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Autobiasing output tubes (6L6)?
Autobiasing output tubes (6L6)?
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Old 10th August 2019, 12:23 PM   #11
disco is offline disco  Netherlands
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Quote:
Originally Posted by Wavebourn View Post
Fixed bias is the best for the sound, but the worst for convenience.
That probably comes down to two regulated bias supplies, allowing some mA current for peaks. Nobody did an universal print for this task yet? Power could be taken from B+ with a series regulator...
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Old 10th August 2019, 06:32 PM   #12
6A3sUMMER is offline 6A3sUMMER  United States
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disco,

Caution.
If you are going to use regulation on your fixed adjustable bias voltage to the grid,
the bias voltage will remain a constant.

But . . . Do Also regulate the B+, or the tube dissipation will go up.
Or . . .
The combination of unregulated B+ and regulated fixed bias is bad for Triode or Triode wired Beam/Pentode output stages.
And with unregulated B+ for Beam/Pentode tubes in UL mode the screen voltage, current, and dissipation increases; and the plate voltage, current, and dissipation increases too.
Even in Beam/Pentode mode, the problem exists if you do not regulate the screen and B+ voltages, but regulate only the bias voltage.

Be sure to regulate the B+ (and screen V) if you are going to regulate the grid bias.

A simpler way:
If you do not regulate the bias, do not regulate the B+, and do not regulate the screen voltage, then when the mains voltage goes up, the plate current and screen remains somewhat constant, since the bias goes up at the same percentage as the B+ and screen V goes up.

Of course, not regulating has its own tradeoffs, just remember if you regulate one supply, be sure to regulate all the other supplies.

I am just sayin'

Last edited by 6A3sUMMER; 10th August 2019 at 06:44 PM.
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Old 10th August 2019, 09:52 PM   #13
kodabmx is offline kodabmx  Canada
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Autobiasing output tubes (6L6)?
Quote:
Originally Posted by Wavebourn View Post
I could not find the better way to auto-bias than bypassed cathode resistors. If to use CCS it is the best for the convenience, but the worst for the sound. It changes bias voltage following by the sound envelope. Fixed bias is the best for the sound, but the worst for convenience.
Ever try something like this? I use them on my monoblocs. They work well if you bias the tubes hot (Monobloc is 320V, 120mA). Don't try and run class B with it.

Module AB-4 for 4 tubes, PP & PPP amps, requires 6.3VAC & bias supply from the amps circuit, with TTL error signal output. TES
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Old 10th August 2019, 10:27 PM   #14
Wavebourn is offline Wavebourn  United States
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Autobiasing output tubes (6L6)?
Quote:
Originally Posted by kodabmx View Post
Ever try something like this?
If to use servo from cathode current sensing resistor, the result is the same as a shunted by a cap CCS. Envelope of the signal causes changes of average current that lead to variable bias that leads to dynamic distortions. Even when the tube works in class A and below clipping so distortions do not sound nasty, still it decreases realism of the sound. And when the amp approaches clipping it makes it worse, more audible.
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Old 10th August 2019, 11:16 PM   #15
MarcelvdG is offline MarcelvdG  Netherlands
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For those who understand Dutch: the linked circuit works in class A, B (with some quiescent current) and AB. Theoretically the bias can drift when you drive the amplifier far into the class B region with asymmetrical square waves, but who wants to listen to loud asymmetrical square waves anyway?

Bias automaat - forum.zelfbouwaudio.nl
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Old 11th August 2019, 12:02 AM   #16
Wavebourn is offline Wavebourn  United States
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Autobiasing output tubes (6L6)?
Quote:
Originally Posted by MarcelvdG View Post
For those who understand Dutch: the linked circuit works in class A, B (with some quiescent current) and AB. Theoretically the bias can drift when you drive the amplifier far into the class B region with asymmetrical square waves, but who wants to listen to loud asymmetrical square waves anyway?

Bias automaat - forum.zelfbouwaudio.nl

Theoretically it does not matter that our hearing is dynamic in nature and hears dynamic changes that can't be conveniently measured?

Practically it does not matter if steady tones have different, relatively high, THD, while theoretically it matters a lot.
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Old 11th August 2019, 05:40 AM   #17
kodabmx is offline kodabmx  Canada
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Autobiasing output tubes (6L6)?
Quote:
Originally Posted by Wavebourn View Post
If to use servo from cathode current sensing resistor, the result is the same as a shunted by a cap CCS. Envelope of the signal causes changes of average current that lead to variable bias that leads to dynamic distortions. Even when the tube works in class A and below clipping so distortions do not sound nasty, still it decreases realism of the sound. And when the amp approaches clipping it makes it worse, more audible.
True, but you get the benefit of self adjustment along with the lack of wasted power in a resistor
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Old 11th August 2019, 06:55 AM   #18
MarcelvdG is offline MarcelvdG  Netherlands
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Quote:
Originally Posted by Wavebourn View Post
Theoretically it does not matter that our hearing is dynamic in nature and hears dynamic changes that can't be conveniently measured?

Practically it does not matter if steady tones have different, relatively high, THD, while theoretically it matters a lot.
What matters most practically is that the people who built it, like it. They typically use it to listen to music.

In the Dutch thread I linked to, you will find all kinds of autobias schemes. The circuits the thread converged to are shown in the first post.

Regarding the various schemes:
Some schemes just look at the sign of the difference between the momentary cathode current and the set value. These oscillate and/or cause huge distortion for low-level signals.

Others try to keep the average cathode currents constant and can therefore only work in class A.

Others adjust the quiescent current at power-on with the audio muted and then keep the negative grid voltage constant. These are the only ones that work for any type of audio signal, but were considered too complicated.

The final circuit is a design by an elderly Belgian gentleman called Anne, with some refinements from me and many others on the forum. It looks at the time average of the cathode current, but has clamping diodes to make it ignore the big current peaks that occur when the amplifier is driven into its class B region. There is a Broskie design that does the same, but that circuit is unnecessarily complicated while Anne's circuit is not.
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Old 11th August 2019, 07:49 AM   #19
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by kodabmx View Post
True, but you get the benefit of self adjustment along with the lack of wasted power in a resistor
Those who worry about wasted power should not make tube amplifiers, period.
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Old 11th August 2019, 12:08 PM   #20
kodabmx is offline kodabmx  Canada
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Autobiasing output tubes (6L6)?
Maybe not, but deliberately wasting heat in a resistor is different IMHO.
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