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High screen grid voltage on 5C1 sort of amp

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  • High screen grid voltage on 5C1 sort of amp

    Actually it is an Octal One which is basically a modernized 5C1. Solid state rectification and 1 more filter cap set-up to run a 6L6GC or EL34. With the 6l6gc in the amp, the screen grid has about +4 more volts than the anode has.
    6L6GC B+355
    Pin3 326
    Pin4 330 screen R at the power tube is 1K, Screen B+ is 334
    Pin8 20.9

    Can I raise the value of the resistor on the B+ power rail that is just before the screen grid tap? It is also 1K. Would raising the r value to 2K2 drop the screen voltage below the plate voltage?

    Thanks in advance,
    Denny

  • #2
    Originally posted by Denny_A View Post
    Can I raise the value of the resistor on the B+ power rail that is just before the screen grid tap? It is also 1K. Would raising the r value to 2K2 drop the screen voltage below the plate voltage?
    How long is a piece of string? try it and see.

    Another 'solution' might be a 470R-1K5 screen grid resistor.
    Building a better world (one tube amp at a time)

    "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

    Comment


    • #3
      Originally posted by Denny_A View Post
      the screen grid has about +4 more volts than the anode has.
      And the problem is...?
      Jon Wilder
      Wilder Amplification

      Originally posted by m-fine
      I don't know about you, but I find it a LOT easier to change a capacitor than to actually learn how to play well
      Originally posted by JoeM
      I doubt if any of my favorite players even own a soldering iron.

      Comment


      • #4
        tubeswell: I already have a 1k screen resistor on the tube socket.

        WA: Isn't the problem tube design limits with the screen voltage higher than the anode, possibly at the risk of burning up the screen grid on the power tube. And won't the amp sound better with the voltage on the power tube anode higher than the screen grid?

        Thanks!

        Comment


        • #5
          Originally posted by Denny_A View Post
          WA: Isn't the problem tube design limits with the screen voltage higher than the anode, possibly at the risk of burning up the screen grid on the power tube.
          You're measuring the "quiescent" screen voltage (i.e. screen voltage under "zero signal" conditions or "idle" screen voltage). 6L6 screens draw very little quiescent current, and as such don't draw enough current at idle for the screen resistor to drop the idle screen voltage below the plate voltage unless you use an ungodly high value screen resistor, in which you start playing with the transconductance of the tube when it's under signal.

          Essentially the DCR of the OT primary (i.e. DC Resistance) is dropping more voltage at the plate than the screen resistor is dropping at the screen when the amp is in a quiesent state. Even though the OT primary DCR is a lower value than the screen resistor, it has much more current flowing through it than the screen resistor does and as such is dropping more voltage than the screen resistor.

          It's very common in 6L6/KT66 amps for the quiesent screen voltage to be a few volts higher than the plate voltage. Nothing to worry about.

          Originally posted by Denny_A View Post
          And won't the amp sound better with the voltage on the power tube anode higher than the screen grid?
          Define "better".
          Last edited by Wilder Amplification; 10-16-2010, 05:04 PM.
          Jon Wilder
          Wilder Amplification

          Originally posted by m-fine
          I don't know about you, but I find it a LOT easier to change a capacitor than to actually learn how to play well
          Originally posted by JoeM
          I doubt if any of my favorite players even own a soldering iron.

          Comment

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