tube amplifier auto bias- accurate?

There are many ways a tube amp can keep self-bias on the tubes. It's neither better nor worse than fixed (Manually adjustable) bias. Some of the tradeoffs are that usually one method (fixed bias) allows a little more power output. For fixed bias an amp usually needs a separate bias supply, which adds to cost/size/complexity. The methods of self-bias vary, the most common is cathode-bias where a resistor biases the tube. Other units (BAT Vk60 come to mind) use a more sophisticated method of bias where its more of an auto-regulated active bias voltage than a simple cathode-bias self-regulating system.

both methods sound good, and adjusting bias manually is not a big hassle. Sometimes it's nice to have fixed bias in that it allows changing the operation point of the tube, which is not generally possible with a self- or auto-biasing amp.

-Ed

I am sorry to disagree with everyone so far. I have owned numerous tube amps from low cost to very expensive. Some auto biasing and others not. There is NO substitute for having your amp biased by a professional with a scope. None.
Ask a tech and see if they don't concur.

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!