Positive Pulse-Selector Stretcher

11-6660-204-25

grid of tube V303B, and the corners of the square

tors R313 and R314 produces a positive voltage at

the cathode of tube V303A greater than the posi-

wave would be rounded instead of spiked. The

positive spike is a desirable feature when this

tive voltage applied to its grid through resistors

wave form is later applied to the univibrator.

R311 and R312. This effectively biases tube

V303A at cutoff so normally, with no signal ap-

2-8. Positive Pulse-Selector Stretcher

plied, tube V303A is at cutoff and tube V303B

(fig. 2-5)

conducts. When a positive input signal is applied

to the grid of tube V303A, the grid becomes posi-

a. The positive pulse-selector stretcher is one-

tive with respect to the cathode. When the input

half of a miniature twin-triode tube, V304A. Its

signal reaches a value of approximately 150 volts,

purpose is to eliminate the negative portion of the

and tube V303A conducts decreasing the plate

applied input signal a:nd to develop a positive out-

voltage and creating a negative swing which is

put pulse, at each of two output points, for each

applied, through capacitor C304, to the grid of

input pulse. One output is used as a trigger for

tube V303B, This negative voltage, in turn, de-

univibrator V305, and the other to drive audio

creases the plate and cathode current of tube

output tube V304B. The stretching is done on the

V303B. Because both cathodes are interconnected

time base to increase the audio component in the

the decreased cathode voltage reduces the effective

pulse applied to the audio tube.

bias on tube V303A and aids the positive grid

b. The rectangular output wave from the plate

voltage of tube V303A. A regenerative action

of tube V303B is applied to an RC circuit consist-

takes place, and leaves tube V303A conducting at

ing of resistor R319 and capacitor C305. The RC

maximum current and tube V303B at cutoff, Any

time constant of this circuit is very small com-

increase of the grid-signal voltage of tube V303A

pared to the width of the input wave and, as a

past the triggering point has no effect on the out-

result, this wave is differentiated. The differen-

put voltage of tube V303B. The two tubes remain

tiated wave is applied to the grid of tube V304A,

in this steady state condition until the signal volt-

whit h is set at the cutoff point by a fixed negative

age at the grid of tube V303A drops below 120

bias voltage applied between the grid and cathode.

volts. At this time, regenerative action again

Negative pulse on the grid have no effect, be-

takes place, because the negative signal voltage

cause the tube is already biased to cutoff. How-

produces a positive voltage across plate resistor

ever, a positive grid pulse produces a plate cur-

R315, which is applied to the grid of tube V303B

rent pulse. This plate current pulse flows through

through capacitor C304. This positive voltage in-

resistor R322 (the cathode resistor of univibrator

creases the drop across cathode resistors R313

tube V305) to charge capacitor C306. Conse-

and R314, and effectively returns the bias on tube

quently a sharp positive pulse, produced by the

V303A cathode to the cutoff condition that origi-

capacitor-charging current, appears across resis-

nally existed with no signal applied; tube V303A

tor R322 and is used 10 trigger univibrator V305.

returns to a nonconducting condition and tube

V303B conducts. The two tubes remain in a

c. The waveform across cathode resistor R321

steady state until the signal voltage applied to the

of tube V304A is shown at jack J313 (fig. 26).

grid of tube V303A is raised to approximately 150

(The second spike is caused by the recovery of the

volts, at which time the circuit is triggered and

univibrator. ) The stretched pulse results from the

the action is repeated. Thus when one tube is held

exponential decay of voltage caused by the dis-

at plate current cutoff and the other tube is con-

charge of capacitor C306 through resistors R321

ducting, a rectangular waveform of fixed ampli-

and R322. The stretched pulse is applied to the

tude is created at the recurrence frequency of the

grid of audio output tube V304B. A momentary

input signal, regardless of input waveform and

contact, INTER S.C. position of switch S203 on

amplitude. `The output of this stage is the signal

the (control panel, is connected across resistor

voltage developed at the plate of tube V303B.

R322 to short the univibrator trigger pulse for

testing purposes. This switch can be used to help

c. Capacitor C304 immediately applies the volt-

isolate trouble in the signal converter circuits and

age developed across resistor R315 in the plate

to determine whether the recorder circuits are

circuit of tube V303A directly to the grid of tube

functioning properly. (Recording at zero is ob-

V303B rather than through resistor R316. With-

t a i n e d when the switch is depressed.) Tube

out this capacitor the steep voltage, rise and fall,

V304A actually functions as a cathode follower to

developed across resistor R315 by operation of

provide a signal for triggering the univibrator at

a low impedance point.

tube V303A would be delayed in application to the

2-7