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

178 High-Power Microwave-Tube Transmitters
+400
A. Measured Constant-Current Tube Characteristics
(1.5 kV screen voltage)
B. Predicted Constant-Current Tuba Characteristics
(7S0 V screen Wtage)
+2U0
‘x
+150
+100
1,= 141 A
i
IP=250A
z ‘w
?$
GO
)0 -Q
g
-50
-1oo
Currenls ara changed in proportbn to
(ratkl of Scrssn vollages)~
-300
-150 ~.-l
___.L,.._,_J
o 1 2 34 5 6 0 0.5 1.0 1.5 2.0 2.5 3.0
Imode voltage (kV) Ads voltage (W)
Figure 10-22. Transiting tetrode voltage/current characteristics for dl~erent screen voltages.
line. To the left of this line screen current dominates anode current, and screen
dissipation becomes excessive. The top segment is the maximum anode current
limit. The curved-line limit is the actual anode dissipation, beyond which the
product of anode voltage and current exceeds 2 MW. The right-hand line, which
rises from the 30-kV anode voltage, is a limit on electron energy, or velocity,
beyond which electrons will penetrate the anode and cause damage.
The voltage/current characteristics of tetrodes, in general, are more cumbersome
to illustrate because there are four degrees of freedom compared to only
three in the triode. This means that there is a unique set of data interrelating
anode and grid voltages with anode, grid, and screen currents for each and every
value of screen voltage. Manufacturer’s published data will rarely show more
than two such screen voltages. Even if no more than one is given, however, it is
possible to synthesize the appropriate data points needed for any other value of
screen voltage. How this is done is shown in Fig. 10-22. The manufacturer’s
information given in Fig. 10-22a and is based on constant-current characteristics
for 1500-V screen voltage. But what is wanted is the same data for a screen
voltage of 750 V. To derive this information, a new set of axes along with new
grid voltage and anode voltage must be generated. In this case, the ratio of 750 V
to 1500 V is 1/2. The original current values for grid, screen, and anode are all
multiplied by the transfer ratio 1/2 raised to the 3/2 power. In the example
shown, the original 250-A anode-current line becomes in Fig. 10-22b 250A x (1/
2)32, which is 86 A, and the original 400-A line becomes the 141-A line by the
same process. Meanwhile the 25-A screen-current line becomes the 8.8-A line
and the 3-A grid-current line becomes a 1.06-A line. An entire characteristic can
be translated, but plotting only a few points at the full-conduction and cutoff
ends usually suffices because we are using these devices primarily as switches.