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Old 20th December 2010, 03:20 PM   #1
Cassiel is offline Cassiel  Libya
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Default Ultralinear but no UL taps.

I don't understand how this works. Maybe somebody can explain it to me.
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Old 20th December 2010, 03:36 PM   #2
SY is offline SY  United States
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The screens are driven by the two 12AU7 cathode follower sections. The input to the cathode follower sections is a tapped down version of the primary voltage, adjustable by the pots. If the 12AU7 plate voltage is high enough, it could work.
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Old 20th December 2010, 03:44 PM   #3
Cassiel is offline Cassiel  Libya
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Thanks, I might experiment with this one. Just out of curiosity.
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Old 20th December 2010, 03:49 PM   #4
Yvesm is offline Yvesm  France
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Yeap, the 12AU7 plate voltage should be much higher than stated !
But you loose the contribution of screen current to output power.
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Old 20th December 2010, 04:08 PM   #5
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The ends of T1 will swing above B+, the triode cannot follow.
It provides for adjustable UL or Triode feedback, but less than
half wave. The results might not behave exactly as intended.

Such feedback will fight the pentode from entering saturation,
but would do nothing to fight the pentode from entering cutoff.
It might be some bizarre sort of Class AB crossing scheme?

Might have been better if a Triode strap resistor fed screen
directly from plate, and 12AU7 shunted excess current away?
No, even that doesn't work full cycle. What was I thinking?

Last edited by kenpeter; 20th December 2010 at 04:17 PM.
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Old 20th December 2010, 06:05 PM   #6
DF96 is offline DF96  England
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I think the quick answer is that it doesn't work. It might work if the triode anode had a much higher voltage, about twice the supply voltage.
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Old 20th December 2010, 06:45 PM   #7
Yvesm is offline Yvesm  France
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Quote:
Originally Posted by DF96 View Post
I think the quick answer is that it doesn't work. It might work if the triode anode had a much higher voltage, about twice the supply voltage.
Glad you agree
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Old 20th December 2010, 06:55 PM   #8
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Bootstrapping the plates of the screen driver to the respective output tube plates might work, but now we're actually consuming power out of the primary, rather than aiding it. A neat thought, but hardly practical as shown. OTOH, I think ARC in fact did something along these lines (UL w/o UL taps), using Mosfets as screen drivers.

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Old 21st December 2010, 03:07 AM   #9
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There was some talk and even a few experiments along these lines in this thread. I breadboarded my circuit, but it still sits on a shelf awaiting further attention.

Adjustable distributed load discussion
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Old 21st December 2010, 01:53 PM   #10
teemuk is offline teemuk  Finland
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Quote:
The ends of T1 will swing above B+, the triode cannot follow.
It provides for adjustable UL or Triode feedback, but less than
half wave.
It is true that the screen voltage won't track the output signal for a full wave but in class-AB the output tube will not conduct current at those times so I don't see how this would really hinder the operation. In fact, doesn't the screen stop drawing any current even in a "real" ultralinear circuit even though the voltage at the screen does swing a full cycle? Essentially, I see the triode being capable of buffering the screen current at the times when it is actually needed to do so. ...In class-AB circuit that is; with class-A bias the incapability to swing a full wave might prove to be a problem.

Last edited by teemuk; 21st December 2010 at 01:58 PM.
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