From: apogosso@tpg.com.au   
      
   "flipper"  wrote in message   
      
   > I hear ya but the simulations aren't showing a problem there.   
      
   Effective input capacitance of a MOSFET source follower consists mainly of   
   the gate-drain nonlinear capacitance plus source-gate capacitance, reduced   
   by the close-to-one gain and dependant on the gain nonliearity.   
      
   In turn gain nonlinearity will manifest when the load changes, when your   
   tube goes into grid current area of operation.   
      
   Capacitance modulation causes distortion:   
   - proportional to frequency;   
   - proportional to the output impedance of a previous stage.   
      
   It is hard to gues how non-linear the combined capacitance will be, but let   
   us assume that it somehow somhow changes by 1pF. If say the MOSFET is driven   
   from 12AX7 with 100K output impedance, then at 20kHz such modulation of the   
   capacitance will create 1% of extra distortion. Whether to ignore it or   
   consider -- up to you. Besides, actual capacitance modulation can be larger.   
      
   Regards,   
   Alex   
      
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