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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-
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.
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-

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