What is Fixed Bias?

Do you have a valve guitar amplifier?

If so you have probably heard of ‘biasing’ and maybe become confused about what is fixed bias.

There are two sorts of bias: Fixed bias and cathode biased. This short article aims to demystify this highly confusing topic.

Let’s make a start. You’re going to like this first nugget. Fixed Bias actually means adjustable bias! Go figure!

So a fixed bias amplifier means that it is one on which you can adjust the bias to get the ‘tickover’ current through the output valves to be the correct level. The term ‘fixed bias’ comes from the fact that once you have adjusted it (set it) then it stays the same (fixed) until adjusted again. Yes, I agree, ‘fixed bias’ is not the greatest name, but that’s what it’s called.

Take home: if you have a fixed bias circuit in your amp, it’s one on which you can adjust the bias voltage. A fixed bias amp can be biased by you or a tech.

Fixed Bias –v- Cathode Biased

In contrast to fixed bias we have Cathode Biased. This could also be called ‘variable bias’ but it isn’t. Now because it’s ‘variable’ don’t get the idea that you or a tech can vary it – you can’t. That’s reserved for fixed bias amps (feeling confused yet??)

In a cathode biased amp, the bias voltage varies up and down automatically to keep the bias correct. In fixed bias amplifiers, once set, the bias does NOT move up and down automatically. And that’s where the ‘fixed’ bit of fixed bias comes in.

For your interest, all preamp valves are cathode biased which is why you never have to bias them. Cathode bias is a kind of self bias circuit.

Some amps (e.g. Vox AC15) have cathode biased output tubes (EL84) and unlike fixed bias amps, these amps don’t need biasing. In fact they can’t be biased.

The vast majority of guitar amplifiers are fixed bias tube amp though. Fixed bias on the output tubes means they are capable of being adjusted to raise or lower the bias current. This is usually done with a pot, or swapping out a fixed resistor.

Summary of Fixed Bias and Cathode Biased Amps

So now you know.

Fixed bias = adjustable (!) but it stays the same (fixed) once adjusted.

Cathode biased = non adjustable and it varies up and down with conditions (so it’s ‘not fixed’).

Posted by Stuart on April 18, 2015