[AMPS] splatter

Mon, 16 Jun 1997 22:21:22 +0000

> >Nor is there any definition of what the minimum grid voltage is. It 
> >might be zero, it might be positive five volts, or negative 1000 
> >volts. The only requirement is the grid can NOT draw current during 
> >the positive portion of the cycle.
> <snip>
> 
> <><><><><><><><><><><><><><><><><><><><><><><><><><><><>
> How can the grid can not draw current if it's positive with respect to
> the cathode?  Or did you mean the positive-going (but still negative)
> portion of the cycle?

That's right Bill, perhaps it would be clearer to say the least 
negative?

Some tubes can have the grids driven positive without measurable 
grid current. I tried to explain that difference earlier. The thing 
that affects cathode to anode resistance is the ELECTRIC field 
between the grid wires, not current flowing in the grid. In tubes 
like the 8877 and 3CX800, Eimac puts the cathode material  on in 
bands. If you look at the cathode, the grid wires are NOT between the 
cathode's emitting area and the anode.

That's why 8877's and 3CX800's have very low grid current even 
when a portion of the RF cycle drives the grid very positive with 
respect to the cathode.  

If you have high amounts of anode potential, if the grid to cathode 
spacing is small, and the distance between grid wires large enough it 
takes more than a small amount of positive grid voltage (with 
respect to the cathode) to cause noticeable grid current!

A classic example of this is a cathode driven 3CPX5000B7 running at 
8-10  kV.  The triode tube can have NEGATIVE grid current under 
certain tuning conditions because actual grid current is so low 
secondary electrons from the anode exceed actual cathode to grid 
current!

3-500Z's, 3CX1200's, 3CX3000's do not have "focused cathodes, and 
so they have much more grid current in normal operation. The grid 
wires are "in the way" of the electron stream.

73, Tom W8JI 

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