View Full Version : Valve Bias Play Time

Sand Dancin Donkey Walker
04-10-2008, 23:15
Hi All

Have been playing with the bias adjustment on my amp this evening. An Almarro 50125A with 6550 valves by Tung-Sol.

Originally I had the bias set at 150mV. This is the only valve amp I have owned that requires manual bias adjustment. Considering how many valve amps I've owned over the years it's a first.

While checking the bias value (which had not changed from the last check) I decided to raise the bias to the max value noted in the manual which is 160mV.

Back on went the music and immediately found the bass quite noticably firmer and punchy and the mids and highs a touch more open or clear may be a better description. The changes are evident on both CD and vinyl.
I hadn't reallised what a change in sound I was going to get with such a small change in bias. I must admit I do like the change in character and presentation of the amp, bass definition and drive from the bass of Marcus Miller and Metallica much improved.

Has anyones else found this of have you adjusted your bias to greater or lesser values than recommended my the manufacturer

Having some thoughts now on looking at the valve compliment in rest of my gear, Pre, Phono, DAC and dirvers for the power amp and wondering if some changes there could have some useful results.

Andy - SDDW

Perceived changes to sounds ect were in no way influenced by the bottle or so of Red Wine I've just consumed. :cool:

06-10-2008, 11:28
hi andy,
bias adjustments can change sonics quite significantly, depending on the type of amp you're using, how much standing current the valves are set at etc, for example, if you have a class A amplifier, this means that the standing current should be equal to the maximum current the amp is ever going to need, ie, it draws the same current all the time whether the amp is being modulated with sound or not! class A is very wasteful, but as far as a lot of purists are concerned, its the only way to go. single ended amplifiers are inherently mostly always class A BY DESIGN. push-pull amps on the other hand can be biased anywhere between a few milliamps, right up to their maximum current draw,as long as the power supplies are up to the job. most push-pull amps are class AB, or AB1 biased, this is usually a good compromise as it will give you a certain amount of class A power, but will also be kinder [in some respects] to the valves and power supplies. the optimum bias point of a class AB, AB1 amplifier will therefore be down to the user, and as you have found your amp in your system seems to sound its best when its biased to the maximum allowable according to the manufacturers information.
however, that may not be the case for someone else using the same amp in a different system.
this is a rather simplified explanation of BIAS, because there are other variables to take into account, but an explanation of these would probably confuse the situation in this instance.
hope this helps.

Sand Dancin Donkey Walker
06-10-2008, 14:42
Hi Anthony
Cheers for the reply and yes it has helped explain a few things, the amp is an AB Push-Pull design. Quite happy with the change in presentation, will be in contact at some point with regard advice on valves, lots on my plate at the moment with regard to job change, see my post 'My Brain Just Will Not Work, Head Up Arse Week ' in Abstract Chat.


Andy - SDDW

06-10-2008, 15:34
hi andy,
already read it, so i understand you must be quite stressed at the moment.
by all means, when your ready, and good luck with your new venture.

06-10-2008, 18:46
Andy, I said that you'd get good advice :)

I've also heard fairly big changes when biasing valves. When I had the Yaqin, with its manual bias adjustment, I could clearly hear the difference between the Svetlana KT88s biased at .58 millivolts, which was spot on, and higher or lower from that point.

'Over biasing' valves tends to have the effect of softening the sound and makes for a 'heavier', more ponderous, presentation. 'Under biasing' valves tends to have the opposite effect, and gives a brighter more 'brittle' sound, with an 'edge' to high frequencies. Get it spot on, though, and the music gels perfectly with no emphasis on any part of the frequency range.


Sand Dancin Donkey Walker
06-10-2008, 19:05
Hi Marco
Cheers for your reply. I think I seem to have hit the sweet spot for the bias value first time, though I might give it some small adjustments over the weekend to see what happens but the 160mV seems to be about right. Will let you know the results.
Wil be updating my 'My Brain Just Will Not Work, Head Up Arse Week ' in Abstract Chat shortly

Andy - SDDW