[Ian Bell[_2_]]

I am having a strange problem with what appears to be distortion due to grid current in a 12AX7.
It's a CC stage with a 3K Rk bypassed about 120K in the plate and a 300V supply. Anode sits at just
over 200V and cathode just below 2V.

I would not expect to see any distortion due to grid current with an input signal of 250mV rms at
2KHz but I do. There's a 1Meg from grid to ground and the test oscillator is fed to the grid via a
100K resistor. Analyzer is connected across grid to gnd. Tube off, there is no distortion with the
100K in cct or shorted. Tube powered up I get about -33dB 2H measured at the grid, -39dB 3H, -59dB 4H.

Short the 100K and the 2H drops to to below -70dB.

I tried changing the tube but the result is the same. What is going on?

Cheers

Ian