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Ampeg SVP-Pro troubleshooting Need Help

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  • Ampeg SVP-Pro troubleshooting Need Help

    Hello

    My Ampeg Preamp SVP-Pro started to distort heavily, so I opened it up I take some voltage measurement.
    They are out especially on V2a and V4(12AU7), but I can't understand why.
    I've already changed the tubes with good known ones with no results, checked resistors and capacitors in the circuits but I didn't found any faulty components.
    I've added the schematic with original Ampeg voltages in BLACK and my voltages in RED.
    I Hope that someone could help me, since I'm running out of ideas.
    Attached Files

  • #2
    That scan is impossible to read the original values.

    Here is a clear one: https://music-electronics-forum.com/...3&d=1501961567

    V2 pin 3 is way off.

    All of the other voltages are off but not by much.

    Did you check the power supply nodes for excess ripple (Vac)?

    Comment


    • #3
      Thank you for your reply.
      Here is a more clear schematic with original voltages added.

      I haven't measured the ripple at voltage nodes yet.
      Do you suspect a bad filter capacitor?
      I had the same suspect too, but the B+ voltage is in the ballpark.

      P.S.
      I've realized I posted the thread in the wrong section.
      I'm trying to move it to the troubleshooting section.
      Attached Files
      Last edited by fuzzman72; 04-25-2019, 05:34 PM.

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      • #4
        V4 Pin 1 is down by 50Volts? Check R5. May have gone high. Also V4 Pin 8 is way off.

        Comment


        • #5
          I don't there is anything wrong with your measured voltages. The "Ampeg" numbers are patently wrong. According to them the current through the 221k plate resistor is (275-124)/221 mA = 0.68mA. That would make the drop across the 3.9k cathode resistor 2.7V. not the claimed 6.5V. Eyeballing the curves for a 12AU7 tells me your measured numbers are OK.

          So, what's causing your problem? Can you feed the tuner out to a different amp? Repeat using the FX send. Now try plugging your guitar into the FX return. Let us know the results.
          Experience is something you get, just after you really needed it.

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