I'm confused? Fixed bias??

[Orphin]

So I've always thought that "fixed bias" is an amp where you set the bias, RV50 etc. and cathode bias is an amp where you don't set the bias, Tiny Terror etc.
But when reading on Eurotubes website I'm getting confused:

Amps using EL84/6BQ5 tubes are almost always fixed bias and because of the type of circuit used are usually O.K. On the other hand, any guitar amp that is not cathode biased and uses 6L6, EL34, 6550 or KT88 power tubes NEEDS to be properly biased to sound it’s best no matter what the manufacturer says!

[Randy Bass]

Based on the context of the second sentence, I think they intended to say "cathode" instead of "fixed". The thing that confuses me is that fixed-bias and adjustable-bias are terms that seem to be used interchangeably. Maybe the difference is whether a bias adjustment pot is present?

[Orphin]

So this is just a typo then?

[jontheid]

It can be confusing about fixed bias, adjustable bias and cathode bias, mainly because fixed sounds like it should be the opposite to adjustable, but it isn't.

For guitar valve amps in general, if it is not a cathode biased amp then it is a fixed bias amp.

Cathode biased amps self bias, the bias level changes dynamically somewhat with variations in the input signal.

With fixed bias amps the bias does not change at all with variation of the input signal, it is always the same, hence fixed.

Adjustable bias amps should really be called adjustable fixed bias amps. The bias point is not affected by the input signal at all, but this point is adjustable by moving a trimmer pot.

Not all fixed bias are adjustable fixed bias amps. Take the Peavey Classic 30 (the three preamp tube model, not sure about the new ones with only two).
This is a fixed bias amp that uses 4 x EL84, but the bias is not adjustable at all, the amp has to be modified to enable this.

[Orphin]

Not all fixed bias are adjustable fixed bias amps. Take the Peavey Classic 30 (the three preamp tube model, not sure about the new ones with only two).
This is a fixed bias amp that uses 4 x EL84, but the bias is not adjustable at all, the amp has to be modified to enable this.

Then I guess this was what they were talking about; a fixed biased amp without a bias pot. I think I recall the Peavey 5150 is the same..

[jontheid]

I think it is a typo though, most EL84 based amps do use cathode bias.

[a.hun]

I think it is a typo though, most EL84 based amps do use cathode bias.

I don't know. Not sure either way. Though we all know (and mostly love) the various usually more or less Vox based 4xEL84 cathode biased amps, there are actually plenty of fixed biased 4 x EL84 amps too. And frankly if these come from the factory biased on the cool side (as most do!) then most of the time you can also pretty safely slot in any reasonably matched modern EL84s and they will be run well within the normal bias range. (I know, no guarantees, but...)

My old tech John is very keen on Mesa amps and has worked on many many of them. Most Mesas (whatever the valve type) are fixed bias, and all of those are non adjustable ie no adjustmant pot. However John has NEVER actually had to alter the bias in one when fitting any good set of valves:
http://www.thegearpage.net/board/showpo ... ostcount=3" onclick="window.open(this.href);return false;
Ditto with the Peavey classic 30:
http://www.thegearpage.net/board/showpo ... stcount=25" onclick="window.open(this.href);return false;

It may be that the Eurotubes quote is kind of lumping all 'Vox type' EL84 amps into one and comparing that with all the various different non cathode biased EL84 types they have to supply for. In which case ironically I can just about understand what they are saying. Though it is important to have the biasing in the right range, if these fixed biased amps are biased fairly conservatively (coolish) from the factory, then in fact they will run almost all valves just fine.

(Sorry if that thought complicates things even more. It may also just have been a typo!!!)


Andy.

[TheOrangeJuicer]

It can be confusing about fixed bias, adjustable bias and cathode bias, mainly because fixed sounds like it should be the opposite to adjustable, but it isn't.

For guitar valve amps in general, if it is not a cathode biased amp then it is a fixed bias amp.

Cathode biased amps self bias, the bias level changes dynamically somewhat with variations in the input signal.

With fixed bias amps the bias does not change at all with variation of the input signal, it is always the same, hence fixed.

Adjustable bias amps should really be called adjustable fixed bias amps. The bias point is not affected by the input signal at all, but this point is adjustable by moving a trimmer pot.

Not all fixed bias are adjustable fixed bias amps. Take the Peavey Classic 30 (the three preamp tube model, not sure about the new ones with only two).
This is a fixed bias amp that uses 4 x EL84, but the bias is not adjustable at all, the amp has to be modified to enable this.

This is correct. The term was badly misued in the supplied quote. It is strange but true that most "fixed bias" amps are in fact adjustable. So as counterintuitive as it seems, the terms fixed bias and adjustable bias can be used interchangably. The only exception are the more rare designs as mentioned, mostly by Mesa these days that employ fixed bias but then don't allow adjustment. Hiwatt also did that. It's a PITA as you have to install tubes that are matshed to best suit the amplifier, which means that tubes are not as easily available when and where you might need them.