Input Impedance Of 12ax7?

[RANDY]

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

[mjmiller]

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

[Steve Conner]

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