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660 volts on the plates?!

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  • 660 volts on the plates?!

    I am working on a Univox 1221 amplifier. This amp uses two 6L6GC tube and claims to get 100 watts (snicker). The schematic show the plates getting a whopping 660 volts!

    I think the voltages shown on the schematic are based on the Japan's 100 volts. I guess they figured 110 USA volts wouldn't effect much since it's only 10% more. Well my outlet voltage is 123 volts and the reading I get on the plates is 739 volts!

    I can't imagine any modern 6L6GC tube lasting very long at these voltages. I am still stumped why they would use a voltage doubler in the power supply. 320 volts would be sufficient.
    Attached Files

  • #2
    I could not open your pdf.

    http://www.prowessamplifiers.com/sch...ivox/1221.html

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    • #3
      Axtman:

      Take a look at this thread - https://music-electronics-forum.com/...ad.php?t=32478

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      • #4
        COnsidering the PT has both 100v and 115v taps, I tend to doubt they thought a 100v amp would be OK on 115v as is.

        They were trying to extract maximum power. Note the screens are running at half the B+.

        Unless they are arcing over inside, I worry more about dissipation than voltage.
        Education is what you're left with after you have forgotten what you have learned.

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        • #5
          I just checked the Tube Bias Calculator at Robrobinette.com.

          For 660V on a 6L6GW (30 watt tube), the results are as follows for Class AB:

          50% Cool = 22.7 milliamp
          60% Average = 27.3 milliamp
          70% Max Safe Dissipation = 31.8 milliamp

          Does that sound right?

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          • #6
            If you consider 50% cool.
            At that kind of plate voltage I consider MusicMan cool at somewhere around 12%.
            Originally posted by Enzo
            I have a sign in my shop that says, "Never think up reasons not to check something."


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            • #7
              The Robinette feature just looks at those three percentages in those terms. It just figures 50,60, and 70% and calls them that. To them, 50% is cool regardless.
              Education is what you're left with after you have forgotten what you have learned.

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              • #8
                320 volts would be sufficient.
                ...but only for an output of around 25W per pair.
                Last edited by Helmholtz; 12-17-2019, 10:58 AM.
                - Own Opinions Only -

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                • #9
                  I had one of these beasts come through.
                  720Vdc.
                  I installed Sovtek WXT's.
                  Biased to 20%.

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                  • #10
                    Biased to 20%.
                    Based on disappearing crossover notch?

                    Just out of curiosity, what was output power?
                    - Own Opinions Only -

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                    • #11
                      Originally posted by Helmholtz View Post
                      Based on disappearing crossover notch?

                      Just out of curiosity, what was output power?
                      Yep.
                      I do not recall the output power.
                      Certainly not 100 watts.

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