From: ruffrecords@yahoo.com   
      
   Ian Iveson wrote:   
   > Ian Bell wrote:   
   >   
   >> I am having a strange problem with what appears to be   
   >> distortion due to grid current in a 12AX7. It's a CC stage   
   >> with a 3K Rk bypassed about 120K in the plate and a 300V   
   >> supply. Anode sits at just over 200V and cathode just below   
   >> 2V.   
   >>   
   >> I would not expect to see any distortion due to grid   
   >> current with an input signal of 250mV rms at 2KHz but I   
   >> do. There's a 1Meg from grid to ground and the test   
   >> oscillator is fed to the grid via a 100K resistor.   
   >> Analyzer  is connected across grid to gnd. Tube off, there   
   >> is no distortion with the 100K in cct or shorted. Tube   
   >> powered up I get about -33dB 2H measured at the   
   >> grid, -39dB 3H, -59dB 4H.   
   >>   
   >> Short the 100K and the 2H drops to to below -70dB.   
   >>   
   >> I tried changing the tube but the result is the same. What   
   >> is going on?   
   >   
   > I could spend a while calculating the drop in grid   
   > resistance and the rise in grid voltage due to signal, and   
   > compare the two, but maybe you've already done that.   
   >   
      
   Yes, we should be well away from where grid current normally starts.   
      
   > What's the grid resistance or current *supposed* to be, and   
   > how should it change with voltage?   
   >   
   > Do all the grids of all the valves in the world behave in   
   > broadly the same way, regardless of their purpose?   
   >   
   > At low anode voltage does grid current rise steeply? Are you   
   > using a typical operating point and load, or have you made   
   > it up yourself? 120k seems very low for an ECC83.   
   >   
      
   120k is about right. The data sheet you quote below gives figures for 47k,   
   100k and 200K so 120K is   
   not unuasual.   
      
   > Do you need the 100k? If so, could you use a lower fixed   
   > resistance multiplied by feedback, and could that   
   > arrangement alleviate the grid current problem?   
   >   
      
      
   The 100K is there as the input arm of an eventual NFB network.   
      
   > The only mention I found of grid current in datasheets is on   
   > page two in this:   
   >   
   > http://www.tubezone.net/pdf/12ax7ecc83.pdf   
   >   
   > Where it appears to note (*) that the distortion shown in   
   > the table is due to grid current.   
   >   
      
   That's a nicely copmplete datasheet. Thanks.   
      
   > Another thing, in passing, is that your test circuit had me   
   > musing on the difference between a grid stopper and a source   
   > resistance. Calculations would be easier if you swapped that   
   > 100k to the other side of the 1meg leak, maybe.   
   >   
   > Ian   
   >   
   >   
      
   Cheers   
      
   ian   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)