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Old 1st February 2012, 10:00 PM   #621
SY is offline SY  United States
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Quote:
Originally Posted by Johan Potgieter View Post

I am simply saying that what difference there is for me, is in the fact that parallel (Schade) feedback still leaves one with an intrinsic pentode, whereas UL leaves one with mainly triode characteristics - through whatever kind of feedback influence.
Is that true? Certainly the source impedance presented to the primary is reduced by the feedback factor. With a low effective plate resistance, I would think that the curves would be more triode-like, and maybe even resemble UL...?
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Old 1st February 2012, 10:22 PM   #622
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Guys,
I think I may have posted these links on a previous discussion.

From Fritz Langford-Smith on Ultralinear.

http://greygum.net/files/radiotronics/UL.pdf
Parts 1, 2 and 3 all in one file.

A better scan of some of this can be found here:
http://www.pearl-hifi.com/06_Lit_Arc...r_Amps_Pt1.pdf
http://www.pearl-hifi.com/06_Lit_Arc...r_Amps_Pt2.pdf

In these articles he addresses the 43% tapping suits all tubes myth.

He also states
"These points all indicate that UL Operation is distinctly different from either triode or pentode mode operation and should be regarded as a separate phenomenon"

and-
I haven't re-read these articles recently but I seem to recall that he stated that UL Operation "observed" performance could not be adequately explained by feedback theory alone.

Ians own observations:
The one thing that people tend to ignore (maybe with good reason but maybe not) is that with UL the screen currents will contribute to the output.

One more "shotgun blast at the barn door" - UL feedback to the screens will NOT be as time accurate as a Schade Feedback connection due to the reactive nature of the output transformer.

Cheers,
Ian

Last edited by gingertube; 1st February 2012 at 10:36 PM.
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Old 2nd February 2012, 02:27 AM   #623
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"I am simply saying that what difference there is for me, is in the fact that parallel (Schade) feedback still leaves one with an intrinsic pentode, whereas UL leaves one with mainly triode characteristics - through whatever kind of feedback influence."

"Is that true?"

I can see where there would be some difference. In the pentode the forward gm is approx. 3/2 power (really more like 2 power for power tubes), while the neg. Schade feedback is linear. So the inherent forward distortion will take large amounts of Schade feedback to correct down to neglible levels.

In the triode on the other hand, the internal neg. fdbk is the inverse of the 3/2 power, and so in theory could correct the forward error with finite internal neg. feedback (just sets the Mu and Rp). In real tubes though, the g1 is more like 2nd power due to "grid proximity island effect", while the g2 is closer to 3/2 power (until g2 current becomes an issue anyway). So real triodes still distort some.

Possibly one could correct the Schade feedback to near triode mode by putting a thermionic diode from the pentode plate through a resistor to B+, and taking the Schade feedback from the top (plate) of the diode. The current in the diode would track the pentode if the load Z is constant. And the voltage variation on the diode would convert the feedback to the 3/2 power. (ie, when the pentode increases it's gm at increased current, the diode increases the Schade feedback in proportion) Interesting experiment anyway. Probably requires some specific diode conductance to track the pentode and generate enough correction. (ie, both pentode and diode need to be operating over the same exact portion of the 3/2 power curve) Likely have to use some flimsy HV dropping diode like a 2AV2 or something similar.

Or alternately, maybe put some resistance in series with the g2 and tap off the Schade feedback from the g2. (since the g2 intercepts approximately a constant fracton of plate current until plate V gets below the g2 V. This method depends on the load R being constant, to get g2 V to track plate V over the linear range) Might be a good way to prevent the tube from drawing too much g2 current too. Of course this would greatly restrict power output.
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Old 2nd February 2012, 03:10 AM   #624
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One more version of Schade corrector:

Put a high current damper diode in series with the pentode's plate, and take the Schade feedback from the damper's plate. So it's self tracking (diode versus pentode) on the 3/2 power curves this way, but less efficient power wise.

A little inconvenient to float the damper diode that way. So I'm thinking maybe put the damper diode in the pentode's cathode circuit instead. That will lower the pentode's gm, but helps enforce 3/2 power law at g1. The pentodes plate still has the diode correction built in since the output OT is referenced to ground. Would have to simulate this to see how well it works I think. Ehhh... Probably can't get enough diode voltage variation these ways without killing the output power. So likely better to stick with the original diode scheme, merely appended to the plate (an R from diode plate to B+), approach.

Edit:
I think I see the way now. A small thermionic diode from the pentode's grid to AC ground through a resistor. So: Diode's plate to the pentode grid, diode cathode to resistor, then that to - bias V. Then the Schade feedback R goes from pentode plate to the cathode of the diode, probably with a series cap to preserve the bias V. Now as the pentode turns on, the Schade feedback makes the diode conduct more (through the previous driver stage plate R) giving tracking 3/2 power feedback. Easy to configure, and efficient. Have to try this out. Will need something like a HV 2AV2 diode to get enough voltage swing correction on the pentode grid I think.
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Old 2nd February 2012, 09:59 PM   #625
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..... by this time it would have been far more convenient to just use a screen tap on the OPT

SY:

I am carefully considering. Look at it the other way round: Take an OPT where the pentode source resistance rp (without feedback) is low compared to the load (an almighty big OPT with a high load, suitable impedance ratio, or simply a constant current load). In a manner-of-speaking, the pentode will still present a set of pentode plate characteristics, not similar to that of a triode. Or if I use a very low load resistor for a triode (say 1 K for a 12AX7) - does the characteristics begin to look like a pentode's - I mean distortion-wise?

(And to those wondering: How does this matter if the driving impedance is lowered? I am thinking of the intrinsic pentode pattern of distortion, with more high-order products than those of a triode. Sure, one is diminishing those by a feedback factor - but so can one with a UL topology. Does that change their ratio to something less prone to listener fatigue? But as said I have to go think ... I have never looked at a constant current loaded pentode with a spectrum analyser. Maybe sit down with Spice before I go on ....)
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Old 2nd February 2012, 10:27 PM   #626
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Quote:
Originally Posted by gingertube View Post
Guys,
One more "shotgun blast at the barn door" - UL feedback to the screens will NOT be as time accurate as a Schade Feedback connection due to the reactive nature of the output transformer.
... and I need to be careful that I am not standing at the other side of that barn door!

But, inasmuch as an output transformer is reactive (or better, its reactiveness plays a role in the audio pass-band), the plate will then also 'suffer' from the resulting phase angle, thus making the Schade feedback also prone to said reactiveness? (This simply means that the OPT design should be such that its reactive characteristics are negligible or at least small within the audio pass-band. This will naturally have to include leakage reactance regarding the screen taps - which is hopefully achieved in the better types of OPT. Aware of this I have looked at phase angles of screen taps relative to anodes, and have mercifully not found anything of sufficient magnitude to worry me.)

It gets worse when using the cathode feedback type of UL, as e.g. in the classic Quad II. Here the 'common' part of the OPT 'sees' a cathode follower action, i.e. working into a low load. Now one does have to worry about phase performance; this OPT design is a little more difficult - but I digress.
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