Figure 2-8. Voltage switch and output gate, simplified schematic diagram.

TM 11-6660-204-25

b. Voltage switch tube V306 is normally held

affect the extent of discharge. When the control-

beyond cutoff by the high grid bias applied to tube

ling pulse is over, voltage switching tube V306 is

V305B through grid-current limiting resistor

a g a i n cut off and capacitor C310 recharges

R330 (fig. 2-7). When a positive pulse is devel-

through resistor R331, tube V307B, and the low

oped by the univibrator, the grid of tube V306 is

impedance filter and load network between the

driven rapidly from cutoff to zero or slightly posi-

cathode of tube V307B and ground. To assure an

tive, and the plate current of switch tube V306

equal amount of energy per pulse being stored in

rises steeply to a steady value. This value of plate

filter capacitors C311, C313, and C314, the values

current and its time of duration are essentially

of resistor R331 and capacitor C310 have been

the same for each pulse. Plate-1oad resistor R331

carefully designed to avoid the variations of re-

is a precision wire-wound type and, therefore, be-

sistance and capacitance that normally occur as a

cause the plate current is essentially the same for

result of changes in temperature and/or humid-

each pulse, the voltage drop produced across resis-

ity; other resistors and capacitors will not be sub-

tor R331 is also essentially the same throughout

stituted for these two particular items. The charg-

each pulse.

ing time for capacitor C310 is normally between

one-fifth and one-tenth of the period of the high-

c. Under static conditions of tube V306, capaci-

est pulse recurrence frequency that will be en-

tor C310 is charged to 300 volts. When a positive

countered, so there is always sufficient time for

pulse is applied to the grid of tube V306, the re-

the capacitor to recover its full charge between

sulting plate-cathode conductively serves as a

pulses.

closed circuit to permit a discharging current to

occur. The low resistance of the tube drops the

d. Because of the high value of capacity in the

plate potential steeply to about 25 volts. The dis-

parallel combination of C311, C313, and C314 (75

charge path on the negative side of capacitor

total), only a few microvolt of charge are

C310 may be traced through tube V307A and re-

built up on each pulse. Choke L301 and its

sistor R332. The time constant of this circuit is

shunt capacitor C312 resonate at approximately

relatively low, so the discharge is completed long

10 CPS, which increases the series filtering effect

before the end of the controlling pulse; thus small

at the low end of the af signal frequency scale. At

variations in pulse duration cannot appreciably

the high end, further filtering is accomplished at

Figure 2-8. Voltage switch and output gate, simplified schematic diagram.

2-11