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Old 18th October 2008, 12:32 AM   #11
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Even parallel bypassed current source(s) are wrong for AB1.
This is a much more complicated problem than I first thought.

Almost like the servo should sample and hold only on the
class A zero crossing, and somehow ignore everything else...
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Old 18th October 2008, 12:58 AM   #12
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"Almost like the servo should sample and hold only on the
class A zero crossing, and somehow ignore everything else..."

Yes, and how might that be done?
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Old 18th October 2008, 01:59 AM   #13
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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.
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Old 18th October 2008, 02:53 AM   #14
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Default 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
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Old 18th October 2008, 02:55 AM   #15
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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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Old 18th October 2008, 03:34 AM   #16
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Call me paranoid... Imagining a problem that doesn't exist.
Something bout that emitter resistor, can't put my finger on it.
Feels "safer" like so?
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Old 18th October 2008, 04:18 AM   #17
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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.

Click the image to open in full size.

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.
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Old 18th October 2008, 12:42 PM   #18
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Default Re: Plan B from outer space.

Quote:
Originally posted by smoking-amp
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.
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Old 18th October 2008, 12:46 PM   #19
SY is offline SY  United States
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Solved problem. There's a perfectly good circuit shown in Morgan Jones's "Valve Amplifiers," 3rd edition.
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Old 18th October 2008, 06:29 PM   #20
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"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.
--------------------

"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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