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

Cathode Pulsers: Hard-Tube Modulators (10) 177
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Emssive screen
current limit
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Excassive e!ectron
velocity Iiiit
o 10 20 30
Anode voltage (l(v)
Figure 10-21. Area of permissible operation for S94000E tetrode switch tube.
This electron-optics technique first emerged in a grid-driven power tetrode for
UHF television transmitter service and resulted in a many-fold increase in the
power-frequency product obtainable from power gridded tubes. The voltage/
current characteristics of the tube for a constant screen voltage of 1000 V are
shown in Fig. 10-19. Note that these curves have a different format than the ones
previously shown. These are constant-current characteristics rather than constant-grid-voltage
characteristics. They illustrate the almost-total screening effect
of the anode from the cathode. This is due to the fact that anode current is all but
independent of anode voltage. The effective perveance, or transconductance, of
the electron optics is such that the full current is obtained when the grid voltage
is never positive with respect to the cathode, and the total “grid base” (the gridvoltage
swing from cutoff to full current) is only slightly more than 150 V. This
condition requires virtually zero driver power. With the grid never positive,
there is no grid current either. The grid that competes with the anode for electrons
is the screen. Note that even with an anode voltage that is only 5% of the
hold-off rating (10 kV compared to 200 kv), the current-split ratio between screen
and anode is 120 A/6 A, or 20, which is superior to the conventional triodes
previously discussed.
Figure 10-20 shows how anode current varies with grid voltage for a constant
screen voltage of 1000 V and constant anode voltage of 12 kV. This figure illustrates
the concept of “projected” cutoff, which is the dashed line. It “projects” a
grid voltage for anode-current cutoff of about -110 V. For a screen voltage of 1000
V, this ndates to a screen-grid amplification factor, or p.,, of 1000 V/110 V, or 9,
which is the nominal screen L. Figure 10-21 shows the dimensions of the tube’s
permissible operating region, which is the equivalent to the safe operating area
(SOA)for a field-effect transistor. The left-hand,sloping limit line is the diode