auto-bias question - how does it work?

[Phil]

Pardon my ignorance, but how does auto-biasing work?  Does it, for example, bias the tubes when you first turn on the unit and stay at that setting until turned off.  Or does it dynamically re-bias as the tubes warm up?

Thanks.

Phil

[richidoo]

I think there are several different schemes BAT, Prima Luna being two I can think of.  Music causes the bias to vary so it can't be continuous, at least not very sensitive. Maybe averaged out over a minute.

http://www.hifi-notes.com/primalunaprologueone24-en.htm

[dangerbird]

I certainly do not know all the ins and outs about autobias,but I know that if you have an amp that does it,,like my Quicksilver GLA,,according to the experts,it is very important to have the tubes "dc" matched, according to Mike Sanders at Quicksilver,I believe that it has something to do with plate current/votage.Kinda makes sense to me,but unfortunately,I cannot put it into words.

[Phil]

Thanks, Rich. That was an interesting read.  I was assuming that was the standard method used but it seems not. 

Phil

[richidoo]

Phil, I tried to find more about the BAT method, but couldn't find technical description. I have read about it somewhere, but can't remember the details.

It's a nice idea, I thought I could hear the difference when my amps were 1mA out of bias. Now I think I must have been imagining it. But that's a good symptom of nervosa...

One time I did A/B some amps that were way off, never biased in 1000 hours since new. One tube was 25% high, one was 30% low. Once corrected it really did sound much better. Using reliable tubes which hold steady bias and checking once a month is good enough for me. New Sensor tubes are famous for reliability, now they make the Genelex resissues which supposedly sound great too.

Another brand that is famous for auto bias is VTL
http://www.vtl.com/pages/Amplifiers/Siegfried/#Features
They use a computer processor to sample and adjust bias during music playback, I think.

[bacobits]

Great question, I never thought of how that works.
As Rich said, when music is played the voltage is all over hence it should be adjusted with no signal volume down. I do that here on an unused input it's an intigrated.
My amp has 2 pots for 4 JJ E34l's one for each channel. I can get close on both pots but I was told not to worry about getting both channels right on. You can pump up the voltage to make things a bit warmer sounding (more heat too I presume) at the expense of tube life. Although the JJ's I'm using can take a bit of push I bias on the low side of the recommended scale by the manufacturer. My house voltage may be and is more than likely different than yours so it should be checked if changing locations.

Isn't this FUN!??
I do check it once a month.

Den