tube amplifier auto bias- accurate?

This is an excerpt from Hank Wolcott in response to questions about how his amplifier works:

The circuit has to be made virtually distortionless through optimal loading before positive feedback can be allowed to increase the gain.

The output of the pentode cathode follower is direct coupled to the output stage and constant current sourced by a pair of transistors forming a current mirror.

The grid circuit of the output tubes is connected to a digitally controlled automatic bias circuit which checks and resets individual tube biases at turn-on and then is disengaged from the signal path

I don't know if that helps, I know Hank very well and could call him and ask for more 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.

My 2 cents: There's a bit of confusion here about autobias and self bias. Self Bias has been around for decades and has been used by many manufacturers over the years.

Autobias is a modern varient of Fixed bias. Fixed bias is the correct way of saying 'adjustable bias' since what is really going on is when one says 'Fixed' is one is referring to the grid voltage being 'fixed' at a particular voltage- often by means of a potentiometer, and now more recently by a modern servo circuit, which might be computor or analog monitored.

Ideally, the bias in an autobias system should be monitored constantly as tubes can drift substantially upon warmup. Computor (or more aptly, controller) based systems are in some ways more adequate to this task as it is easier to prevent the resulting servo circuit from 'hunting' back and forth across the ideal bias point. In an analog-based system, the timing constants have to be chosen to lag slightly behind the tubes and the voltages measured so that stability in the system can be maintained.

Note that as a result in an autobias system the system will rarely be at the ideal bias point but will always be close by. In practice, many users are not attentive enough to monitor the bias level of their equipment (proper use of test equipment being a major limiation on convinience) so often the fixed bias system may be further off at any given time.

So the result is while an autobias system is probably not going to be exact, it probably will be closer in day to day life then a fixed bias system.

Of course, a self-biased setup will not always be correct either, as tube variations usually mean that the self biased circuit will be off by the variation in that tube. But it will always be off by that variation and none other. So if component values are carefully chosen (for example- matched tubes) a good compromise can be reached.

Ultimately: the autobias system is more likely to be the closest to correct from day to day of the three systems currently in use for push-pull amplifiers. The exception is in SE amplifiers, where the tube is operated self bias and class A. In this special case where no other power tubes are involved, the bias level is very accurate.

Sorry for the long diatribe...

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.

Hi Eldartford, actually semiconductors are nice for self bias as they allow for a higher operating voltage. Since the bias voltage for the power tubes is usually developed across a cathode resistor, that means that the B+ available was the power supply voltage minus the cathode voltage. A tube rectifier would knock an additional 50-75 volts off of that- meaning you had to have 500+ volt power supplies to make enough power with certian tube types. Nowdays the job is easier because you can do the same work with a 450+ volts supply. 450V filter caps are a lot easier to get then 500V! The transformers are a little easier to make too.

The real problem with self-bias is it is really a lot harder to make a high powered amp. And high power is a big deal these days. By high power, I mean over 80 watts. You just don't see that many 80 watt amps that are self biased!