Power Amplifiers

TM 11-6660-204-25

sistor, the resistance of which depends on its fila-

V308B is supplied through a decoupling circuit

ment voltage to control feedback between the

consisting of resistor R359 and capacitor C333.

fourth and fifth voltage amplifiers. As the fila-

Capacitor C321 provides a feedback path, so the

ment voltage increases, the plate resistance of

output voltage at the plate of tube V309A (fig.

tube V310A decreases and vice versa. When the

213) is fed back to the cathode of tube V308B.

line voltage is steady, this circuit provides a fixed

This feedback circuit is degenerative and reduces

value of negative feedback from the plate of tube

the possibility of oscillations caused by stray posi-

V310B to the cathode of tube V309B. When the

tive feedback.

plate signal of tube V310B goes in a positive

c. The third voltage amplifier (fig. 2-13) is

direction, more current flows through the feed-

one-half of a

twin-triode tube V309A which

back circuit (capacitor C324, resistor R349, and

receives the signal from the plate of tube V308B

tube V310A ) and causes a larger degenerative

through coupling capacitor C320. The signal is

effect across cathode resistor R346 which reduces

developed across grid resistor R342 and applied to

the gain of that stage. If the line voltage in-

the grid of tube V309A where it is amplified and

creases, the filament voltage in tube V310A will

appears across plate-load resistor R344. Self-bias

increase, thus decreasing the plate resistance and

is provided by cathode resistor R343, which is

increasing the negative feedback to reduce the

unbypassed for negative feedback purposes.

gain of tube V309B. The reduced gain of tube

V309B, will compensate for the overall gain of the

d. The signal from the plate of third amplifier

remainder of the servosystem. Conversely a de

tube V309A is fed through coupling capacitor

creased line voltage produces less gain but less

C322, and appears across SENSITIVITY poten-

negative feedback, which acts to keep the overall

tiometer R213. The SENSITIVITY control is a

gain up to its normal level. Resistor R348 is the

screwdriver-adjusted potentiometer mounted on

plate load resistor for tube V310A. Capacitor

the front control panel. The wiper arm of this

C327 is provided to bypass any extraneous high

control is connected, through current-limiting re-

frequency (hf) voltages picked up by the cabling

sistor R345, to the grid of fourth amplifier tube

between the output of the fifth voltage amplifier

V309B. This adjustment permits sufficient sensi-

and the power-amplifier circuits in the power sup-

tivity for rapid servo action without jitter of the

ply chassis.

pen while tracing. The fourth voltage amplifier

uses one-half of a hi-rep twin triode tube (V309)

(fig. 2-13) where the signal is amplified and ap-

21 5. Power Amplifiers

pears across plate-load resistor R347. Bias is ob-

(figs. 2-15 and FO-6)

tained by the plate currents of amplifier tube

a. Two power amplifier tubes, V405 and V406,

V309B and line-compensating tube V310A (fig.

are mounted on the power supply chassis. The

214) flowing through cathode resistor R346. The

tubes are connected in parallel, but are shown as a

cathode resistor is unbypassed for degenerative

single tube in figure 215 for simplicity of presen-

purposes. Capacitors C335 and C336 (fig. 213)

tation. Tubes V405 and V406 supply excitation to

are bypass capacitors which provide a low-imped-

the control winding of two-phase balancing motor

ance path to ground for any voltages that are

B501 (para 213 ). The power amplifiers also sup-

induced into the cabling between the control panel

ply a signal for operation of the pen-lifter circuit

in the signal data unit.

(para 2-16).

e. The signal from the plate of fourth amplifier

b. Plate voltage is obtained from the positive,

tube V309B is fed through coupling capcitor C323

unregulated, 500-volt supply through voltage

(fig. 2-14) and developed across grid resistor

dropping and plate decoupling resistors R417 and

R350. The signal is then applied to the grid of the

R418, and the parallel-tuned circuit which con-

fifth amplifier tube (V310B ) which is one-half of

sists of capacitor C409 connected across the con-

a hi-P grid twin-triode, through current limiting

trol winding of the balancing motor. Capacitor

resistor R360 where the amplified signal is ap-

C408, together with resistor R417 and R418,

plied across plate load resistor R312. Self-bias and

forms an RC filter which presents a low imped-

some degenerations are provided by cathode resis-

ance path to circuit ground for any hf voltage

tor R351.

inducted by the line winding of B501 and reduces

the 120-cycle ripple remaining in the 500-volt

f. Line-compensating feedback tube V310A,

power supply circuit. Screen-grid voltage is ob-

which is one-half of a twin-triode tube, is con-

tained from the positive regulated 300-volt supply

nected as a diode and functions as a variable re-

2-17