Manual vs. Auto Bias

The same amp that has manual bias can be upgraded to have an autobias by quite a few examples from simple shared cathode resistor to a complexed current mirror (using transistors, zenner, shottke diodes). After such upgrade the sound does not degrade at all in the same amp. The cost of labor increases and probably steals the manufacturer's profit, hence there's a reason to say that it can degrade the sound.

Do you know that 2 matched new tubes will age differently and even if you check bias of your amp often, you might still miss the moment when tube's going out of bias beyond its operational tolerance even before reaching its half-life? I'm very savyy with DMM of any kind, but learned to appreciate tube amps with autobias implemented.

Yes, the whole thing is confusing!

'Fixed bias' refers to any method of biasing the amp where the changes to the bias are affected by adjusting the voltage on the grid. This voltage 'fixes' the bias.

'Self bias' (also sometimes called cathode bias) refers to the use of a cathode resistor to bias the tube, the grid being tied to ground through a high resistance.

'Manual bias' is a method of fixed bias that is user adjustable.

'Autobias' is a method of fixed bias that is adjusted by a circuit (servo) in the amp and does not need user attention unless a tube has failed.

The two former terms are considered engineering terms. The two latter terms might be considered marketing terms.

My bad. I ASSuMEd auto-bias meant auto-adjusted by the amp itself, fixed-biased meant there was no adjustment knob/screw to change the bias, and manual-bias meant the adjustment could be made manually by adjusting a control knob/screw.

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.

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.

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 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.

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.

Tube auto bias circuits are SO cool...I wish they had 'em in guitar amps. They get rid of an issue I've always had with high end tube amps.

Manual bias adjustments here with a 2 minute soft start (Lamm ML2.1).

Need a decent true RMS voltage meter (I use a Fluke) and one has to make adjustments to both plate voltage and current on each monoblock. The measuring jacks and adjustment pots are right on the top, with the required values written adjacent - very easy access. It's a ten minute job and the drift over time is small. It's not a problem.

Regards,

Not sure why the discussion of auto bias vs. fixed has come about since the OP is seeking info on auto vs. manual bias, where neither is fixed. Anyway, my manual bias amps also have the soft-start feature, which certainly is good to save tube life.

More over I'd say that if I ever go back to tubes, I'll make sure I'll buy an auto-bias amp or if price is convincably low for fixed-bias unit, I'll DIY implement it(most-likely with SS parts for less space and money).

Marakanetz,
Thanks for bringing up that point. The builder of my Coincident amp as I recall did mention soft startup for the tubes in this design, and said tube life is very long.

Autobias has a main advantage over fixed bias that tube life is much longer than in fixed bias. During powering up an auto-bias self-adjusts the quiescent current thus giving a tube a 'soft' start.
Sonically It's also more beneficial since stabilization of quiescent current decreases distortion.

I have one "auto bias" amp, a Cary SLI 50 and I think it sounds great. My other amp is a VAC PA100 which is a manual bias via LED's on the frt panel. It is very easy to bias. Which type of bias circuit is used is not a make or break deal for me. However, amps requiring manual bias that require removing panels and covers, which would be difficult and or dangerous to bias, are off my ownership list. Some amps in this category are models from ARC and Jadis.

Anjowin,
I have a push-pull 100 watt amp(6550/KT88 tubes) that requires manual biasing(very simple to preform) that sounds very good. I also own an auto biasing 300b SET amp that`s the best sounding amp(without question) i`ve ever owned thus far.If a auto bias circuit (in theory) adds distortion, that has`nt been my experience at all.

My experience has been some amps are easy to bias some are not so easy. Check the instructions on how to bias the amp before buying anything. Personally I like to set the bias if the process is reasonably easy.

I have auto bias. It sounds good and I barely know what bias is. I pay to hear the band, not to make the music.

I like a manual bias as it allows for fine adjustments to squeeze the last drop of performance from the tube and circuit.

I don't have any preference. I go by what has the sound I want.

I have had a cj tube amp required biasing using the lights mounted on the amp and now cary using a meter, both are easy. Although I guess I prefer to use a meter so I know exactly where the bias is set. Both amps required biasing and were not automatic but not a big deal to adjust it infrequently.

Chuck