From: eikelan@gmx.net   
      
   Big Bad Bob wrote:   
   >  On 2020-03-03 02:18, Eike Lantzsch, ZP6CGE wrote:   
   > > When the EL84 start pulling grid current the difference of   
   > > impedance of the cathode side and the anode side of the phase   
   > > splitter comes into play. To balance the grid current the anode   
   > > side grid resistor is of lower value than the cathode side   
   > > resistor. That is careful design. The engineers probably found   
   > > out that, for consumer grade equipment, the distortion due to this   
   > > effect are negligable compared to the the distortion due to   
   > > overload and canceled the resistors to save some cents.   
   >   
   >  interesting.  normally if you care enough to add the series resistors on   
   >  the output tubes, you'd have both halves of a 12AU7 driving them so that   
   >  it isolates grid current issues from the splitter circuit.   
   >   
   >  But yeah that's one more tube in the design, and extra $$.   
   >   
   Yes that would be a better design. For more costly amps with more   
   oomph it was applied - such amps equipped with EL34 or KT88 and the likes.   
   But for the more economic ones with around 15W output and EL84 tubes the   
   engineers usually didn't do that. But the latter were good amps   
   anyway.   
      
   >  For a guitar amp, I'd do it this way (with the extra 12AU7) becaue a   
   >  'totem pole' type of splitter does really weird things at the overload   
   >  point.  if you WANT that sound, go with it.  Otherwise, the somewhat   
   >  cleaner "final stage distortion" of the splitter -> 12AU7 -> power tube   
   >  combination would be better.  Also a series resistor on the splitter's   
   >  grid would be needed.  Then you'll have very nice predictable behavior   
   >  at the clipping point, and WAY beyond that.   
   >   
   >  aside from that, the different resistor values on the grid resistors is   
   >  a bit silly, probably why they were just removed, later.  "Muntzed"   
   >   
   >   
   >   
      
   Kind regards, Eike   
      
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