Fixed Bias

Bern_397 · Post by **Bern_397** » Mon Sep 13, 2004 6:56 pm

Hello,

I've got an Orange OR 80 Combo (1975).
Does it need to be Biased. What is "Fixed Bias"?

Thanks.

Andy H. · Post by **Andy H.** » Mon Sep 13, 2004 7:27 pm

Hi again, Bern 397.

Yes it does need biased.
Ironically 'fixed' bias means that it can be adjusted(!), but a tech will possibly have to replace one or more resistor(s) inside or twiddle a trim pot (if fitted) to do this.

The other type of biasing system is called 'cathode biasing', and is designed to allow tubes to be changed without always having to rebias the amp. There is some leeway built in for different valves. But it has drawbacks too. It's less power efficient - so power loss is a problem, and it is usually only seen on lower powered amps. The Vox AC30 and the Orange AD30 are examples that do use this system.

For some more info, check these out:
http://www.aikenamps.com/ (Tech info section. This is nice and clear stuff, mainly...)

http://aga.rru.com (FAQs section. A bit more detail.)

Hope that helps, and feel free to keep asking away. That's why we're here!

Andy.

Edited by - Andy H. on 13 Sep 2004 19:28:26

fiveightandten · Post by **fiveightandten** » Tue Sep 14, 2004 4:34 am

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>
But it has drawbacks too. It's less power efficient - so power loss is a problem, and it is usually only seen on lower powered amps. The Vox AC30 and the Orange AD30 are examples that do use this system.
<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

One thing i've never understood is how the AC-30 is so freaking loud. The AD-30 too. But my AC-30 is even, volume-wise with a Peavey Classic 100 head I had a while back (8 fixed biased EL-84's). It's as loud as a 50W JCM800. I'm curious to put that amp on the scope and see what it's actually putting out. I may do that sometime this week.

I think a lot of the "lowered" powered EL-84 based amps have low valued coupling caps and concentrate on useing their output power to ampify midrange and treble frequencies, not wasting power on low end response.

-Nick

irish_admiral · Post by **irish_admiral** » Tue Sep 14, 2004 9:49 am

Yeah, let us know what you find. I'd be quite interested in trying to mod my AD30 to get a little more low end out of it, even if it was at the expense of some volume.

Don't tell me the best way, just tell me the cheapest way...

Fixed Bias

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