Understanding tube amp biasing methods

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Understanding tube amp biasing methods

Postby housekustoms on Sat Mar 01, 2014 3:57 am

Hi again! I have another noob question. This one is about biasing methods for tube amps. :?

I have read all kinds of explanations about amp biasing but I still have some difficulty sorting out and getting a good mental picture of cathode, fixed and adjustable biasing. I guess the trouble is although I can understand what I'm reading I walk away with no good insight into when one is more appropriate than another and why. So here are a few of my questions if anyone is willing to try to increase my understanding. I understand technically what the bias is doing as far as throttling or allowing the electron flow from cathode to plate via the grid bias but I can't seem to see the forest for the trees so to speak.

1. The adjustable bias seems kind of self-explanatory I think. If I’m not mistaken it is cathode bias with a pot rather than a fixed resistor. Is that the case?

2. Fixed vs. cathode isn’t as obvious to be as both seem “fixed” so I know I am missing the point. How is one really different than the other?

3. A real world question whose answer should be obvious if I understood the topic would be what drives a designer to choose one method over another? There must be specific pros and cons to each method but they are not obvious to me I suppose since I don't really grasp the preceding questions.

I thank you in advance for any help in bettering my understanding of the topic.
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Re: Understanding tube amp biasing methods

Postby Clanger on Sat Mar 01, 2014 10:06 am

To put it briefly there are two common methods of acheiving bias;

1, Cathode bias - a resistor and bypass cap in between cathode and earth. Enables a solid backstop to what a tube can be exposed to voltage wise, but due to voltage loss across it the acheivable power output will be lower than other designs.

2, Grid Applied voltage bias, where the cathode is directly grounded and the bias is acheived by applying neg voltage to grid with signal supeimposed, - more power output available as no cathode resistor voltage loss. Downsides are that the bias must be designed or set right otherwise tube may fry!

Each design could be made adjustable, but more commonly used in method 2.

Hope this helps.
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Re: Understanding tube amp biasing methods

Postby housekustoms on Sat Mar 01, 2014 2:30 pm

It does, thank you. So with the cathode bias method then, how is the relationship between the grid voltage and cathode voltage determined and set? Is there a formula or rules to help make the determination of what is optimal?
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Re: Understanding tube amp biasing methods

Postby Clanger on Sat Mar 01, 2014 3:23 pm

Again briefly,

Well you'd start off with 1, the spec sheet for the tube in question 2, your ideas for what you are trying to build - do you want power or reliability.

Design limits might be 1) the minimum power you want to acheive 2) the maximum cathode curent the tube can take reliably 3) the amount of distortion you can live with (more of a hifi concern). 4) where do you wish the valve to operate e.g. load lines.

Once that is settled the first stop would be good old V= IR, B+ and the valve characterisitics , to set the size of cathode resistor that will make the grid sufficiently negative wr/t the cathode. Don't foget bypass cap DC leakage. Look at spec sheets to get clues as to the right ballpark for your design.


Others no doubt can expand on this very breif outline....
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Re: Understanding tube amp biasing methods

Postby housekustoms on Sat Mar 01, 2014 3:49 pm

I appreciate your taking the time to provide this additional framework, thanks!

:thumbsup:

I realize the topic is bigger than a forum reply but this helps me a great deal and provides a great framework for consideration of the various variables and choices that exist. I've never tried to start from scratch before but wish to do so in time and this is the type of practical information I can't seem to glean out of technical definitions and theory. My experience in this area is, to say the least, limited.

:cheers:
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