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5F1 Bias Resistor and Voltages?

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  • 5F1 Bias Resistor and Voltages?

    I just finished a 5F1 clone and am wondering if I need to lower the 470 Ohm bias resister to get the current up.

    Here are my voltages:

    Pin 8 to Ground: 20.7v
    Pin 3 to Ground: 370.9v
    Pin 8 to Pin 3: 351.0v

    If I am not mistake the cathode voltage is pretty low and the watts dissipation on the JJ 6v6 is about 7 watts, half of what it can take. I am using a 5Y3 NOS rectifier. Thanks in advance.

  • #2
    With the cathode at 20.7V with a 470R, you'll have 20.7/470 = .044A (44mA) tube current

    And if the plate is 370 and the cathode is 20.1, then you'll be dissipating (370-20.7)*.044 = 15.3W! (including screen) You want to aim to get somewhere between 12.5W to 13.8 W (assuming you have a modern brand of 6V6)

    So you need to get the plate voltage down a bit (say to about between 340 and 360). Best way is to to lower the B+ by putting in some reverse-biased zener diodes in series with the HT winding center tap to ground. Try a couple of 12V 5W diodes in series. (Make sure the banded end is pointing to ground.)

    Incidentally, What brand of rectifier have you got? Some '5Y3GTs' are not 5Y3GTs if you get my drift.
    Last edited by tubeswell; 06-04-2008, 08:50 PM.
    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

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    • #3
      Is the high plate voltage do to the sort of universal Weber power transformer that I am using?

      I was the looking at the MM PT's and they had ones that were rated at 260-0-260v (Mid 50's) and 330-0330v (59 Tweed) which might be better all together. My 5e3's plate voltage is only 360v. 370v seemed pretty high for the 5F1 circuit.

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      • #4
        You could just increase your cathode resistor a bit. 600 ohms is
        not unheard of. I have 376v at the plate and use a 680 ohm
        cathode resistor, which gives me ~13w dissipation at idle.

        Paul P

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        • #5
          I was way off on the 7 watts of dissipation. Wow! Well, I will just bump up the bias resistor and see how it does. Thanks

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          • #6
            Bumping up the bias resistor will have a different effect from lowering the B+/plate voltage. If you continue to have high plate voltage, it will be harder to overdrive the tube (which is okay if you like a cleaner amp).
            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

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            • #7
              Clear Guy,

              You DON'T have a problem.

              You DON'T NEED to drop B+ voltage (obviously you can if you want), but 370v is not high for this kind of amp. You do not have high plate voltage. No champ ever had a 260-0-260VAC PT secondary. 350-0-350VAC is probably more realistic!

              You DON'T NEED to change your cathode resistor value. ALL champs had 470ohms. Unless your plate voltage (measured to ground) was significantly over 400vdc I'd stick with 470ohms. 660ohms should tame even the highest voltage champs (as Paul P intimates).

              In a design like this (SE, cathode biased), even a no name brand 6V6 will handle 20W plate dissipation, they do it all the time, for many reliable years in SF Champs with plate voltages in the 410v-430v region. I'm not saying that this is something to aim for (I actually have an 800ohm cathode resistor in my SF champ, but that runs at 440v from plate to ground with a GZ34), just putting a realistic perspective on things. 15-16W plate dissipation is no problem at all, yours is less than 15W as you have not accounted for screen current.

              Dividing cathode voltage by cathode resistor is not a pin point accurate way of measuring plate current/dissipation.

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              • #8
                I may just leave it alone or use a 6l6 in there which would be fine. Its only getting used at low volume in my living room. Let it fly!

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                • #9
                  If you switch to 6L6, be sure that the OT can take the additional plate current (6L6 will draw more current than 6V6 with same cathode resistor), use a 10W cathode resistor and a 100v cathode bypass cap on the power tube.

                  I'd leave it alone, the 6V6 is not in any danger.

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                  • #10
                    Thanks MWJB for the advice. Leaving it alone.

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                    • #11
                      Tubeswell. Why banded end to ground? I ask for lack of understanding. Zeners are generally oriented with the band towards the V+. So I'd put it in the opposite way as I understand it...which must be wrong. Thanks!

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                      • #12
                        The zener will 'lift' the full wave bridge rectifier from ground by the zener voltage.

                        Oriented the 'proper way', you will simply get the diode drop.

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                        • #13
                          Yes that I understand. It's the orientation that's a ....surprise.

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                          • #14
                            try it and see.
                            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


                            • #15
                              Zeners are generally oriented with the band towards the V+.
                              No, positive supply zeners are wired that way. Look at a typical +15 and -15 zener supply. You will find the negative supply zener wired reverse of the positive. Zeners are wired for reverse bias, so they break down at their zener voltage and conduct in reverse.

                              In your typical 5Y3 supply, the current only flows one direction through the tube, and so only one direction through the transformer CT.. We want the zener to prevent conduction until a bite is taken out of the voltage.
                              Education is what you're left with after you have forgotten what you have learned.

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