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5f4 with 5881's

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  • 5f4 with 5881's

    if i put 5881's in a 5f4, do i need to use 470 ohm screen resistors?

  • #2
    Originally posted by scole View Post
    if i put 5881's in a 5f4, do i need to use 470 ohm screen resistors?
    I would....yes.
    Bruce

    Mission Amps
    Denver, CO. 80022
    www.missionamps.com
    303-955-2412

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    • #3
      thank you sir

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      • #4
        so why do 5881's use 470 and 6l6's use 1500?

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        • #5
          Originally posted by scole View Post
          so why do 5881's use 470 and 6l6's use 1500?
          I'm not sure Scott but are you confusing the control grid with the screen grid?
          The control grid (G1) usually has a resistor on it called a grid stopper that introduces lumped R and L to the tube's internal C in order to suppress amplification at very high frequencies, like ultrasonic oscillations or RFI.
          The screen grid (G2) resistor can do this too but mostly it is used as a way to limit some of the current flowing on the actual screen grid.
          If you don't use a large enough resistance, it really doesn't do very much.
          You can measure the voltage drop across a screen resistor when the tube is in use at idle and by simple Ohm's law, get the real direct current flowing.
          If you then run a big signal through the power tube you can measure again to see what screen current there is while the tube is making power.
          Depending on what power tube is being tested, ie., EL84, 6V6, 6L6, KT66, EL34, KT88, 6550.... etc., the measured high or low screen current value can be quite startling to the uninformed.
          Bruce

          Mission Amps
          Denver, CO. 80022
          www.missionamps.com
          303-955-2412

          Comment


          • #6
            i was looking at the 5f6a because it uses 5881's. it has a 470 ohm at pin 4. nothing on pin 5. i want to put 5881's in a 5f4 wich has 1500 ohms at pin 5. the 5f6 has no 1500 at pin 5, so do i put the 470 at pin 4 on a 5f4 build if i use 5881 tubes?

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            • #7
              Originally posted by scole View Post
              i was looking at the 5f6a because it uses 5881's. it has a 470 ohm at pin 4. nothing on pin 5. i want to put 5881's in a 5f4 wich has 1500 ohms at pin 5. the 5f6 has no 1500 at pin 5, so do i put the 470 at pin 4 on a 5f4 build if i use 5881 tubes?
              I would do that.
              Use the octal, tube socket layout from a black face Super Reverb.
              Bruce

              Mission Amps
              Denver, CO. 80022
              www.missionamps.com
              303-955-2412

              Comment


              • #8
                cool, thank you sir.

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                • #9
                  sorry for bringing this old topic back up but I just stumbled across it and I wonder what kind of problems I would run into if I use 5881s and don't change the screen resistors. I have been using new production Tung Sol 5881 tubes in both my 5F4 and my 5E5A and they both sound great and have been working trouble free for many years now with the original octal layout and values. What would be the changes in sound or would it be a matter of reliability?

                  thanks!

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