Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Yosser on January 16, 2026, 11:53:45 am
-
I found this instruction advice on fixed biasing using a one ohm cathode resistors, which I think is really fantastic:
https://www.duncanamps.com/technical/lvbias.html
This might seem obvious but,
He mentions "You can play your guitar through the amp each time you adjust the bias, and see how you like it. You can even adjust the bias by ear" - does he mean
cranked up really loud?
( In other words, is the difference in bias adjustment perceivable at lower volume levels or only when the amplifier output stage driven in high distortion ie SRV v ACDC)
-
It's not obvious at all from that description.
For fixed bias, you are aiming for power dissipation at idle less than 70% of the max rating of the tube. I normally bias around 60% more or less, and have great tone and long life from my tubes -- but some folks argue that higher is better. I don't think so, but it is a personal choice.
For me, I would listen to your amp at the sound level you normally play. That will be different based on your situation -- eg, my buddy does a lot of gigging, and would listen to his amp at club volume. I play at home, so listen at that lower volume.
If you want to prolong the life of your power tubes, one method would be to bias at 70% and listen at your selected volume setting. Then lower your bias by 5%, and listen again, and repeat this until you find your the bias setting where you can hear a difference. For me and my ears, that is usually about 50% +/- but each set of tubes is different. I would then set bias about 10% above this.
Everyone has their own preferred method.
RobRob has a method very similar to the DuncanAmp page, but it is confusing to me as well. I DO like RobRob's Tube Bias Calculator, though it is not as obvious as it could be IMO.
-
This sounds like the way to go - thanks
I did wonder though what would be the consequences of using and amplifier bias optimized for low volume use at high volume levels or visa versa
-
I did wonder though what would be the consequences of using and amplifier bias optimized for low volume use at high volume levels or visa versa
No adverse electrical consequences if you stay at 70% or less (this is for fixed bias). You could probably go higher if you test your plate current at various volume settings, but IMO it is not worth it.
I have not been able to hear any difference (high v low volume) when I bias at the low end. Presumably there are folks that can.