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Hmmmm, bias point...

FractalAudio

Administrator
Fractal Audio Systems
Moderator
For as long as I can remember I've always biased my amps around 60% of maximum dissipation. But I just ran across a site that claims 70% is optimal for a Class-AB amp. So I tried it on my go-to Plexi patch and I do believe it sounds better.

Set Power Tube Grid Bias to 0.7 to do this.
 

trancegodz

Forum Addict
For as long as I can remember I've always biased my amps around 60% of maximum dissipation. But I just ran across a site that claims 70% is optimal for a Class-AB amp. So I tried it on my go-to Plexi patch and I do believe it sounds better.

Set Power Tube Grid Bias to 0.7 to do this.
I just tried it and it does sounds good. I like it better.
 

Adman103

Veteran
And you don’t have to risk your life draining filter caps to poke around in the amp with a screwdriver
Yea, no kidding. I got zapped by a Traynor YGL-3 a couple of decades ago, and that is an experience you don’t forget. I was damn lucky. It’s amazing to be able to get a great time without having to bother with 50 year old wiring, tubes, etc.
 

jakel

Veteran
I built an 18W Marshall from a kit once, and it sounded best biased around 70% IIRC, maybe even higher.
 

ccroyalsenders

Fractal Fanatic
For as long as I can remember I've always biased my amps around 60% of maximum dissipation. But I just ran across a site that claims 70% is optimal for a Class-AB amp. So I tried it on my go-to Plexi patch and I do believe it sounds better.

Set Power Tube Grid Bias to 0.7 to do this.
Curious for this site/article!
 

Promit

Inspired
So the story I heard is that the 70-75% bias point yields a slightly angrier sounding amp, but the 60-65% bias is preferred for tube longevity. Obviously tube longevity isn't an issue in an Axe Fx, but is the story true?
 
So many cool tools! Thanks for the tip.

I remember conversations about this years ago on the AX84 forum where I came to understand that the 70% number was just a rule of thumb that was not specifically "right" and not guaranteed to be "safe" although it often was safe for production amps. It seemed many production amps biased colder because it was safer, especially if consumers replaced tubes themselves, and an amp tuneup often involved just biasing the amp.

I can't remember all the methods people used but it seemed like this was an area where there was a lot of wives tales that weren't based on engineering. (EDIT: Randall Aiken had what seems to be a good article on it- The Last Word on Biasing . I don't think we can really replicate this in the Axe-FX since we don't measure the voltages and could adjust this value wherever it sounded best since we can't fry the virtual amp)
 

Robboman

Fractal Fanatic
So the story I heard is that the 70-75% bias point yields a slightly angrier sounding amp, but the 60-65% bias is preferred for tube longevity. Obviously tube longevity isn't an issue in an Axe Fx, but is the story true?
I'm gonna say no, because it can't be generalized that easy. Totally depends on the amp in question, though it may well be true for many amps.
 

Monstercastle

New here
That's so crazy that you can read an article about IRL amps and make the change in Axe-Fx III space and it all translates. Mind blown.
Credit the good design forethought in how they approach modeling at Fractal for this. I agree it's awesome. Loving this device and the approach to supporting it.
 

mikeyg

Inspired
Have no idea what you are talking about, but I sure will give it a try :)
I only sorta do... but some amp companies bias their amps cold to get maximum tube life. To my ears, this sounds harsh and cold, almost shrill. At 70 % you'd get less tube life, but likely warmer tone.

And of course in the virtual world, there are no tube concerns!
 

yyz67

Inspired
Obviously tube longevity isn't an issue in an Axe Fx
And of course in the virtual world, there are no tube concerns!
BUT it might be fun to have a virtual tube age parameter where 0% is no aging (default) and 100% would be farty, spitty, grainy angry tubes!

AFAIK Cliff hasn't modeled "degradation" -- short of controllable things like mismatched tubes, transformer/impedance matching, and delay distortion.
 

jakel

Veteran
I only sorta do... but some amp companies bias their amps cold to get maximum tube life. To my ears, this sounds harsh and cold, almost shrill. At 70 % you'd get less tube life, but likely warmer tone.

And of course in the virtual world, there are no tube concerns!
That's exactly my experience; I like warmer tones with some compression, and the higher bias got me there. I like the sound and feel of the tube saying, "I'm trying, but I can't quite get there."
 

cobbler

Fractal Fanatic
Yea, no kidding. I got zapped by a Traynor YGL-3 a couple of decades ago, and that is an experience you don’t forget. I was damn lucky. It’s amazing to be able to get a great time without having to bother with 50 year old wiring, tubes, etc.
So true! In one numbskull moment I found myself thrown across the room. Can feel it just thinking about it. LOL
 
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