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Theory Of Operation; Amplifier Circuit Operation; Regulated Power Supply Operation; Dc Heater Supply - HP 450A Operating And Servicing Manual

Hp 450a amplifier serials prefixed: 010
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Model 450A
SECTION III
THEORY OF OPERATION
Sections III and IV
Paragraphs 3-1 to 4-5
)
3-1.
AMPLIFIER CIRCUIT OPERATION.
3-2. The amplifier circuit consists of two stages of
high-gain voltage amplification and a cathode follower
output stage connected as shown in the schematic
diagram.
Pentode tubes are used in all three stages
for wide bandwidth with low noise. The triode con-
nected cathode follower presents a relatively low
source impedance at the OUTPUT terminals.
De-
generative feedback is carried around the entire
amplifier to stabilize gain.
The amount of degen-
erative feedback is adjusted by the GAIN switch to
obtain 20 or 40 db amplification. The resistive feed-
back circuit is shunted by a small adjustable capacitor
for gain compensation at high frequencies. Resistance-
capacitance coupling is used between each stage.
Cathode bias is used at each stage.
3-3. Degenerative feedback is taken from the ampli-
fier output through a resistive divider consisting of
R3 and R6 to the cathode of the first stage VI. The
R3A portion of the divider is shorted by the GAIN
switch Sl to decrease feedback and increase gain to
40 db.
Resistor R6 provides fine adjustment of gain
for calibration purposes.
Capacitors Cll and Cl2
provide gain compensation at high frequencies.
3-4.
REGULATED POWER SUPPLY OPERATION.
3-5. The power supply for the amplifier is electron-
ically regulated to stabilize operation during changes
in line voltage and to minimize line frequency modu-
lation of the output signal.
The regulated supply
consists of power transformer TI. rectifier V4.
series regulator tube V5, regulator amplifier V6 and
vOltage reference V7.
The series regulator is a
cathode follower whose cathode supplies the regulated
voltage to the load consisting of VI, V2 and V3. The
series regulator serves as an adjustable impedance
controlled by amplified feedback from V6.
Ampli-
fier V6 samples the regulated voltage and amplifies
any difference between it and the reference voltage
provided by V7. Voltage comparison is accomplished
by applying the sample voltage to V6 grid and the
referen~e
voltage to V6 cathode.
If the regulated
voltage tends to rise, V6 amplifies this increase and
applies it to the grid of V5 causing the impedance of
V5 to increase, thus instantly and exactly counter-
acting the original tendency to increase.
This grid
control automatically holds the series regulator
cathode voltage constant.
The high plate resistance
of the series regulator tube assisted by amplifier
feedback attenuates ripple and stabilizes the output
voltage during changes in line voltage and rectifier
output.
The high transconductance of V5. assisted
by the same feedback, stabilizes the cathode voltage
during changes in load current.
The sample of the
regulated output is obtained from resistive divider
R24 and R26.
Resistor R25 is selected to adjust the
value of the regulated voltage to +210 volts.
3-6.
DC HEATER SUPPLY.
3-7.
DC
vOltage is supplies to the heaters of VI, V2
and V3 to prevent line-frequency modulation of the
output signal through heater-cathode leakage in the
tubes. This voltage is obtained from a 9-volt winding
on the transformer rectified by a full-wave bridge
rectifier CRI. The rectified voltage is filtered by C7.
Resistor R27 provides adjustment of the heater voltage
to accommodate aging changes in rectifier resistance.
SECTION IV
SERVICE INSTRUCTIONS
)
4-1.
WARNINGS AND CAUTIONS.
4-2. The amplifier contains a selenium rectifier.
When selenium rectifiers burn out due to overheating,
poisonous fumes are released. Ventilate immediately,
and do not inhale these fumes.
Do not handle the
rectifier until it has cooled.
4-3. Do not remove VI or V2 with the amplifier
turned on. These tubes are supplied with unregulated
dc heater voltage.
If
VI or V2 is removed with the
amplifier operating, the heater voltage on the re-
maining tube will rise sharply and possibly damage it.
00001-1
4-4. EQUIPMENT REQUIRED FOR MAINTENANCE.
4-5. General troubleshooting requires an electronic
multimeter such as the Hewlett-Packard Model 410B.
Other multimeters can be used if they have 20,000
ohm/volt sensitivity or greater. To calibrate the gain
of the amplifier requires an ac signal source and an
accurate ac voltmeter of the required frequency range,
such as the Hewlett-Packard Model 650A Oscillator
and 4000 Voltmeter.
Other test instruments can be
used if they provide the necessary frequency range
and accuracy. To measure distortion from theampli-
fier requires a signal source producing a signal with
3-1
4-1

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