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Marshall jcm 900 SL-X 2100 Preamp Problem

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  • Marshall jcm 900 SL-X 2100 Preamp Problem

    Got a personal Marshall JCM 900 SLX 2100 with Grid and Cathode Down
    on V1, V2 & V3

    Replaced BR3 when purchased few months ago as it was cooked
    and all Electrolytic & Filter Capacitors

    It's been running beautifully for the past few months as just started
    to sound poor

    Ran some voltages and checked all the Gird and Cathode resistors to the
    best of my limited knowledge

    Really do appreciate any and all help

    First the tubes

    V1

    1 197V
    2 0
    3 1.9V
    4 7.1V
    5 7.1V
    6 284V
    7 197V
    8 188V

    V2

    1 299V
    2 0
    3 2.9V
    4 7V
    5 7V
    6 183V
    7 0
    8 1.9V

    V3

    1 198V
    2 0
    3 1.8V
    4 7V
    5 7V
    6 319V
    7 198V
    8 188V

    R3 & R6 23.2 Ohms

    Chassis Ground to Chassis 1 ohm

    This amp has a ground setup I'm not familiar with nor quite
    understand

    Have a quad of TAD 6L6GC Biased

    http://www.drtube.com/schematics/mar...slxa-61-02.gif

    http://www.drtube.com/schematics/mar...2100-60-02.gif

    http://www.drtube.com/schematics/mar...2100-60-58.gif

  • #2
    Voltages are good. Rather high compared to a 50 watt version I have here but does not indicate a problem. I have 6.0V in heathers and 430V in the plates with the amp biased (32mA/tube).
    You have not described the voltages of the fourth 12AX7 (driver). They should be something like this (somewhat higher probably, but maintaining the relationship):

    226
    33
    36
    -
    -
    201
    22
    36
    -

    You need to explain in more detail what is "poor". In an amplifier like this (without clean sound) it can be more difficult to interpret.

    Comment


    • #3
      Last night using some wooden chopsticks found one of the
      plates for the PI(V4) needed to be replaced, R23

      R23 100k, the outside pair of tubes V7&V8 was flashing
      a bit brighter blue than first pair V5&V6

      The best way to describe the tone problem was the notes
      wouldn't sustain, dead flat and the Bass was breaking up.

      Guess could say the Bass frequencies were flubby.

      Sounded very anemic,weak and thin

      Sorry, haven't gotten to purchasing a oscilloscope yet,
      plan on for this kind of work. Know it's needed very much
      and can't give a proper description of what the signal
      is doing.

      Haven't had a chance to test it since changing R23, it was
      split in half.

      Changed all the preamp 12AX7's with known quality
      before changing R23

      Here is voltages for

      V4
      1 297V
      2 20V
      3 41V
      4 3.9V
      5 3.9V
      6 288v
      7 14V
      8 41V
      9 3.9V

      Thanks for helping very much!

      Thought the voltages might be off as this problem
      was posted @ Marshall forum and someone there
      was helping me, didn't tell me those were in the range.

      Will post back results later today after work

      Comment


      • #4
        Pedro,

        Seems replacing the PI plate resistor corrected
        the amp and it sounds good again.

        Thank you so much for letting me know the
        preamp voltages were normal

        Comment


        • #5
          Good news. I'm glad.
          Greetings.

          Comment

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