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Old 25th November 2009, 12:39 AM   #91
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AV7 cathode resistance is like 1/.0061mho=164ohms ???
Plus 2K2 external SRPP resistor gives us 2364ohms...

So, I'm thinking... Split this like 1K2 on the bottom, and 1K on top,
with the load and feedback tapped between. This appears as 600
ohms in series with 40K (SE plate resistance of two parallel AX7's,
one virtual). And the slew capabilities are now symmetrical.

I am aware this level of nitpick is probably silly if the circuit works.
But there are situations where it might not be silly, and you were
already halfway there to having discovered this symmetry of the
assymetrical on your own. Figured I'd nudge you over that edge...

Big gates can be challenging too.

Last edited by kenpeter; 25th November 2009 at 12:50 AM.
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Old 25th November 2009, 01:32 AM   #92
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Default Well............

I ran a few sims with that proposition and the distortion increases about 10 fold. I think there is a significant difference between what you propose to do and what this circuit is doing, and yes, there is always improvement to any circuit, but that approach is not an answer.
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Old 25th November 2009, 03:19 PM   #93
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I wouldn't normally have gone for a mere 2:1 Gm difference...
But I'm a little surprised to hear it actually made things worse.

Is my theory totally broken now, or is AV cathode resistance
doing something strange??? Or merely letting the AX7 plate
directly drive half (double apparent Z) of the real load???

What is the load anyways???
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Old 25th November 2009, 06:53 PM   #94
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Nevermind... I may need totally re-evaluate the way I've been
de-rating output impedance in circuits with asymmetrical pull-up
and pull-down impedances. Its not necessarily that the worser
of the two is the weakest link, as I had till now assumed... It
may be that only Iq and Load are critical factors in pull down.

Last edited by kenpeter; 25th November 2009 at 07:02 PM.
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Old 29th November 2009, 02:45 AM   #95
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Default Mmmmmmm........

You see, in the Vacuum tube preamp using two triodes as a transimpedance amplifier, (ref: WSF detail), the uppermost triode U3 is not a simple pullup but a dynamic amplifier operating in counterphase with the lower triode U2, there is a voltage modulation across the 2.2 k ohm U3 cathode resistor. U3's cathode current is split between the U1 plate current and the the counterphase U2 current. The upper triode is therefore a gain element acting as a modulatable resistance in the plate of the lower triode. Where this differs from the models you are using is in the use of a third triode U1, feeding current into a summing node. This theme is also used to sum together multiple sources all having a common combinatorial node where all summed plate currents have a complimentary current fed back to to the same node through a single Fbk resistor. The U1 can then be any number of triodes. The resulting DC bias condition can then be removed, without causing any distortion component, through the use of a positive current source into the same node. A single triode feed to the summing node is manageable without cancellation unless a larger feedback resistor was needed, then a cancellation current source would be used to prevent clipping. Dad always said that "all these circuits are the same" "they just keep mixing up all those parts to confuse you, just gotta know which parts go where"

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Last edited by Roadbagger; 29th November 2009 at 03:00 AM.
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Old 30th November 2009, 04:28 AM   #96
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Quote:
Originally Posted by kenpeter View Post
I am aware this level of nitpick is probably silly if the circuit works.
But there are situations where it might not be silly, and you were
already halfway there to having discovered this symmetry of the
assymetrical on your own. Figured I'd nudge you over that edge...
A split cathode resistor is used when a triode totem pole must drive relatively low load impedances. The load induced I x R product across the lower portion of the upper cathode resistor is bidirectional and generates counterphase gain modulation of the upper triode and improved symmetry of source/sink currents. No news here. This technique as well as the transimpedance amplifier gain stage were both common circuits used extensively in analog computing arrays in the early 1950's. Nowhere, however, have I ever encountered the term "anti-triode" in literature review.
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Last edited by Roadbagger; 30th November 2009 at 04:31 AM.
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Old 30th November 2009, 10:34 PM   #97
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Anti-triode refers to the imaginary parallel triode faked by the upper circuit.
Especially so when the upper half abuses an overwhelming Gm provided by
a solid state depletion mode circuit. Thus no real Triode ever needed in the
anti-triode emulating upper half.

Or in transformer push pull, if you were to sit both devices atop a pair of
matched CCS bridged by a cap (again sound familiar?). One a Triode, the
other an overwhelming Gm device of whatever solid or hollow state breed
might suits your fancy. This topology works with enhancement mode too.

No matter which side (or both) you chose to drive, the superior device
of this pair forces itself to become anti-complimentary. Takes upon itself
a strangely mirrored, yet somehow opposite curvature of the inferior.
Its really a folded cascode that forces equal and opposing AC currents.
And voltages swing equal and opposite too, caused by the transformer.
These two working together theoretically misbehaves like single ended.
But the matched (and AC coupled) CCS bridge makes it easy to cheat
and abuse a cheapie cheapie toroid transformer that won't tolerate DC...

Thats what I'm basically calling "Anti-triode". There may in some situations
be an advantage that "help" given to the real triode takes a natural shape.
That of a phantom twin. Not to accidentally trick our real reference device
into sounding weird by giving it unexpected help that helps too strangely.
Not to say that strange help is always bad...

But anti-compliments so forced into being by deliberately mismatched
pairings don't need any vacuum device for reference. You can easily
anti-anything (Nelson Pass' Aleph one famous example of such abuse).

I have made some errors in calculating output impedance that made this
arrangement look always "better" to me than Mu-Follower style SRPPs.
Process now of reevaluating a lot of things I had until recently thought
slightly outside the wrong boxes.

---------------------

I don't have much technical reference material from the 50's. My Dad's
widow hated him, and me, and anything technical. Seems she decided
to destroy every trace of his life that didn't pertain directly to her. Mint
Radiotron and all that good stuff gone, most likely shredded... We saw
his office empty, and huge boxes marked "shred". Then again, he was
retired CIA toy factory elite, so who really knows??? Maybe cluelessly
following unclear final instructions tainted by her own misinterpretations
and general hostility.

I'm not going to be capable of any "literature review" outside what I
can find online...

Last edited by kenpeter; 30th November 2009 at 10:47 PM.
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Old 30th November 2009, 11:00 PM   #98
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Quote:
Originally Posted by Roadbagger View Post
The U1 can then be any number of triodes. The resulting DC bias condition can then be removed, without causing any distortion component, through the use of a positive current source into the same node.
Well, I thought I had mentioned that notion earlier in this same thread??
About a current in parallel with the lower triode could allow the upper to
swing more current than the lower does.

But you will recall I also said that added current could/should also be a
counter-counter-modulated helper. Thus do we find ourselves right back
to an optimum balance at N*(Triode-Antitriode)??

-------

Why is an external RFeedback to the grid better than simply choosing
a lower Mu triode to begin with?? I would expect that sort of external
Schade (Zenmod read "Zen") network only necessary to emulate Triode
curves in the case of a statically screened Pentode or FET. Also has the
unfortunate side effect of lowering the input impedance, unless thats
somehow helpful?

Are we forgetting that Mu, the effect of distributed space charge in a
Vacuum Triode or solid state SIT already performs a feedback function,
and probably much more locally and faster than an external path? Are
you preferring an external feedback resistor over lower Mu for reasons
related to linearity?

Last edited by kenpeter; 30th November 2009 at 11:25 PM.
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Old 1st December 2009, 02:36 AM   #99
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Default in other words

Yes..........
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Old 1st December 2009, 05:32 AM   #100
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Default keep it up

I've found that the sum total of your posts when digitized and compiled can be used as a significant random number generator that predicts lottery numbers with the same odds as the precision of predicting rain drop locations in a storm, so keep it up.........................
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