[underdog64]

I have a wonderful sounding VTL ST 150 which doesn't have autobias.Companies like Prima Luna tout their autobias feature as reducing distortion up to 40%.My question:does the complexity of added auto bias as a feature hurt overall performance or help a tube amps sound?

[FullRangeMan]

My 2 cents:
Some Auto Bias are used as marketing banner by some manufacturers.
I dont like auto bias, it control the tube bias all the time(annoying the tube) and it prejudice the sound quality.
I like to adjusting the bias to know how the tube is behaving, so I prefer Manual Bias or better still Self-Adjusting Bias as used by Decware(only possible with low power circuits) it never need bias the tube.

AutoBias is great to women or childrens amps.

P.S.: Auto-Bias dont reduce the distortion per se, to do it the P-L autobias must have anything more inside.

[Quiet Earth]

Hmmm.... I thought auto bias (cathode bias) was less complicated, not more complicated.

Read this and tell me what you think : http://www.idealinnovations.biz/biastubes.htm

[Ericus Rex]

QE is right, fixed bias is actually much more complicated with far more components.  Don't get fooled; 'auto-bias' does not mean there's an active circuit continually monitoring the tube's performance.  It's just a resitor and paralleled cap usually tied to the cathode to 'balance' things out.  Maybe Prima Luna has developed some new sort of auto-bias system but I doubt it; there's nothing new under the sun, as they say.  Probably just Marketing.  Roger Modjeski in the Music Reference Circle once said that auto-bias limits the dynamic range of the tube.  Search his circle and that very old topic should come up.

[FullRangeMan]

Hmmm.... I thought auto bias (cathode bias) was less complicated, not more complicated.

Read this and tell me what you think : http://www.idealinnovations.biz/biastubes.htm

Nice link thanks. A pity this site not mention the Manual Bias of cathode.
The Prima Luna auto-bias is a complex PCB with various ICs.
The more simple auto-bias as this site inform also tune or control the bias tube all the time, in my humble opinion.

The Fixed Bias they refer if occour some short-circuit in the amp really may burn all, until finish the short-circuit as the Bias is fixed and also permanent.

[Lord Soth]

Your question is a very good one and very similar to the auto vs manual car debate.

Just like driving a manual car, the fixed (i.e. adjustable) bias offers more control.
This has been verified by many others in that if you run your output tubes just slightly before red-plating occurs, you will get the best possible sonics out of your tube amp.
The drawback is shorter tube life but whilst your tubes are running, they will provide you with the best possible sonics.

For auto-bias tube amps, your output tubes are likely to be run at the manufacturer's "safe" settings. The sonics can still be great but they won't be the best possible.

For the above reason, I also prefer an (adjustable) fixed bias tube amp coz' I can squeeze every drop of sonics out of my tube amp.

[opnly bafld]

The type of biasing is only one aspect of the overall circuit/design and there are a number of exceptional amplifiers that are self biasing.
IMO the only time the type of biasing should be the deciding factor is when the potential owner wants self biasing because he is dead set against adjusting it himself.

[rollo]

  There is an advantage to manual biasing. One could lower the setting and produce a fuller sound. Or higher for a brighter sound. Usually around 10% in either direction should produse a discernable difference in tonality.
    One could also set the bias lower to save tube life. Many advantages without disadvantages. Less circuitry, the ability to fine tune. Makes sense to me. Just keep it simple not simpler. I'll stick with Einstein, thanks.


charles

[Quiet Earth]

It is more circuitry, not less.

[Quiet Earth]

The type of biasing is only one aspect of the overall circuit/design and there are a number of exceptional amplifiers that are self biasing.

I agree. If you like the way your amp sounds, then don't worry too much about it.

[FullRangeMan]

I agree. If you like the way your amp sounds, then don't worry too much about it.

Who does it are only the music lovers, that listen only the music, not the equip.
Usually musiclovers persons buy SolidState amps to get free of retubing, tube versions/brands or adjusts.

Audiophiles dont do it, they want to get a better tuning of the amp, better sound or long tube life as previously mentioned.
Some do tube rolling, even when his tubes are new, just to know how the new tubes will sound like.