"Fixed" bias versus "auto-biasing"

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

That is one of the other advantages of auto bias..you can replace one tube when it goes and not a whole matched set of 4. It becomes less critical to have your tubes matched..but I would still recommend matching so that the bias circuit doesnt have to compensate too much.

Most amps are now fixed bias with auto biasing less popular today. Mike Saunders at Quicksilver is a proponent of auto and uses this type in many of his amps without problems.Ive used several of his nice amps.

I echo Banksfriend's question of what amp(s) are you looking into?

With auto bias, the tubes will be continually biased as you go..sonic degradation will occur towards the end of the tube life and then the tube will be so weak, that the auto bias circuit will not be able to bias it anymore and it will sound that way! or just give up the ghost.

You will have to adjust the bias on a fixed amp on occasion, usually a check every 3-6 months is good and adjust as necessary.Some tube types like the 300B require check fairly often...others like the EL84/6BQ5 much less often