Manual vs. Auto Bias

Viridian, the term 'auto-bias' refers to a form of fixed bias operation, wherein the bias current is monitored in some form by either an analog or digital servo circuit which allows the amp to react to changes which may come from the tubes or perhaps line voltage.

The term 'cathode bias' or 'self bias' refers to an amplifier that is biased by a resistor in the cathode of the power tube circuit. Now it is true that such an amplifier does not need adjustment, but how that differs from an auto-bias amplifier is that the bias in an autobias amp is applied to the grid and is not derived from the cathode circuit.

This issue is indeed confusing!

The reason that autobias arose is there are those that think that self bias does not sound as good. Self-bias also requires higher power supply voltages in order to get the same amount of power.

I thought "fixed bias" meant you could NOT adjust the bias...no pot available in the design for that. I think most tube amps allow for bias adjustment, and auto bias merely does it for you on the fly...and that's good.

Kevin Hayes at VAC does not like any of the auto bias circuits out there. He says sound is compromised somewhat and that is why he does not have it.

I still say buy it for sound, unless there's a reason for not doing it yourself. Some may be to hard to do, but check out if you could do manual bias first on an amp you may like. After having at least 20- 30 or more in my system, I have heard great both ways. And that number I gave can be low. Over the years, 4-6 of us bought and swapped amps.

You don't need a Fluke True RMS meter for this task. I have a couple of Fluke True RMS meters, and you could use a regular good digital multimeter to do it. The True RMS is for measuring AC only or a EE, Designer, Tech or other to check its output. Not something for the average person to do. And most don't have or need the other equipment to do that.

I suspect that excellent sound can be achieved with either auto or manual bias, given the superb tube amps available in both categories.Certainly auto bias has`nt been detrimental as implemented in my 300b SET amplifier.