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idea for a simple bias servo for AB PP

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The purpose of this servo is to maintain steady *combined* bias current for a PP output stage in AB. Balancing bias current between the tubes has to be done manually. But it is simple enough to be used where dedicated fixed bias winding is unavailable and therefore proportional g2 voltage and bias voltage is difficult to obtain. Stabilization of g2 voltage is not required with this circuit.

First the simplified block diagram illustrating the idea. D1/C1/R2 captures the bottom of each current cycle (the trough of the 2nd harmonic of the signal) which corresponds to the quiescent current in both steady state, class A and class AB operation. C1/R2 time constant is set to sub-audible frequency range. The a simple servo uses this voltage to set the bias voltage

An externally hosted image should be here but it was not working when we last tested it.


And the possible real implementation with a single pnp transistor servo and all necessary power supply circuitry. R6 trim pot used to manually balance bias current. Note that wiper disconnect event on R6/R7 lowers the bias voltage so it is safe. Zeners provide stabilized positive rail for reference voltage and negative rail for bias voltage (negative rail is stabilized only to limit max collector voltage). A current-limited AC feed from B+ or some other suitable winding provides the power for the servo.

An externally hosted image should be here but it was not working when we last tested it.
 
I built a circuit like this last year and showed it at VSAC this spring.
One difference is that I applied it to class A using 300B tubes,
but the principle is the same. I track the troughs of the
combined cathode current waveform using a negative peak
follower circuit and fed that to an error amplifier with a LPF,
and back to control the cathode voltage. I also tried a balancer
circuit in the loop.

http://db.audioasylum.com/cgi/m.mpl?forum=tubediy&n=142992&

My goal is to provide the "stiffness" of fixed bias in class A with
the stability of cathode bias or even CCS bias.

I had trouble with stability at large signal conditions, and I have
not completed the investigation into why. Some could have been
implementation issues with my opamp based circuit (basic debug)
but also there were some things with the behavior of the cathode
current that I didn't anticipate. Basically, I went back to ruminate
on it some more and switched the amp to resistor bias for more
measurements.

I like your approach of a simpler circuit, though I haven't thought
through your particular circuit through yet.

I'll dig back through my notes and see if there's anything that might
help. One suggestion I do have is to make a lot of observations
under different signal conditions so you really understand what it
is you're controlling ;-). I haven't looked at class AB for a while but
it could be a whole different set of issues.

Anyway, it sounds like a great idea and project and I look forward
to hearing about your experiences.

Michael

PS some of the problem is class A was due to signal current unbalance
 
Any AB biased amp has a region of class A operation. So I did some simulations introducing deliberate imbalance in both gain and static bias running at different signal levels (including class A operation). Imbalance leads to imperfect suppression of the main tone at the common cathode resistor and some interesting trough shapes. However the resulting bias estimation error was really small (below few percent).

I think what you may have seen is an interaction between your bias balancing servo and common bias control servo if you had both loops running simultaneously.
 
I thought that early on, so I disabled the balancer and it still had
the same instability. At some level of signal, the controlled current
would oscillate (signal generator input) or jump up (music peaks)
at the time constant of the servo LPF.

As I recall, the sensed cathode current would briefly drop to near
zero on one side only, leading to the servo over reacting and driving
the current up on both sides by about 1.5X until the servo settled
again. I could observe this drop in cathode current across the cathode
resistor, so I concluded that my MOSFET voltage regulator was
probably glitching for some reason. I tried the usual remedies
like feedback caps etc. but never really tracked it down.

Michael
 
I've done some more simulations with the loop closed (needed to tweak some values to make it work). I do not see signs of instability with closed loop either (again trying different levels and imbalances). There is some 2nd harmonic feed into grids but it is symmetrical so it does not affect amp output much. But maybe this feed has a potential of creating some instability via non-linear effects when the servo gain is too high.
 
Here was mine, from Alligator project (SE, of course class A):

alligator301.gif


Edit: for class AB I was thinking about a similar device with switched time constant, like a S/H: shorter time constant on low signal levels, and very long one with some signal applied. It should work well. Also, I saw some guys implemented PIC device that used to "reset" bias when the amp is switched on each time.
 
R1 is a resistor. Held at constant current (by servo or whatever)
means a constant voltage drop. Replace with a string of LEDs
and be done with it.

If you wanna get fancy, an adjustible current source bypassed
with a good audio cap will get you the same result.

I'm not sure why the need to return your servo to the grids
rather than to the cathode? Just seems rather complicated...
 
kenpeter - I think you are missing few points. First of all your suggestion of shunted CCS is not suitable for AB operation. As for your suggestion of a fixed drop in cathode circuit - there are a lot of ways to generate stabilized fixed bias, all of them require stabilized g2 voltage as well for a pentode design which this is all about.

BTW Michael Koster circuit does exactly the same thing (I had little doubts that I am reinventing the wheel here since I am new to the tubes) and the servo feedback goes to the cathodes but more complex servo amp is required.
 
Sorry, most my designs have twin parallel current sources with
some sort weird cathode bridging or bypassing. Got blindsided
by the not-so-weird single tail.

I agree, with only one common tail for DC, the single bypassed
current source does not bias optimally for AB.

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You have been less than clear in some details. How exactly does
G2 supposedly figure into it?
 
The zero crossing is also the minimum voltage regularly seen at
the tail resistor... Does that make it simpler? So the servo might
track and act only upon the minimum observed... I dunno, maybe
its a completely daft trainwreck of thought? As-if I'd never been
previously accused.
 
Plan B from outer space.

Using the minimum voltage with a negative peak detector would seem to be the approach already taken above.

Another way would be to fill in the missing part of the class AB waveform for each tube (making simulated class A). Using some series LEDs in each of the cathode circuits, sufficient to generate the class AB bias, would allow some small remnant of class A operation to still exist in a compressed form as the resistance of the LEDs increase at low currents. (ie, the tube would now turn off the last tiny bit slowly as if still in class A, never dropping completely to zero current. This is using the non-linear transfer curve of the LEDs.)

By putting the same series of LEDs in the feedback path of the "detector" (now better called the "decompressor") Op. Amp., the shrunken "missing" class A signal can be expanded or "restored" for servo control by simple summation and LF filtering. One decompressor needed for each side of P-P. (Could also view this as compressing the current signal from the class AB part to match the remnant class A part for each tube. )

Not much different really from the present scheme, just need matched LEDs for each side. This may not be practical due to the precision needed. Would have to simulate this first to get the ideal case working, then try selected real LEDs in a real circuit. Operation would likely require matched current levels thru the feedback LEDs and cathode LEDs. Loop gain will vary between the class AB portion and the expanded missing remnant, possibly causing stability problems.

Don
 
I was wanting to "simplify" by controlling bias at the cathode(s).
Yet sensing the "problem" at the same cathode(s) node as the
"fix" might be applied, makes the "problem" seem twice worse.
I didn't seem to be getting anywhere at first...

But the cathode isn't the only place I might sense the current or
voltage drop minimum that indicates a zero crossing. The center
tap up top can supply this same information. Applying the "fix"
at the bottom now doesn't positive feedback onto the measure.

I am sleepy, and this may be utter nonsense...

"Sense" the minimum voltage drop, low-pass it, and generate
a high impedance voltage controlled current to tweak the bias...
Whatdya get when you cross an Elephant with a Rhino? Elephino...
 

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Some slight modification (and with better thermal stability). When simulating I realized that R7 has to be much lower to provide stiff rectification. Direction of D1 may be misleading but the sole purpose of it is to provide fixed voltage drop matching Q1 Vbe.

An externally hosted image should be here but it was not working when we last tested it.


To minimize the cathode feedback one can use pnp transistor in the cathode ciruit fixing cathode voltage in a cascode-like arrangement. If it is also used to control cathode voltage this is essentially how MichaelK circuit works. Also I think an adjustable shunt reference can be made to work in this circuit somehow for better gain and thermal stability.

I want to emphasize that such servo is not necessarily the optimal solution for all cases, sometimes stabilized g2 and fixed bias could be easier. I am actually contemplating using one in my minimalistic 829b amp however depending on power transformer choice will go with either the servo or stabilized g2/bias voltages.
 
Re: Plan B from outer space.

smoking-amp said:
Using the minimum voltage with a negative peak detector would seem to be the approach already taken above.

Perhaps I misunderstand the mechanism, but looks like the
"approach already taken" does not exclude positive peaks,
therefore may "breathe" as volume changes from class A to
class AB1.

I am suggesting you have to detect the class A minimum,
once per crossing, that never drifts. Rather than the class
B maximum value, or even an average, that varies with
the music content.
 
"does not exclude positive peaks,
therefore may "breathe" as volume changes from class A to
class AB1."

Hmmm, I'm trying to picture this but not getting it. Seems to me that negative going peaks while in class B territory (the current sum is always positive) will not be lower than the minimum seen at crossover in class A territory. Minimum voltage (or current really) detected with a negative going peak detector should be catching the smallest current level at the crossover. Can you give an example of where another negative going peak would be lower?

I can see where non-linearity in the class A region might cause a false minimum. This is because the two tube currents nominally sum to a constant thru-out the region, so any sudden fall off in gm of one or both tubes could drop this level below the level at crossover. But for reasonably matched tubes, the usual tube non-linearity makes for slightly increasing current sum as the crossover point is departed.
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"Solved problem. There's a perfectly good circuit shown in Morgan Jones's "Valve Amplifiers," 3rd edition."

I looked thru the book this morning, but no luck. Any hint on where this is at?

Don
 
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