From: apogosso@tpg.com.au   
      
   "Ian Bell"  wrote in message   
   news:i6r73c$ai7$1@localhost.localdomain...   
   >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   
      
   Hello Ian,   
      
   Out of curiosity, could you short out the plate load of 12AX7? (AC component   
   by a 10uF from plate to ground.) If the distortion reduces, it is Miller   
   effect amplifying the distortion of the tube. If the distortion remains   
   about the same -- then it is some kind of weird non-linear conduction of the   
   grid. Grid current should not appear with the bias more than --1.4...1.5V, I   
   would think.   
      
   And what if you reduce the heater voltage?   
      
   Regards,   
   Alex   
      
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