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

"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

"I wonder if Macs were the first ones to implement autobias successful in their vintage gears like the MC240s."

Doubtful. bias as a concept is as old as tubes, e.g. early part of the 1900s, 1910-1920 is when most basic concepts and methodologies were developed. Not much has changed since then. Things rediscovered, e.g. Parafeed, etc.

-Ed