3.6M north10.pdf - Dean-O's Toy Box

Cathode Pulsers: Hard-Tube Modulators (10) 147
plied to a grid is positive with respect to the cathode, some electrons will be
intercepted. (This is almost always true for the screen grid and, in a high-power
gridded tube when full current is being demanded, for the control grid as well.)
One of the goals of tube design is to minimize the control-electrode current
because it serves no useful purpose and subtracts from the current that can be
useful. The total cathode current in a tetrode can be expressed as:
I =
cathade I anode + lco.troIgrid + Is.reen grid = ‘:’
where K is the effective perveance. K can also be evaluated as
~= 2.33x 10+(A)c
(Sdiade)’ ‘
where A is the cathode emitting area in cmz, c is a constant dependent on tube
geometry, S~l~e is the equivalent diode spacing and given in cm, and V is the
effective voltage. The fractional exponent 3/2 applies to most tube geometries.
Effective voltage can be further defined as
v v
v=Vcontro,tnd + ““en “d +J=-,
where Psg and ~ are the amplification factors of the semen grid and anode,
respectively.
The perveance of electron tubes can vary from less than 10~ perv., or 1 ~perv
(the equivalent to 1 pA/V3t2), for a high-performance millimeter-wave TWT, to
0.01 perv, or 10,000 ppervs (the equivalent to 0.01 A/@JZ), for a large, gridded
power tube. The screen-grid and anode amplification factors, or p~g and ~, are
factors that quantify the strengths of the electric-field contributions at the cathode
produced by voltages applied to screen grid and anode as they relate to the
electric field produced by the control grid. If, for instance, a tetrode had a screen
amplification factor of 10, it would require a change of screen-grid voltage 10
times as great as that of the control grid to have the same effect on cathode
current change. Similarly, if the anode amplification factor were 100, the change
in anode voltage would have to be 100 times as great as the change in controlgrid
voltage for the same cathode-current change. The screen-grid and anode-p
factors reflect the facts that not only are the screen grid and anode farther away
from the cathode than the control grid, but the field produced by voltage on the
screen grid must penetrate the shielding effect of the control grid, and the field
produced by voltage on the anode must penetrate the shielding effects of both
grids. We can see, then, that for positive voltage applied to both screen grid and
anode, there will be a negative voltage that, when applied to the control grid, will
cancel the field-strength contributions of the others and result in no current leaving
the cathode. This balance point is called the cut-off grid bias for a given
Psg
Pa