tube amplifier auto bias- accurate?

Most auto control processes are as good or better than you can do manually. This would obviously be a requirement on the designer.

In airplanes, even little ones, the autopilot flies the plane more accurately than a pilot. This is hard for macho pilots (like me) to accept, but fuel consumption on a long trip proves the point.

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.

Ed_sawyer...Actually, you can use a scope to measure voltage if you are too lazy to go to the cabinet and dig out a voltmeter. Or, if you want to impress an audiophile.

I also know how to add on a slide rule (remember those).
And count to 35 on my fingers. (Hint...use two-place base 6).

Bin...Actually it was Ed_sawyer who suggested that a voltage measurement was all that is required, and I merely pointed out that you can utilize a scope as a voltmeter. It doesn't make sense to do that, (like adding on the log scale of your sliderule) but it can be done. Most bias setting is done using a voltmeter, often a meter that is built into the amp.

Your method sounds good, although it's not obvious that the end result would be any different from using a voltmeter.

Tubes108...In military equipment that I worked on we had a similar situation.... we were adjusting amplitude and phase of an AC power supply to account for variables of the load. The way to do it is to run the adjustment process until the parameter you are adjusting is within some tolerance, (and then a little longer so you don't always get hung up ON that tolerance). Then, quit the adjustment process unless the parameter drifts outside of a larger tolerance. In other words, introduce "hysteresis".

This is how automatic tube bias adjustment onbviously ought to be done, but...? Does anyone know the actual details?

Albertporter...Setting bias just at turnon would seem to be a problem for amps that are left on all the time. Also, if the amp is turned on and off, one would expect the settings to change somewhat as the amp warms up, so that a reset procedure, such as I described, would be useful.

Atmasphere...Your comments are worth at least 4 cents.

One final comment...the demise of self biased tube operation relates to the development of inexpensive solid state diode rectifiers. In the bad old days, generation of an additional dc voltage for tube bias required another rectifier tube, and all the stuff that goes with a tube. Most peoople didn't think that the improvement was worth the cost. Today it's no big deal.