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‘81 Princeton Reverb Bias Question

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  • ‘81 Princeton Reverb Bias Question

    This is circuit AB1270. I’d like to use the Robinette biasing method but I’ll not sure where the output transformer center tap is located. The pics show a red, blue & orange-brown wire coming from the output transformer, the blue & orange-brown going to the plates of the 2 power tubes. Is the red the center tap?
    Attached Files
    Last edited by Perkinsman; 03-11-2020, 05:29 AM.

  • #2
    Originally posted by Perkinsman View Post
    This is circuit AB1270. I’d like to use the Robinette biasing method but I’ll not sure where the output transformer center tap is located. The pics show a red, blue & orange-brown wire coming from the output transformer, the blue & orange-brown going to the plates of the 2 power tubes. Is the red the center tap?
    Yes, the red wire is the center tap.

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    • #3
      Thanks, I checked voltage there and it's about 100v lower than the 420v on the schematic, thats 25% off. I took voltages on the power tube plates & a few of the preamp tubes and they're low too...when I checked the bias, one 6v6 was 61ma and the other was 80ma, I believe 34ma is recommended. Can someone please point me in the direction I need to be going to find the source of the problem here. Thanks.

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      • #4
        What is the bias voltage at the tubes and how clean is it? meaning how free of ripple?

        Your B+ is 100v low? How free of ripple is that?

        I attached the 1981 schematic.
        Attached Files
        Education is what you're left with after you have forgotten what you have learned.

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        • #5
          Thanks for doing that. I subtracted each of the PT plate voltages from OT center voltage. Is that the bias voltage? I don't know how to check a signal on the scope yet. The initial complaint was that the amp distorted after being played for a couple hours, after cooling down it was fine for a couple more hours.

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          • #6
            The bias voltage is the negative voltage on the control grids of the power tubes - pin 5 of each. It controls the current we measure when adjusting the tubes. If the voltage is too low, the tubes work harder and drag your B+ down. It also needs to be smooth DC - no ripple.

            Your 420v measures really low? Could be the filter cap not doing the job. The resulting ripple will cause a low reading.

            Problems with either power supply COULD cause distortions.

            Please explain in detail exactly how you measured the tube current.
            Education is what you're left with after you have forgotten what you have learned.

            Comment


            • #7
              Originally posted by Perkinsman View Post
              Thanks for doing that. I subtracted each of the PT plate voltages from OT center voltage. Is that the bias voltage? I don't know how to check a signal on the scope yet. The initial complaint was that the amp distorted after being played for a couple hours, after cooling down it was fine for a couple more hours.
              filter caps if original,its time to go,also check the rectifier tube and any resistor on the power supply string

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              • #8
                Originally posted by Enzo View Post
                The bias voltage is the negative voltage on the control grids of the power tubes - pin 5 of each. It controls the current we measure when adjusting the tubes. If the voltage is too low, the tubes work harder and drag your B+ down. It also needs to be smooth DC - no ripple.

                Your 420v measures really low? Could be the filter cap not doing the job. The resulting ripple will cause a low reading.

                Problems with either power supply COULD cause distortions.

                Please explain in detail exactly how you measured the tube current.
                My bias voltage is -12.3. The filter caps are the original, 1981’ish can. They (20/20/20@450) measured from 20-24uf at full voltage on my Sprague cap tester and charged-discharged properly. I calc’d bias current by measuring & subtracting each plate voltage from the OT center tap voltage & dividing by the resistance from each plate to the center tap.
                Last edited by Perkinsman; 03-12-2020, 07:16 PM.

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                • #9
                  My bias voltage is -12.3.
                  That's the reason for your excessive idle currents and will cause low B+. Grid bias should be around -33V.
                  What's the voltage across the bias filter cap?
                  - Own Opinions Only -

                  Comment


                  • #10
                    Originally posted by Enzo View Post
                    The bias voltage is the negative voltage on the control grids of the power tubes - pin 5 of each. It controls the current we measure when adjusting the tubes. If the voltage is too low, the tubes work harder and drag your B+ down. It also needs to be smooth DC - no ripple.

                    Your 420v measures really low? Could be the filter cap not doing the job. The resulting ripple will cause a low reading.

                    Problems with either power supply COULD cause distortions.

                    Please explain in detail exactly how you measured the tube current.
                    Voltage across the bias cap is -12.5

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                    • #11
                      Voltage across the bias cap is -12.5

                      Comment


                      • #12
                        Originally posted by Perkinsman View Post
                        Voltage across the bias cap is -12.5
                        I guess the bias cap is leaking. But also check the 100k and 22K voltage divider resistors.
                        - Own Opinions Only -

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                        • #13
                          Yep, the cap was bad. All voltages seem correct now. I’ll measure everything & report back. Thank you guys.

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                          • #14
                            Originally posted by Enzo View Post
                            The bias voltage is the negative voltage on the control grids of the power tubes - pin 5 of each. It controls the current we measure when adjusting the tubes. If the voltage is too low, the tubes work harder and drag your B+ down. It also needs to be smooth DC - no ripple.

                            Your 420v measures really low? Could be the filter cap not doing the job. The resulting ripple will cause a low reading.

                            Problems with either power supply COULD cause distortions.

                            Please explain in detail exactly how you measured the tube current.
                            Bias voltage now at 37.5vdv. Brought B+ up & calc’d 18.9ma for bias current, which is about 70% of max dissipation so I believe I’m good now. Thank you.

                            Comment


                            • #15
                              I would still check for ripple on 40 year old filter caps. Do you have an o’scope? Do you have any hum?

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