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Input Impedance Of 12ax7?

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  • Input Impedance Of 12ax7?

    What is the input impedance of a 12ax7? (at the grid) THANKS!

  • #2
    It depends on the 12ax7 - look up the datasheet from the manufacturer of the tube in question. The datasheet will have TONS of other useful information as well.

    I think the standard value used for a generic 12ax7 is 1 megohm

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    • #3
      It's just whatever the grid resistor is, but at high frequencies you have to take into account the Miller capacitance. That is a capacitance that appears to be from the grid to ground, and its value is equal to the grid-to-plate capacitance of the tube multiplied by the voltage gain of the stage. With a 12AX7 it can be of the order of 100pF, which is significant at high audio frequencies.

      http://www.aikenamps.com/MillerCapacitance.html
      "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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