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Old 20th February 2006, 01:56 AM   #1
HFGuy is offline HFGuy  Canada
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Default Fixed bias vs. auto bias

iam sure this question has come up before or is even unanswerable but i will ask regardless.

Iam planning to build a kt88 amp, i found the circuit for the modified williamson circuit on the Plitron website. A friend mentioned to me that fixed bias was for guitar amps and i should convert to auto bias. Should i keep the fixed bias or convert it back to the williamson? Your thoughts, opinions and past experiance would be greatly appreicated.
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Old 20th February 2006, 03:14 AM   #2
Trout is offline Trout  United States
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Default Re: Fixed bias vs. auto bias

Quote:
Originally posted by HFGuy
iam sure this question has come up before or is even unanswerable but i will ask regardless.

Iam planning to build a kt88 amp, i found the circuit for the modified williamson circuit on the Plitron website. A friend mentioned to me that fixed bias was for guitar amps and i should convert to auto bias. Should i keep the fixed bias or convert it back to the williamson? Your thoughts, opinions and past experiance would be greatly appreicated.

Fixed bias for guitar amps? Guitar amps historically speaking have been and continue to be both. Look at all the old Fender designs,They used both types. I generally prefer auto bias.
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Old 20th February 2006, 06:45 AM   #3
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I use fixed bias with EL34 triode-connected PP, mainly because I don't have enough B+ for cathode bias. However, l do also get the benefit of being able to choose the operating point and go deeply into class AB if I want to.
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Old 20th February 2006, 11:53 AM   #4
SHiFTY is offline SHiFTY  New Zealand
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Fixed is best for high-power, high voltage amplifiers such as KT88 PP, as you get a lot more control and more power. Also you don't bleed off a lot of power through large cathode resistors.

If you are going for triode-connected with a lower B+ , then it might be ok as autobiased.

I recommend fixed bias with individual adjustments for each tube, that way you can use unmatched tubes and adjust the idle current to be equal. This is very important with PP amps as the output transformers do not like unbalanced DC in the primary- even more crucial for toroidals like the plitrons I believe.

Its a bit more work but worth it. Also you should try regulated screens (g2) using a tube regulator if you don't like ultralinear...
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Old 20th February 2006, 04:05 PM   #5
kevinkr is offline kevinkr  United States
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I generally prefer fixed bias in my designs both SE and PP as it allows me to easily fine tune and find the "sweet spot" operating point for the output tubes I use, and perhaps more importantly it eliminates the need for high quality cathode bypass capacitors. This also makes more efficient use of the available B+ and wastes less power than autobias. (The power dissipated in the cathode resistor is wasted energy and can be a very significant % of the overall power consumed in an output stage. )

Good fixed bias designs recover much more gracefully from sustained clipping where the output tubes in a pp autobias design may be driven so far into class b that when signal drive is reduced to normal (not clipping) levels the output tubes actually cut off for some period of time - this is called "blocking" and sounds truly terrible. (Larger cathode caps needed for better bass response exacerbate this due to the longer time constant of the bias network) Note this is a simplification and this phenomena can be made even worse by bad driver stage design, but in general fixed bias designs can and do recover more gracefully from overload.

I have designed power amplifiers around everything from a 6BQ5 to K90 as well as dhts such as the 45/2A3/300B using fixed bias.

IMHO Autobias amplifiers to me usually (not always) sound more euphonic and less detailed, but for this reason may be more forgiving on bad sounding material.

edit: typo, and content
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Old 20th February 2006, 08:20 PM   #6
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Hi Kevin, it sounds like the blocking you're talking about is the cathode bypass capacitor charging up?

The blocking that I've always believed to be a problem is the coupling cap charging when the grid is driven positive causing grid current to flow. It gets charged such that the grid becomes more negative (the same result you spoke of.) Obviously, it takes some time for the cap to discharge through the grid resistor (with some time constant RC.)

Anyway, the way that I've understood (and believe) it is that a self biased amp will tend to increase the bias under clipping, thus reducing the amount of blocking caused by the coupling cap. This is the popular explanation for why self-biased amps often seem as though they can play louder than a similar fixed-bias amp even though with fixed-bias the maximum output can be measured as being higher.

A grid choke or tranformer coupling will provide a low DC resistance for the coupling cap to discharge with a resulting reduction in the time it takes to recover from blocking.

-- Dave
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Old 20th February 2006, 08:46 PM   #7
kevinkr is offline kevinkr  United States
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Hi Dave,
Yes, that's what I was talking about, I have run into this scenario in amplifiers I have troubleshot for friends that used really large (>470uF) cathode bypass capacitors.

The time constants I use in my grid coupling circuits usually don't result in audible blocking under any circumstances I have encountered, however the mechanism you mention does exist, and can be a major problem if very large values of coupling capacitance are used and/or the grid bias resistors are large values. (I have seen just this problem in Williamson based amplifiers with inappropriately chosen first and second stage phase splitter time constants when oscillating at subsonic frequencies.)

Interesting your experience with autobias has been more or less the exact opposite of my impression for a given steady state power rating, I have always felt that the fixed bias amplifier played louder, and dynamic headroom on burst waveforms has generally has measured better on amplifiers running fixed bias in my experience. (only) I suspect that other factors are also playing a role here, my regulated power supply designs are very stiff, and my amplifiers employ fixed bias, whereas the comparison amplifiers had beefy, but unregulated supplies and autobias.

What you say is true in terms of grid cap blocking, however I think the actual mechanism with autobias is that the effective increase in bias may actually prevent positive grid current from flowing.

I am definitely with you on the grid choke thing, I have designed such amplifiers (SE) for others and they have a lot of other advantages as well.. Using a 50 DHT this is the only practical way to go other than a IT..

Kevin

edit: fix typo, add comment

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Old 21st February 2006, 04:41 PM   #8
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Why you don't try DC servo bias?


See:
http://www.normankoren.com/Audio/TENA.html


[]s
Joo
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Old 21st February 2006, 06:19 PM   #9
HFGuy is offline HFGuy  Canada
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well i guess my quick answer would have to be, if i wanted overly complex i would have gone with a multistage SS design
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Old 21st February 2006, 06:53 PM   #10
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Quote:
Originally posted by kevinkr
I generally prefer fixed bias in my designs -
Good fixed bias designs recover much more gracefully from sustained clipping where the output tubes in a pp autobias design may be driven so far into class b that when signal drive is reduced to normal (not clipping) levels the output tubes actually cut off for some period of time - this is called "blocking" and sounds truly terrible. (
Kevin, I agree. I made a autobias based on 30 mA CC source with 200 muF on a EL84 (in fact ECL86 on a Revox Modell 40) and observed that after clipping the capacitor could load up to 30 volt - and stay there for the next cycle! . So Dave, the -Vbias is obtained by filling the cap fast and slowly, throttled, release. In my case, very slowly.

So what I did is bypass the whole CCS with a resistor taking another 5 mA (at the normal op point of 10V bias/300 V anode) and this really was able to bleed the capacitor in a gracefull manner .
I use the CC source just to force the tube into balanced mode because transformers are less forgiving for inbalance; should the tubes, which are now paired, be replaced in some futrure, then the owner will not be harmed by imbalance.

I also observe that small caps have just the same punch as large ones: even interstage capacitors can be smaller than one would like on the calculator.
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