is the Atma-Sphere which is biased with a different method than most amps. Still manual but IMO more efficient.
Others with more time and tube knowledge than myself will surely flame me and tell you how wrong I am, but you asked for many responses.

Bias is the voltage applied to the tubes grid. Manufactures design the circuit for the tube to have certian voltage applied that results in optimum life, sound, and safe operation of the amp.

Basically the difference is that with fixed bias, you adjust the bias externally using a mulitmeter( or the amp has meter or led lights) and either an adjustment knob on the amp, or screwdriver slotted to fit an adjustment hole either inside on a board, or outside the amp to adjust the current mA reading that is recommended by the amp manufacturer, so that the tubes run at a desired bias point for long life and good sound.(deep breath ;) Fixed bias is only "fixed" until after YOU adjust it to the right setting or your prefered setting as some do..( bias for more power/ shorter tube life...or less power and longer life. Both are not good as they have thier obvious problems, and sonics tradoffs. Go with the maufactures recommendation here.

With auto bias, the amp has a built in circuit( feedback comparator or servo mech.) so that when the tubes are in place and the amp is on..it will automatically set the mA current level of the tube circuit, compensating for tube differences and thus run them at the recommended point.

The advantage of the auto is that it makes the replacement of tubes very easy to just slip in and go. The disadvantage is that extra circuitry can fail and cause damage to the amp when it does. The servo can also require its own adjustment so that it does its job and brings the tubes in line accurately. Most auto bais is quite reliable however. There is another type of biasing, I think its called cathode self bias...its basically works like a fixed bias, in that you still adjust manually, but from what I know of them, you cant adjust the bias skewed for higher power, like true fixed bias.

This is what I remember just off the top of my noggin'.hope this help you.

Ken

Thanks to both Kehut and Banksfriend for your responses. I appreciate the input. Can either or both of you tell me briefly about tube replacement when the amp is auto-biasing. That is, I assume that a single tube could be replaced if necessary rather than having to replace a pair or a set of four? With an auto-bias amp, is it even necessary to purchase "matched" tubes should one tube only go bad? What kind of tube-life should someone expect, generally, or is tube degradation something I'd recognize without much trouble?