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Low Power from Cathode Biased 2x6V6 Magnatone

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  • #31
    Screen voltage drops about 7 volts from idle to max clean output.

    Screen shot of scope with clipped output. I didn't know how clipped you wanted it, so I just grabbed this shot with both positive & negative swings clipped. (wtf, picture keeps coming out sideways)
    Click image for larger version

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    • #32
      Thanks.
      Picture shows almost symmetrical clipping. No signs of OT saturation.
      Scope needs adjustment of trace rotation.
      Please always post scope settings with pictures (vertical sensitivity and time base coefficient).

      Your cathode voltage with 250R is 25% higher than specified in the schematic. That could be caused by a leaky coupling cap. To check for leakage, disconnect one end of the cap(s) and watch for changes in cathode voltage.

      Otherwise no new ideas. Is there any spec regarding amp output power?
      Last edited by Helmholtz; 03-23-2020, 01:10 AM.
      - Own Opinions Only -

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      • #33
        Originally posted by Helmholtz View Post
        Thanks.
        Picture shows almost symmetrical clipping. No signs of OT saturation.
        Scope needs adjustment of trace rotation.
        Please always post scope settings with pictures (vertical sensitivity and time base coefficient).

        Your cathode voltage with 250R is 25% higher than specified in the schematic. That could be caused by a leaky coupling cap. To check for leakage, disconnect one end of the cap(s) and watch for changes in cathode voltage.

        Otherwise no new ideas. Is there any spec regarding amp output power?
        None that I can find, there doesn't seem to be any old ads or marketing or product literature that I can find. I'm just considering the tubes and the voltages involved, and power just seems low. Even a 5E3 Delux is good for at least 8 - 12 watts.

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        • #34
          Even a 5E3 Delux is good for at least 8 - 12 watts.
          The primary impedance of a 5E3 OT is around 8k/8R. Your measurements indicate that your OT has significantly higher primary impedance. To find out you need to determine the voltage transfer ratio.
          - Own Opinions Only -

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          • #35
            Quick check would be to run it into a 2 ohm load if you have one. Power out should increase.
            "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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            • #36
              Hmmmmm, I'll have to see if I can rig something up. I've got 2 x 8 ohm load resistors for testing, that gives me 4, 8 & 16 ohms depending on how I connect them. Never thought I'd need more than that.

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