From: ruffrecords@yahoo.com   
      
   flipper wrote:   
   > On Fri, 17 Sep 2010 10:48:50 +0100, Ian Bell   
   > wrote:   
   >   
   >> 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.   
   >   
   > What distortion did you get on the plate and what were you expecting   
   > it to be?   
   >   
      
   Exactly the same as at the grid - about 2H at -30dB or worse. If I short the   
   input resistor 2H at   
   the grid drops below -70dB and at the anode falls to -40dB which is about what   
   I would expect. So I   
   was expecting -40dB 2H at the anode and got a lot higher so traced back to the   
   grid and found it was   
   the same but that it wnet away with near zero source impedance.   
      
      
      
   Cheers   
      
   ian   
   >> Cheers   
   >>   
   >> Ian   
      
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