JTM45 Red Plating

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by mitchguitar13, Apr 13, 2015.

  1. mitchguitar13

    mitchguitar13 Member

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  2. pdf64

    pdf64 Member

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    Tubes will tend to red plate unless they have sufficient bias voltage applied to their control grid (terminal 5).
    Have you assessed if the magnitude of the bias voltage is large enough to keep the plate current / plate dissipation at a suitable level?
    eg has a bias probe been used to check on the power tubes, some other method to measure plate current?
    I like the platting of the OT secondary wires!
     
  3. mitchguitar13

    mitchguitar13 Member

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    I'm getting about 430 on pin 3 of V4 and V5 and the reading is -30 on pin 5 on V4 and V5
     
  4. pdf64

    pdf64 Member

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    I suggest that:
    - read up on the topic eg http://www.aikenamps.com/index.php/what-is-biasing
    - the magnitude of the voltage on terminal 5 is increased, may require doubling for Chinese KT66.
    - the output tube plate current is monitored, eg add 1 ohm resistors to the power tube cathode ground return and measure the static voltage across them, or measure the OT primary resistance (CT to each plate) and the voltage drop across those sections of the primary under static conditions.
    - adjust the terminal 5 voltage so as to achieve about 35mA plate or cathode current per power tube.
     

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