[Ultralight]

Read up a bit on bias but did not find an answer to my questions.

My understanding with tube amps is that there are three types of bias:
1. Fixed Non-adjustable (other than changing a physical component)
2. Manually Adjustable
3. Automatically adjusting

I find the above confusing.

Questions:
1. Why are not ALL units automatically biasing?  That would seem to be the easiest when it comes to tube rolling.   

2. What the advantages of each?

3. Is it true that auto adjusting has tonal drawbacks as a custom make claims?

4. With fixed bias, wouldn't that create a problem when rolling tubes - limiting tube options as, presumably, one has to find tube that will work just right with the fixed bias?

Thanks,
UL

[FullRangeMan]

I would add a fourth Bias type: Self-Adjusting Bias, as usel in the Zen Triodes amps.
http://www.decware.com/newsite/tubes.html

[JohnR]

Usually, the terms used are self-bias, where a resistor in the cathode circuit provides a DC offset voltage which effectively becomes the bias voltage, and fixed-bias, where a "fixed" negative voltage is applied to the grid. That voltage is typically adjustable by a potentiometer, sometimes one for both tubes (in a push-pull amp), sometimes one for each tube. I don't think there would be too many fixed-bias amps that wouldn't allow adjustment.

Self-biasing amps are a simpler circuit but dissipate power in the cathode resistor. Fixed bias requires a negative supply - typically another winding on the transformer - and the bias rectifier and voltage divider circuit. In the "classic" amp days self-biasing was typically used with lower power amps while fixed biasing was used in higher-power amps.

The terms cathode bias and grid bias are also used.

[FullRangeMan]

Thanks for the explanation.
I wonder if tipically in a modern tube amp(as PrimaLuna etc) the Automatic-Bias system constantly adjust the tubes (say thousands times per second) or it adjust the tubes some times per minute??
Sorry if looks a dumb question.

[Elizabeth]

Small signal tubes usually have a permanent bias. Certainly when I would change a preamp tube, no bias adjustment is needed nor expected.
The power amp power tubes are different. They can explode, and all sorts of awful things if they get too far off on the bias.
(I am not certain if they can do those awful things even if the bias is correct?)

[Nels Ferre]

Small signal tubes usually have a permanent bias. Certainly when I would change a preamp tube, no bias adjustment is needed nor expected.
The power amp power tubes are different. They can explode, and all sorts of awful things if they get too far off on the bias.
(I am not certain if they can do those awful things even if the bias is correct?)

I've never seen one explode, but I have seen a power tube glow red hot, and one glow white hot- smelled awful too. Fortunately, there was no damage to either amplifier.

[Nels Ferre]

Fixed bias means one must generally replace the output (power) tubes as matched pairs or matched quads.

I prefer individually adjustable manual bias. This way, as the tubes age, they can be adjusted to perfection. Also, in the event of a tube failure, one can pop in a single spare, bias it, and be good to go.

[Steve]

Thanks for the explanation.
I wonder if tipically in a modern tube amp(as PrimaLuna etc) the Automatic-Bias system constantly adjust the tubes (say thousands times per second) or it adjust the tubes some times per minute??
Sorry if looks a dumb question.

Automatic/self is DC bias and continuously adjusts bias, at any instant in time.

Fixed bias (output stage, tube) typically allows for greater output power because the full B+ is applied across plate to cathode. The cathode is typically grounded (or close) and negative voltage is applied to the grid. Thus maximum plate voltage swing is achieved for a given B+.

Cathode bias, on the other hand has the grid grounded, with bias voltage applied from cathode to ground. Thus optimum use of B+ is not achieved since the bias voltage is subtracted from B+ at any given instant, thus the tube sees less plate to cathode voltage.

There are variations such as cathode bypass caps vs no bypass caps, which affects the output power.

Hope this is clear; it is simplified.

Cheers.

[FullRangeMan]

Very thanks Steve for your detailed information.  Always good to clear a doubt.
Regards, Gustavo

[Ericus Rex]

Roger has talked about self-biasing amps over on the Music Reference circle.

http://www.audiocircle.com/index.php?topic=53177.0

I found his answers to be very informative.

[Ultralight]

Thanks for all the comments.  Appreciate it.

Comment below deleted - I thought my questions were not answered but JohnR's response below made me reread the posts slowly and thinking about the comments.  Apparently were all answered. THANKS!

UL

[JohnR]

You had all these questions answered for you. What specifically do you want to know??