tube amplifier auto bias- accurate?

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!

As one who has built both auto-bias and fixed (pot-adjustable)bias amps (EA230 etc.)over the years, I can tell you that the only advantage to fixed bias is low cost. This assumes that the auto bias is implemented properly. For auto bias, I have used both digital control (my EA2-150 circa 1976) and analog control with high-speed sample and hold techniques (ZH270). Either works well and holds the proper tube operating points with far better precision than the fixed bias approach. In the case of the ZH270, I even use the bias information developed from the auto bias circuit to tell the power supply shut down if tube parameters are out of spec.

But here is the sonic reason to control bias in typical push-pull amps. Non-gap output transformers used in P-P amps experience core saturation with as little as 1/4 mA current imbalance between the two halves of the push-pull. Toroid transformers, because of their tight coupling, can saturate at half this current. There is no way that hot output tubes will hold their bias to this tolerance without a good automatic controller. The sonic effects of mismatch over 1/4 mA are loss of transparency in the highs and increased distortion in the bass.