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  1. Replies
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    So the circuits are different but their EQ is...

    So the circuits are different but their EQ is similar? :confused:
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    It looks like they were trying to increase gain...

    It looks like they were trying to increase gain (does it need more gain?) Shorting R18 increases the gain of V1B by bypassing its cathode resistor with C26, A job already done by K1A in the position...
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    Sticky: Could you use the online version? - Image2PDF...

    Could you use the online version? - Image2PDF
    I use this one - To PDF
  4. Replies
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    The voltages I posted were across the 220k/110k...

    The voltages I posted were across the 220k/110k resistors but I forgot to increase the cap values (it was late), When I calculated the 5D5 output impedance I used huge caps. Doing the same with...
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    I'm not sure. Asymmetrical clipping, increased...

    I'm not sure. Asymmetrical clipping, increased distortion before clipping, reduced output power?
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    47n, 25k build-out 55158

    47n, 25k build-out
    55158
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    I surely did :)

    I surely did :)
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    220k AC load Left triode 2.62V, Right triode...

    220k AC load
    Left triode 2.62V, Right triode 2.53V

    110k AC load
    Left triode 2.256V, Right triode 2.173V

    Edit:
    220k Left triode, 110k Right triode
    Left triode 2.62V, Right triode 2.248V
  9. Replies
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    Yes, I could do that. Which schematic did you...

    Yes, I could do that. Which schematic did you have in mind?

    I calculated the output impedance of the 5D5 second triode in sim and it is 12k as Zollner stated.
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    I tried it in sim on your circuit and 25k was the...

    I tried it in sim on your circuit and 25k was the optimum value for the build-out resistor.

    25k build-out resistor
    55154

    no build-out resistor
    55155

    Edit:
    The above plots were with 2u2...
  11. Replies
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    5D3 has the second triode's cathode unbypassed. I...

    5D3 has the second triode's cathode unbypassed. I think that halves open loop gain and doubles output impedance. Would that account for its high output impedance?

    There's also a 56k from the...
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    I don't think there will be any differential...

    I don't think there will be any differential ripple signal with that cap removed. It removes the second triode's NFB making its output impedance the same as the first triode and the second triode no...
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    To use unmatched tubes?

    To use unmatched tubes?
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    You may well be right :)

    You may well be right :)
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    True, the autoranging fooled me. I just noticed...

    True, the autoranging fooled me. I just noticed the increased phase shift and the differential signal now being larger than the grid to ground signal. There goes another theory. I'm still thinking...
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    Sorry, they are the caps between the PI plates...

    Sorry, they are the caps between the PI plates and the power tube grids.
  17. Replies
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    Green and blue power tube grids wrt ground, red...

    Green and blue power tube grids wrt ground, red grid to grid differential.
    First plot 47n, second plot 470n.

    55135

    55136
  18. Replies
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    By interstage do you mean the cap connected to...

    By interstage do you mean the cap connected to the virtual earth triode's grid? When I did the sim that one didn't have much effect. Try increasing the value of the caps to the power tube grids and...
  19. Replies
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    I didn't want to risk one of them being connected...

    I didn't want to risk one of them being connected to a PI output.
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    I think now I understand why the long tail pair...

    I think now I understand why the long tail pair PI has much better bias supply ripple rejection than the floating paraphase. The floating paraphase by its very nature converts the common mode signal...
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    I no longer have access the those sorts of...

    I no longer have access the those sorts of things. I have to make do with minimum test equipment and a small table in the spare bedroom (and LTSpice of course) :)
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    I'm not sure I understand. Connect the earth...

    I'm not sure I understand. Connect the earth clips where, to chassis? If neither point being measured is ground that's all you can do.
  23. Replies
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    The PI outputs are low impedance compared to the...

    The PI outputs are low impedance compared to the 220k resistors from grids to bias supply so there's a potential divider which reduces the ripple at the grids when the PI is connected.
  24. Replies
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    A scope is usually used to measure a signal wrt...

    A scope is usually used to measure a signal wrt ground. Differential mode is where both channels are used to measure the difference signal between two points not wrt ground. Set the scope channels to...
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    I couldn't understand why it didn't work as I...

    I couldn't understand why it didn't work as I described (badly) in post #55 (outputs out of phase) so I decided to simulate it by injecting a 50Hz sine wave where the bias supply connects. The first...
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    There's some phase shift there. Perhaps that's...

    There's some phase shift there. Perhaps that's why the ripple isn't rejected more?
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    The level of ripple in post #42 doesn't seem...

    The level of ripple in post #42 doesn't seem unreasonable. I'd expect getting on for 1V p-p on the raw bias supply with a diode and 100u capacitor straight off the winding. Try adding a 100u from...
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    When I was trying to figure out how they worked I...

    When I was trying to figure out how they worked I assumed that the broken line between the wipers of the two sides indicated that they were ganged not electrically connected. I thought they would...
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