Radar System Engineering

SEC. 10.9] THE HARD-TUBE P ULSER 369
care in processing and use it is possible, nevertheless, to arrive at a satisfactory
and highly flexible pulser desigu using the oxide-cathode pulse
tube up to 20 kv. Where extreme reliability and relative immunity to
overload are required, the designer will prefer the thoriated-tungsten
cathode switch tube in spite of its greater cathode power requirements.
Switching tubes of the oxide-cathode variety usually can be counted upon
to emit 300 to 600 ma/watt of heating power while the emission of the
best thonated-tungsten cathodes is only about 100 ma/watt.
The oxide-cathode tube is limited in power output by the onset of
grid emission. It is impossible to prevent thermionically active cathode
material from contaminating the grid structure. Great care must be
taken, therefore, to cool the grid by fins, heavy rods, and other means, for
it can emit a few milliamperes even at a sub-visible temperature. A few
milliamperes may sound small in comparison with several amperes pulse
current, but a few milliamperes flowing all the time can cause serious
heating of the anode and screen, which are at relatively high potentials.
Usually the result is a runaway condition in which grid control is lost.
The most successful oxide-cathode high-power switch tube is the 715B
developed at Bell Telephone Laboratories. The 715B has a gold-clad
grid which poisons and “absorbs” act ive material, thereby lowering its
efficiency as a thermionic emitter.
TABLE 1O.4.—TYE’ICALOIV:RATING cmm~c’rmus’mcs
Tube
type
3D21
3E2!l
715B
5D21
304TH
6C21
6D21
527
Cathode
type
Oxide
Oxide
Oxide
Oxide
Th-W
Th-W
Th-W
Th-VV
C’athode
plate
Max.
>owei-, voltage,
watts kv
10
14
56
56
125
140
150
770
Emiscur-
sion,
rent, amp/
amp watt,
3.5 5 0.5
5.0 8 0.57
15.0 15 0.27
20.0 15 0.27
15.0 6 0.05
30.0 15 0.11
37.5 15 0.10
30.0IPulse
60 0.08
cutoff
grid
vOltage*
– 70
– 100
– 500
– 500
– 900
–1000
– 500
– 1200
plate
drop,
Volts
400
600
1500
1500
2000
1500
2000
1500
9, creen
voltage
800
Soo
1200
1200
2000
‘ositive
grid
drive,
volts
150
150
200
200
200
1500
1500
1500
* 0,2-ma cutoff.
A good output pulse tube must possess a sharp grid cutoff. For
efficient operation, the plate current must be one milliampere or less when
the grid is biased to some reasonable value. Because the anode is at very
high voltage between pulses, any “leakage current” delivers excessive
power to the anode. As an example, consider a pulse tube normally
giving 20-amp pulses with an anode drop of 3 kv at a duty ratio of ~m.
Such a tube will display a peak power anode loss of 60”kw, but an average
power loss of only 60 watts. If the leakage current at “ cutoff” amounts