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What the heck is bias anyway?

I'm getting a new tube amp that will need to be biased. The process of doing it seems fairly straight forward, but I am curious: what are you actually doing when you bias an amp? In english, for us technical Ludites.
grimace
phaelon
1,053 posts
 
"Lets say for max power the current needs to swing plus and minus 40 mA from the bias point. If the bias current is set too low (say 30 mA) then it won't be able to make a full swing down since it wants to go down 40 but will hit zero and clip before it gets there.”

I have to say, Herman, that I think your rebuke of Elizabeth was undeservedly harsh. Good intentions deserve better. That said, I’m not new to tubes, but I’ve never felt like I really grasped the importance of "bias” until I read your explanation. Thank you.
6550c
393 posts
 
Yes, I think if the signal current(music) exceeds the bias current, then there is no sound or just tons of distortion. Is that how they make the Ibenz tube screamer?

Solid state needs bias too, we just dont roll transistors too much. :)
kirkus
502 posts
 
I dunno Trelja, I'm going to have to stick by it. Amplifiers and and cars are certainly different, but the main, general pragmatic, neophyte-level points between bias adjustment and idle throttle angle correlate well:
- They both determine the level of energy dissipated by the system while it's performing no work, but in an active state waiting to be used
- The proper setting is one that is neither too high nor too low, depending on the machine's design and intended application
- A misadjustment of either rarely results in a catastrophic condition, but results in reduced performance and/or increased stress on the system
- Both are traditionally adjusted by turning a little doo-hickey with a screwdriver, in a manner hidden from the casual end-user
- A similar level of mechanical/electrical aptitude and tools are required to adjust either to a factory-specified setting
- Deviation from the "stock" setting is ill-advised, unless you have the technical competence to evaluate the many interrelated system parameters

Above all, I think this analogy gives a good insight to one who is unfamiliar, whether it's something he/she should be comfortable twiddling with.
atmasphere's avatar
atmasphere
12,085 posts
 
Lesse... the setting of the bias has *no effect* on the power output. It affects the distortion and the heat of the tube (or transistor).

I think if the signal current(music) exceeds the bias current, then there is no sound or just tons of distortion.

Once properly set, if the audio signal exceeds the bias value there are no worries. That's kind of the point of being able to amplify.

Distortion pedals used for guitars use a variety of techniques to make distortion and that is a subject for a different thread.
trelja
3,226 posts
 
Kirkus, where the analogy of idle speed falls apart is with an engine, regardless of whether I set the idle speed at 500 (for the sake of example - too low), 800 (recommended), or 1650 (too high) RPM, performance will be identical at 2500, 3500, or 6000 RPM.

With a tube amplifier biased at 20 mA (again, for the sake of example - too low), 50 mA (recommended), or 75 mA (too high), its performance will be different in readily apparent ways when playing music.
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