My friend bought a "Dared" 6L6 PP amp. He says it's Class A AND Auto-biasing. What is the methodology of "Auto-Biasing." Anyone heard of "Class A" Dared PP amps? Yes I could look up the Class A part, but I'm truly curious about this Auto-Biasing, and I don't want to read a vendor's description! Thanks in advance for your time answering my questions!
its a current sense feedback to the voltage bias... very simple.
however the more i understand output transformers and output valves, the more I think a perfect balanced bias does not matter.
The tolerance depends on the transformer,
the most important is not to have 0 null current at the B+ tap.
What is important is to keep the transformer inductances (both phases) to the maximum values.
Any drop in the OT inductance caused by some effects of current is very detrimental to the whole THD, sound and bass accuracy.
if the windings require -1.5mA bias or 2mA as a resultant, then it is the best bias.
Note too that as things warm up, the optimal bias point can shift somewhat.
An auto-bias should take the warm up in consideration which is around 15-30 minutes on a well ventilated amplifier.
however the more i understand output transformers and output valves, the more I think a perfect balanced bias does not matter.
The tolerance depends on the transformer,
the most important is not to have 0 null current at the B+ tap.
What is important is to keep the transformer inductances (both phases) to the maximum values.
Any drop in the OT inductance caused by some effects of current is very detrimental to the whole THD, sound and bass accuracy.
if the windings require -1.5mA bias or 2mA as a resultant, then it is the best bias.
Note too that as things warm up, the optimal bias point can shift somewhat.
An auto-bias should take the warm up in consideration which is around 15-30 minutes on a well ventilated amplifier.
There's multiple methods I suspect. One is the so called "servo - amplifier" which is basically an op-amp set up with a very long time constant, usually implemented using a capacitor. There's controversy whether that capacitor is in the audio path - and I recall a friend 20+ years ago ripping out all the caps and replacing them in his servo amp designs.
There's other methods where the website selling them puts in all caps NOT IN SIGNAL PATH IN ANY WAY or some such claim - maybe they use a microprocessor or something to create the instrument that does this. So I guess this part of whatever implementation is bothersome and I'm sure there's 101 ways to skin that cat.
I'm finding out that the output tube bias isnt so critical in terms of precision - it's like the inflation on your cars tires, where getting all 4 to within a fraction of a psi automatically is more of a convenience, than something that's going to blow you away in terms of how the car drives with such a gizmo installed.
There's other methods where the website selling them puts in all caps NOT IN SIGNAL PATH IN ANY WAY or some such claim - maybe they use a microprocessor or something to create the instrument that does this. So I guess this part of whatever implementation is bothersome and I'm sure there's 101 ways to skin that cat.
I'm finding out that the output tube bias isnt so critical in terms of precision - it's like the inflation on your cars tires, where getting all 4 to within a fraction of a psi automatically is more of a convenience, than something that's going to blow you away in terms of how the car drives with such a gizmo installed.
I use individual self bias resistors and caps for each tube, and professionally matched output tubes. The plate currents match very well. That way, the OPT gets "babied". I do not use regulated filament supplies, and do not use regulated B+. But as the mains range from 117V to 123V, the plate currents still do match. Simple and effective. The match stays good as the tubes age together. My tube vendor, eurotubes.com, re-tests the JJ tubes (multiple tests for noise, match, etc.). They are local, and I drive over and get what I want.
Last edited:
Some do not like to use a self bias resistor and bypass cap, because they are worried about the time constant, and the resultant bias shift with very large and or long transients . . .
But some of those same people really like the idea of a microprocessor system that uses a current sense resistor (which requires a capacitor or other form of integrator). Well, to that I say . . . there is a time constant there too, and you will get the same bias shift results.
Same results, which is easier?
But some of those same people really like the idea of a microprocessor system that uses a current sense resistor (which requires a capacitor or other form of integrator). Well, to that I say . . . there is a time constant there too, and you will get the same bias shift results.
Same results, which is easier?
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...
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'
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:
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
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.
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
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.
True, but you get the benefit of self adjustment along with the lack of wasted power in a resistor
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.
Those who worry about wasted power should not make tube amplifiers, period.True, but you get the benefit of self adjustment along with the lack of wasted power in a resistor
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.