tube amplifier auto bias- accurate?

Does manual biasing always drift over time? So the amp is always slightly off without auto-biasing. So this could be very tube depended. Some output tubes can last quite a while without drifting while other change on a weekly or monthly basis.

I am a bit confused by the terminology. When I was involved with tube power amps there were three schemes which went like this.

"self biased" ....Bias voltage derived by a fixed resistor from the plate voltage. No provision for any adjustment. Output power reduced by about 10 percent compared with adjustable.

"adjustable bias"....Bias provided by a separate voltage generated by the power supply, and having adjustment capability by a pot.

"AutoBias"....Same as adjustable except with the pot replaced by a circuit that makes measurements and adjusts voltage accordingly.

Bin, yes there is a substitute. I'm using one!

BIN,
ofcourse a tech is going to concur. How would he get paid otherwise?

"adjustable bias" - this is actually called Fixed Bias (yeah, great naming convention, I know).

within the realm of 'auto bias' there are many possible ways to do that concept. Same with fixed bias - eg. separate bias power supply, winding, or tranny, various rectification schemes for bias supply, battery-biasing, etc.

bin:
biased with a scope? What the hell are you talking about? Bias is a voltage, nothing more or less. A good voltmeter is all that is needed. it's either accurate or its' not. There's nothing to graph on a meter. Tech's must have been having a laugh at your expense, methinks.

-Ed