From: ruffrecords@yahoo.com   
      
   Ian Bell wrote:   
   > 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   
      
      
   I just spent some time perusing the 12AX7 data sheets in more detail,   
   particularly the part about RC   
   coupled amplifiers which gives the maximum output before the onset of grid   
   current under various   
   conditions. Based on that data I calculated the cathode voltage and also the   
   peak input voltage   
   before grid current for several values of Ra, Rk and Va. Subtracting the two   
   gives the grid bias at   
   the onset of grid current and this seems to be pretty constant at about -0.6V.   
   It also seems that   
   with the 300V HT I have there is really only one operating point that will   
   give me the 102V peak   
   output swing I need without reaching grid current and that is with a 220K   
   anode load, 2.2K up the   
   cathode leg and a quiescent current of close to 0.6mA. I will try those values   
   and see what I get.   
      
   Cheers   
      
   Ian   
      
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