From: ruffrecords@yahoo.com   
      
   flipper wrote:   
   > On Thu, 16 Sep 2010 21:58:37 +0100, Ian Bell   
   > wrote:   
   >   
   >    
   >   
   >>   
   >> Yes, I have come across the same effect in my work with mu followers. I was   
   just surprised to see it   
   >> acting so soon. generally you can allow the signal to take the grid up to   
   -1V before worrying about   
   >> grid current bu that seems not to bee the case with the 12AX7 - today I fed   
   it with a mere 25mV and   
   >> it produced 1% distortion. There's something funny going on here.   
   >>   
   >   
   > You are chasing phantoms as it isn't 'positive grid drive'.   
   >   
   > It's called a "grid leak" resistor for a reason: there's grid (leak)   
   > current. (This, btw, is what determines the 'recommended maximum' grid   
   > leak resistor.) It's small but there nonetheless and, being roughly   
   > proportional to plate current, varies with signal as well.   
   >   
   > As for the size of it, RDH4 recommends that, for cathode bias 'high   
   > mu' triodes, the grid leak be no larger than 3x the (DC) plate load,   
   > and no larger than 4x the preceding stage's plate load, but that's a   
   > bias shift consideration and larger values were common practice.   
   >   
   > As for distortion, what you've got on the grid is a summing junction   
   > of your input and the 'grid leak' signal plus, of course, inter   
   > electrode capacitances so any measurement there will, of necessity, be   
   > 'distorted' relative to your source. Lower the source impedance and   
   > you increase it's proportion of the sum so the relative distortion   
   > decreases but it will never be '0' unless you turn off the tube (all   
   > of which you've observed).   
   >   
   > Why are you trying to measure distortion on the grid anyway?   
      
      
   Only because I traced back to there starting from the ouptut.   
      
   Cheers   
      
   Ian   
      
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