Univibrator

11-6660-204-25

b. Tube V305A has its grid connected to the

b. The grid of tube V304B is directly connected

through current-limiting resistor R320 to the

+300-volt supply through resistor R328 so that it

is normally in a conducting condition when no sig-

cathode of tube V304A and is supplied with a

nal is applied. Biasing is accomplished by the flow

fixed negative bias of 12.6 volts. When the posi-

of grid current through resistor R328, and develops

tive streched pulse appears at the cathode of tube

V304A, it is applied to the grid of tube V304B

a bias which is sufficient to produce a voltage drop

across R328 almost equal to the voltage supply.

through grid-current limiting resistor R320. Be-

Normally, with no signal applied, the grid rests at

cause of the large signal amplitude swing, the

a potential of +4 or +5 volts. Because tube V305

tube draws a slight amount of grid current on the

is a high-gain triode, it requires only a small volt-

signal peaks through resistor R320 effectively

age for cutoff (approximately --5 volts at 300

clipping the spiked portion of the signal and pro-

plate volts ). With a positive voltage on the grid,

ducing a pleasing sound in the speaker. The signal

the tube conducts to produce a plate current flow

is amplified and fed through audio output trans-

former T301 to the speaker mounted on the con-

of approximately 4 milliamperes (ma), and a

cathode potential of approximately 4 volts. In op-

trol panel. A simplified diagram is shown in figure

2-6. The cathode of tube V304B is returned to

eration the grid voltage is slightly higher than the

developed cathode voltage, so the tube normally

circuit ground through unbypassed potentiometer

R214. This potentiometer has no shunt capacitor

operates with a small positive bias. The grid of

tube V305B is directly connected to the plate of

and effectively controls the amount of negative

feedback, tube gain, and speaker volume,

tube V305A through resistor R324 and is biased

beyond cutoff by a voltage divider arrangement

2 - 1 0 . Univibrator

which consists of resistors R324, and R325, and

(fig. 2-7)

R326, between that + 50-volt plate of tube V305A

and --175 volts, This bias is adjustable through

a. The univibrator uses both sections of a mini-

variable resistor R326. Resistor R322 is the cath-

ature twin-triode tube, V305 (fig. 2-7). The univi-

ode resistor for tube V305A, and R329 is the cath-

brator is essentially a one-shot multivibrator trig-

ode resistor for tube V305A. The input trigger

gered by the output of the preceding pulse select-

voltage is applied to the cathode of tube V305A

or circuit. Its purpose is to generate a positive-

and the output signal is taken from the grid of

going rectangular pulse of constant time width to

operate voltage switch V306 (fig. 2-8).

tube V305A and applied to voltage switch tube

Figure 2-6. Audio output, simplified schematic diagram.

2-9