From: ruffrecords@yahoo.com   
      
   Ian Iveson wrote:   
   > Ian Bell wrote:   
   >>>>   
   >>>> To tackle the "mystery" of the distortion on the grid I   
   >>>> suggested to   
   >>>> temporarily block the plate of 12AX7 to ground by an   
   >>>> electrolytic, say   
   >>>> 10uF.   
   >>>> This will reduce the gain to practically zero. After   
   >>>> that remeasure   
   >>>> distortion on the grid. If it reduces to nearly zero (as   
   >>>> with the tube   
   >>>> pulled out or cold) then it will show that the Miller   
   >>>> effect creates (or   
   >>>> rather magnifies) this virtual distortion. If the   
   >>>> distortion is still   
   >>>> high,   
   >>>> then it is some non-linear conductance of the grid   
   >>>> current or   
   >>>> nonliarity of   
   >>>> the grid capacitance due to pulsating electron cloud   
   >>>> (space charge).   
   >>>>   
   >>>> Apart from a scientific curiosity you can derive   
   >>>> something result from   
   >>>> this   
   >>>> experiment. For instance, it it proves Miller effect   
   >>>> cause, then it will   
   >>>> further inspire you to try EF86, which will not have any   
   >>>> Miller effect at   
   >>>> all.   
   >>>>   
   >>>   
   >>> I would be surprised if it were a Miller effect but I   
   >>> will do the   
   >>> experiment you suggest and let you know the results.   
   >>>   
   >>> Cheers   
   >>>   
   >>> Ian   
   >>>   
   >>   
   >>   
   >> The trouble with experiments is you get results and this   
   >> time they are not as I expected, nor indeed I suspect as   
   >> anyone expected. To recap with 380mV on the grid at 2KHz I   
   >> got the following distortion figures at the grid:   
   >>   
   >> 2H -49, 3H -62, 4H -70, 5H -77   
   >>   
   >> I added a 22uF 400V electrolytic from plate to ground and   
   >> the figures became:   
   >>   
   >> 2H -60, 3H -70, 4H -77   
   >>   
   >> So the distortion is reduced but by no means to zero. 2H   
   >> is reduced by 11dB, 3H by 8dB and 4H by 7dB. I am not sure   
   >> what the mechanism is - all I feel safe in saying is that   
   >> adding a very low value ac plate load appears to reduce   
   >> the distortion measured at the grid.   
   >   
   > I keep making my point about grid current rising as anode   
   > voltage falls but no-one's picked up on it.   
      
   I thought I did in one of my earlier posts where I did tests at various   
   operating point. The anode   
   voltage varied by over 100V but there was no discernable change in grid   
   distortion.   
      
   Cheers   
      
   Ian   
   A grid is a   
   > grid, be it input or screen. Doesn't it have a knee? Where's   
   > the knee on an EF86?   
   >   
   > As usual, I could be totally wrong...   
   >   
   > Listen to Alex more. His idea may not have solved your   
   > problem, but it's made a key contribution to analysis.   
   >   
   > Ian   
   >   
   >   
      
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