3.6M north10.pdf - Dean-O's Toy Box
3.6M north10.pdf - Dean-O's Toy Box
3.6M north10.pdf - Dean-O's Toy Box
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168 High-Power Microwave-Tube Transmitters<br />
500<br />
A. Type 76W triotle B. Type 6547 Mode<br />
500<br />
I<br />
400<br />
400<br />
r<br />
I w F , ,<br />
I I I A I I<br />
100 I<br />
100<br />
I 1 /1 1 , , 1 1<br />
0<br />
0<br />
01 234_56789 io 012345678 9<br />
Plate voltage (l(V)<br />
Peak current (A)<br />
Figure 10-12. Voltagdcurrent characteristics for two power triodes: type 7560 (lefi) and type 8547 (right),<br />
which dt~er only in that the 7560 anode is fartherfiom its grid.<br />
line,” which is the line (not exactly straight) that slopes upward and to the right<br />
from the origin. It is also the line where all of the anode current lines merge.<br />
This diode line quantifies the virtual cathode and has a much steeper slope in the<br />
closely spaced electrodes of the 8547 than in the widely spaced ones of the 7560.<br />
If we select operating points for both tubes that meet the arbitrary but sensible<br />
90%-anode-efficiency criterion-at which the average output power could<br />
conceivably be as high as 175 kW/O.1, or 1.75 MW-we can determine how well<br />
grid current is managed in the two tubes. For the 7560, the conduction drop<br />
selected is 5 kV, which is 107o of 50 kV, and for the 8547 it is 1.7 kV, 107o of 17 kV.<br />
With positive grid voltage of 2000 V, the anode current of the 7560 is 380 A, and<br />
its grid current is 130 A. Therefore, the ratio of the two values is 2.10. Of the<br />
total 513-A cathode current, which is the sum of anode and grid currents, 7470<br />
became anode current and 2670 became grid current. This breakdown suggests a<br />
grid-screening fraction of 179’0, which would put the 7560 in the medium-p category.<br />
(Its p, in fact, is 45.) With 50 kV between anode and cathode, the grid<br />
must be negative with respect to cathode by approximately 1.1 kV for “projected”<br />
plate-current cutoff. (The actual plate-current cutoff requires an additional<br />
safety factor of negative grid bias.) The 8547, with a positive grid voltage<br />
of 2000 V, an anode current of 390 A, and a grid current of 100 A, has a ratio of<br />
3.10. Of the 490-A cathode current, 80~0 made it to the anode, and 2070 to the<br />
grid, suggesting a 13’70 grid-screening factor and lower p.. We know, however,<br />
that the tubes are identical except for anode-grid spacing, so the grid-screening<br />
factors are the same. The y is lower, however, because the anode is closer to the<br />
cathode (p.= 14).