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  • Bias query

    Hi folks - in this bias circuit, how would one go about increasing output to around -50V?
    I am unfamiliar with the use of transistors here
    Thanks--
    Click image for larger version

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  • #2
    Increase R55 to 7.13K.
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
    REMEMBER: Everybody knows that smokin' ain't allowed in school !

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    • #3
      .............
      Juan Manuel Fahey

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      • #4
        OK thanks. Are Q1 and Q1 working as regulators or something?

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        • #5
          Originally posted by hylaphone View Post
          OK thanks. Are Q1 and Q1 working as regulators or something?
          Yes. It appears to be a series pass regulator.
          The divider network R56/ R55 (with the help of R57) sets the base voltage of Q2/ PNP transistor.

          The two transistors are set up as a darlington pair to pass a higher current (as opposed to simply one transistor)

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          • #6
            Before you start changing too much, go to R57 and see how much voltage is there. That is the highest voltage available in this circuit. If the 50v caps there are well specified, you may not even HAVE 50v to adjust up to.
            Education is what you're left with after you have forgotten what you have learned.

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            • #7
              Originally posted by J M Fahey View Post
              .............
              Can I sig this?

              "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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              • #8
                Originally posted by Enzo View Post
                Before you start changing too much, go to R57 and see how much voltage is there. That is the highest voltage available in this circuit. If the 50v caps there are well specified, you may not even HAVE 50v to adjust up to.
                Yeah, it looks like the input voltage should be about -84V but it doesn't make sense to have C31,C32 at 50V and C30 at 150V. It should be the other way around.

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                • #9
                  All are correct here - the caps are 150V rated - looks like they have the V/MFD backwards on the schematic
                  Bumped R55 to 8.2K and now have a nice usable range at the bias pots
                  Thanks for the help!!

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                  • #10
                    I should have just accepted LT's advice, I seldom have reason to doubt him.
                    Education is what you're left with after you have forgotten what you have learned.

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                    • #11
                      Originally posted by Enzo View Post
                      I seldom have reason to doubt him.
                      I'll agree with that

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