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Unsteady Bias on 5F6A

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  • #1

    Unsteady Bias on 5F6A

    I have 5F6a that built. I set the bias using the transformer shunt method.
    I set my meter to milliamps and measure one side of the output transformer. Even after the amp has warmed up the bias drifts between -36.1mA and -34.8mA. It 's never steady. I do use a zener diode on the PT center tap to drop voltage. I have replaced the bias supply power resistor, diode and filter cap. The cap is 100UF 100V. Checked connections with a meter and connections test good. Used different tubes. Amp works fine. I'm wondering if the power transformer or zener diode are bad . What could be my problem??

    Rymac
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  • #2
    I'd measure the bias voltage and see if it's drifting over time. You may also try removing that zener diode and see how it behaves.

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    • #3
      I'm with Diablo. Check the bias voltage. If it's not drifting, then it's the tubes. Unless...Have you checked to see if your wall voltage drifts? Does your meter drift? Mine does, a little. How's the battery in your meter? Whatever the case, it's tiny. You have 1.3 ma of current drift at idle. That could happen as the amp warms and cools with a breeze coming through the window. Don't worry about it until something breaks, then fix it.

      JM2C

      Chuck
      "Take two placebos, works twice as well." Enzo

      "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

      "If you're not interested in opinions and the experience of others, why even start a thread?
      You can't just expect consent." Helmholtz

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      • #4
        Bias voltage? I'd watch your mains voltage. With 480VDC B+, and a 120VAC mains supply, every 1 volt change in mains voltage will mean a 4v change in B+. Whatever your B+ moght be will operate proportionally the same way. SO if the mains are drifting a few volts, your B+ will be drifting 10s of volts, and that is certainly enough to drift your bias current a few ma. And Unless it runs one direction with no signs of stopping, I wouldn;t sweat a few ma up and down.
        Education is what you're left with after you have forgotten what you have learned.

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        • #5
          I removed the zener diode from the center tap. The bias is steady now.
          My B+ voltage before was 428vDc and now it's 448vDc.
          I'm going to leave the diode out. It's a 5F6A circuit.
          I think I'll try to change the power supply to lower the voltage to the plates on the preamp tubes closer to bassman specs.

          Rymac

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          • #6
            Your amp is already below 5F6A Bassman spec voltages. 470vdc +/- 10v is typical. You'll never see a stock vintage bassman that runs 432vdc.

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            • #7
              yep, my kendrick bassman-clone runs at 472v when tubes are biased to 35mA.

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              • #8
                On the Kendrick what are the plate voltages on the V1 and V2 with 12AX7 tubes??

                Rymac

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                • #9
                  Can't speak for the Kendrick specifically, but 230vdc +/- 10v at V1 pins 1 & 6 is typical on a 5F6A with a 12AX7 in V1 & tube rectifier, RI's with a solid state rectifier can exceed 250vdc.

                  You might see 350 to nearly 400vdc at V2 pin 6, around half that at V2 pin 1.

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