Pulse Shaper and Amplifier Circuit - TM-11-6665-209-400014

TM 11-6665-209-40

Pulse Shaper and Amplifier Circuit. The pulse shaper and amplifier circuit con-

sists of variable resistor R117, tubes V103 and V104, transistor Q105, switch section

S102B and associated resistors and capacitors. This circuit converts the random-

a m p l i t u d e , random-duration pulses from the detector circuit into pulses of constant

amplitude and duration and feeds them to the indicator circuit. Regulation of the plate

supply voltage of tubes V103 and V104 is necessary to keep the amplitude and duration

of the output pulses to the indicator circuit constant.

(1) Resistor R117 is a potentiometer which adjusts the voltage on the grid of V103

to the optimum bias point. This adjustment is outlined in paragraph 3-4b, but

will be briefly outlined here.

(2) The grid voltage of V103 is measured by means of a vacuum-tube voltmeter,

and resistor R117 is adjusted until the circuit is just out of the free-running

condition. The voltage at this point is noted, and R117 is adjusted until the

voltage is 0.5 volt higher, which is the optimum bias point. Hum is picked up

at the input circuit and amplified. Hum neutralization is provided by applica-

tion of a voltage of opposite phase to the transformer core, which due to the

special construction of the transformer, is capacitively coupled only to the

cathode circuit through the filament winding. The major portion of any ripple

voltage is thus effectively canceled out, assuring stability of the circuit and

also very low background noises in the headphones.

(3) Tubes V103 and V104, connected as triodes, make up a one-shot multivibrator.

Transistor Q105 is connected as an emitter follower and is mainly an im-

pedance matching device to insure proper pulse shape and height even with a

relatively low resistance in the grid circuit of V104 (as on the 50 mR range).

It also allows headphone output from the emitter, which makes possible a

l a r g e - s h u n t - c a p a c i t a n c e free cathode circuit, to eliminate sensitivity losses

and deterioration in a waveform. In addition, CR107 clamps the grid of V104

t o ground preparing the multivibrator circuit for another pulse immediately

following the completion of a preceding pulse.

(4) In the multivibrator dormant state, no pulses from the detector circuit are

received. In this condition, V103 is conducting and V104 is cutoff. Resistor

R121 and transistor Q105 are the plate load for V103. Resistors R116 and R117

form a voltage divider which establishes the steady-state grid bias on V103.

The bias and the plate voltage of V103 are such that V103 is conducting. The

control grid of V104 is connected through one of the resistance paths of

switch section S102B to ground. Tube V104 is cut-off because its cathode is

held positive by being connected to the cathode of the conducting tube V103.

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