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Old 25th October 2004, 04:55 AM   #1
hacknet is offline hacknet  Singapore
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Default benifits of auto bias

hi, i`ve been looking around and i`ve noticed that quite a number of amps have chosen to use auto bias with a voltage on the grid compared to having a resistor on the cathode.

is there anything special about having an auto bias compared to a simple fixed bias?
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Old 25th October 2004, 08:20 AM   #2
garbage is offline garbage  Singapore
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Default Re: benifits of auto bias

Quote:
Originally posted by hacknet
hi, i`ve been looking around and i`ve noticed that quite a number of amps have chosen to use auto bias with a voltage on the grid compared to having a resistor on the cathode.
i think you have mistaken... or maybe i am.
what you described is fixed grid bias.

auto bias is mostly implemented with cathode resistor.
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Old 25th October 2004, 11:01 AM   #3
hacknet is offline hacknet  Singapore
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dang... i got it mixed up....
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Old 25th October 2004, 01:14 PM   #4
SY is offline SY  United States
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Fixed bias (a confusing term- it's actually adjustable) trades off some DC stability and complication for more power and lower distortion. Take your pick.
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Old 25th October 2004, 04:36 PM   #5
hacknet is offline hacknet  Singapore
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oh... okay... what sonic effects would i expect? more bass, finer highers?

Thanks a million!
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Old 25th October 2004, 04:53 PM   #6
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Quote:
what sonic effects would i expect? more bass, finer highers?
Not an easy question. Fixed bias has obviously better bass in all configurations i've tried. The rest may vary upon the specific implementation.

Some power triodes are more willing to draw grid current with fixed bias. This obviously increases demands towards the biasing circuit. I have always been suspicious towards the sonics of the 25W cathode resistors but replacing those (and the cathode caps) with a fixed bias circuit does not necessarily bring all-round improvements. Often the sound becomes too dry and solid-state-like; probably the sonics of the fixed bias network become obvious.

In my current amp this dillema is even more acute - the cathode bypass cap is replaced with an 'ultracap' style capacitor and i just don't see how a fixed bias arrangement can possibly emulate the sonic advantages of the ultracap connection.

At the end, as usually, the best approach is to try several biasing arrangements and choose the best sounding one in the context of your output tubes, your system and your ears.
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Old 26th October 2004, 12:57 AM   #7
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Hi,

Quote:
Often the sound becomes too dry and solid-state-like; probably the sonics of the fixed bias network become obvious.
Exactly.
The PS for the fixed bias supply is often the last place people look at when improving an amplifier.
IME_ I'm not alone either_ it's well worth the extra time and effort to put in some quality components and improve the circuit with whatever means available.

Some circuits will show more audible improvements then others but that's nothing new either...

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Old 26th October 2004, 02:02 AM   #8
SY is offline SY  United States
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The main differences are not because of fixed versus automatic per se; it's really more a question of implementation and how much attention the designer has paid to overload recovery. This bias of the output tubes is the very spot where most verifiable audible problems start. Blocking distortion is very recognizable and very offensive.
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Old 26th October 2004, 02:18 AM   #9
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Hi,

Quote:
Blocking distortion is very recognizable and very offensive.
A major cause of output tube failure too IME.
OTL amps or rather the type of tubes these amps use are particularly sensitive to blocking distortion; they don't seem to handle this well at all which is why some designers resort to huge gridstopper resistor values IMO.

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Old 26th October 2004, 02:50 AM   #10
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Sy wrote:

Fixed bias (a confusing term- it's actually adjustable) trades off some DC stability and complication for more power and lower distortion. Take your pick.

(Oh, BTW. Hi Sy! How have you been!?)

My reply:

HUH!!!!!????????????????

I do not understand the above. "Fixed" bias is adjustable only inasmuch as when one replaces tubes, one adjusts bias to the proper spec... then leaves it alone... or... fixed.

Depending on where one places the fixed bias, and yes I agree that one usually finds fixed bias on amps where the tubes are being pushed for maximum power output, the sound quality can be every bit as good, if not better than standard or cathode bias.

Just talk to the folks at Conrad Johnson, who both uses solid state rectification and voltage regulation, and also fixed bias. I have heard their amps. I for one would not argue with them! You know, proof in the pudding and all that.

An amp with lower distortion... sounds good! Yes, if the bias is fixed to the maximum of the tube, where it is virtually class B, then it does tend to sound more solid state. But one can fix bias at a more AB1 bias and if one would pardon the expression have one's cake and eat it too (the afore mentioned Conrad Johnson). So one can get better dynamics and tighter deeper bass and have that famous tube sound.

Want more power? Use bigger tubes and higher B+. Me, I want better sound. SS rectification, voltage regulation and fixed bias give that to me. But the audio path must see grids, cathodes and plates.

Now, amps that draw grid current for more power also need fixed bias, but are only designed that way if the engineer wants it that way, hence the class A2, or AB2, etc. But that's another argument.


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