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Uneven resistance on plate

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  • Uneven resistance on plate

    I've seen a couple of schematics that had resistors on one plate in each pair in a quad power stage. (I think it was some Marshall amps.) What is the point? Is it to prevent some sort of oscillation..? Just curious, if anyone would chime a little wisdom here...
    In this forum everyone is entitled to my opinion.

  • #2
    Can you provide an example.
    That makes no sense.
    How can the current be shared , being in parallel, if there is a resistor on one tube of the pair?
    FYI: I looked at every Marshall schematic that I have.
    None have a resistor on the plates of the output tubes.
    Excepting the 200 watt Major.
    In that case all four KT88's had a 100 ohm resistor on the plates.

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    • #3
      Did you mean the power tube grids?
      Originally posted by Enzo
      I have a sign in my shop that says, "Never think up reasons not to check something."


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      • #4
        Eh ok, scrap the Marshall stuff I wrote. I had to search for schematics with this feature... Here's one.

        Click image for larger version

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        In this forum everyone is entitled to my opinion.

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        • #5
          I don't see any resistors connected to the plates. Grid and screen resistors are the same for push & pull. Please specify the component numbers you mean.
          Originally posted by Enzo
          I have a sign in my shop that says, "Never think up reasons not to check something."


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          • #6
            I don't see it.
            Other than the spike suppression diodes, it's a clear connection from tube plate to tube plate/ pair.

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            • #7
              I think the issue is with the screen resistors on only 2 of the 4 tubes.

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              • #8
                I had this question with a Traynor YGL-3 Mark III. This amp also has a quad of power tubes.

                Enzo provided the answer:

                "Provides some isolation between the tubes.It prevents the screens from interacting"

                http://www.webphix.com/schematic%20h...ii_ygl3_3a.pdf

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                • #9
                  those are the screen resistors.

                  Normally you'd want 1 per tube.

                  they used one per push/pull pair.

                  profit - trillions!

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                  • #10
                    Thanks Markr, now I see it, 2 screens with screen resistors, 2 screens straight to the screen supply.
                    Originally posted by Enzo
                    I have a sign in my shop that says, "Never think up reasons not to check something."


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                    • #11
                      The point is, i think, to mix two different compressions curves >> "richer sound"

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