Western Electric 6071 B User manual

RADIO
Western
AMPLIFIER
INSTRUCTIONS
Electric
FREQUENCY
No.
Cu'eljr-rva
FOR
B
6071
USE
Printed in U.S.A. Instruction Bulletin No. 728

The equipment described in this Bulletin
was designed and developed for the
Wes/47r71 Electric Company
by.
BELL TELEPHONE LABORATORIES

RADIO FREQUENCY
AMPLIFIER
No. 607 1 B
CONTENTS
Page Page
INTRODUCTION 1 Installation of Amplifier Tubes.... 12
Power Requirements (Table) 1 Bias Voltages and Plate Current
(Table) 13
GENERAL DESCRIPTION 2
Amplifier Input Circuit 2 TUNING ADJUSTMENTS 13
Amplifier Tubes 2 Preliminary 13
Amplifier Output Circuit 2 Amplifier Input Circuit 13
Harmonic Suppression and Antenna Neutralizing and Amplifier Output
Coupling 2 Tuning 14
Monitoring Circuit 2 Coupling Circuit Tuning 14
Power Circuits 2 Adjustment of Output Transformer
Power Control and Protection Cir- Turns 15
cuits 3 Antenna Tuning 15
Formula for Coupling Capacity 15
INSTALLATION 3 Final Tuning Operations 16
General 3 Typical Meter Readings (Table) 16
Ground System 4 Summary of Adjustments 16
Antenna 5 Formula for Antenna Current 17
Power and Control Connections... 5 Operation at Reduced Power 17
Radio Frequency Input Connections 5 Formula for Efficiency (Foot-
Audio Frequency Connections 5 note) 17
Transformer Connections 6
Miscellaneous Installation Notes.. .
Change of Transmitter Nameplates 66 MODULATION
At Normal Power 1818
At Reduced Power 19
PRELIMINARY ADJUSTMENTS OF
POWER CIRCUITS
Power Supply Circuits
Fuse Ratings (Table)
Preheat Rectifier Tubes
Delay Relay
Grid Bias Load Resistances
667779
MONITORING
Monitoring Levels
Maximum Monitoring Levels
(Table)
Monitoring Circuit Impedance
1919
1919
Door Switch and Filament Relays.. 9
Plate Rectifier Contactor, Surge and OPERATING PROCEDURE 20
Filter Resistance Relays 9 Starting the Equipment 20
Grid Bias Marginal Relay 9 Stopping the Equipment 20
Overload Relay 10 Overload Relay Reset 20
Normal and Overload Current Operating Use of Meters 20
(Table) 10 Operating Use of Load Resistance. 21
General 12 Definition of "Resistance Intro-
duced" (Footnote) 21
PRELIMINARY ADJUSTMENTS OF
RADIO FREQUENCY CIR- CONNECTION TO OTHER EQUIP-
CUITS 12 MENT 22
General 12 Connection to Transmitter Other
Adjustment of Antenna Impedance 12 Than No. 12B 22
Value of Antenna Condenser Connection to Amplifier of Higher
(Table) 12 Power 22
I

MAINTENANCE
General
Care of the Cabinet
Cleaning Variable Air Condensers
Relays and Magnetic Contactors 23
Vacuum Tubes 23
Vacuum Tube Sockets 24
Repair of Thermocouples 24
Additional Routine 24
CONTENTS (Continued)
Page Page
22 APPARATUS INFORMATION 27
222323
LOCATION OF TROUBLE 25
General 25
Power Control Circuit 25
Fuses 25
Reduced or No Radiation 26
Distortion 26
Noise 27
II
SPARE PARTS 27
General List of Spare Parts for the
6071B Amplifier 27
Special List of Spare Parts for the
1000 Watt Equipment 28
Special List of Spare Parts for the
500 Watt Equipment 28
Special List of Spare Parts for the
250 Watt Equipment 28
ENGINEERING SERVICE AND IN-
FORMATION FOR ORDERING
REPLACEMENTS 29

RADIO FREQUENCY
AMPLIFIER
No. 6o7113
Introduction
The Western Electric No. 6071B Amplifier is a radio-frequency ampli-
fier designed for connection to a Western Electric No. 12B or other Radio
Transmitter capable of delivering 100 watts. It may be operated at any
frequency from 550 to 3000 kilocycles and will deliver 250, 500 or 1000
watts of completely modulated carrier into a suitable antenna. It is com-
posed of a No. 71B Amplifier plus miscellaneous apparatus such as
vacuum tubes, fuses, etc., required for operation.
The No. 71B Amplifier consists of a metal cabinet, similar to that of
the No. 12B Radio Transmitter, containing the radio-frequency circuits,
complete power supply equipment and all the necessary control and pro-
tective circuits. As the amplifier is completely a-c. operated, no motor-
generators, batteries or other external power equipment are required.
The amplifier is designed to operate on 220-volt, 50- or 60-cycle, three-
phase power supply. The following table gives the total power required
for operation at the three output ratings in the standby and operating
conditions. The power factor is 90 per cent.
Output Rating
1000 watts
500 watts
250 watts
Full Operation
4000 watts
2400 watts
1500 watts
A schematic diagram of the amplifier circuit is shown in Figure 1.
The amplifier consists of two tubes operating in a balanced push-pull cir-
cuit, a circuit for suppressing radio-frequency harmonics, and provision
for efficiently coupling the output to an antenna. Separate rectifiers pro-
vide the necessary direct current to the plate and grid circuits. Control
and protective circuits interlock with those of the associated radio trans-
mitter to provide protection to the equipment and the operating personnel.
The Nos. 302C, 303C. and 304C Radio Transmitting Equipments (250,
500, and 1000 watt output, respectively) each consist of a No. 12B Radio
Transmitter and a No. 71B Amplifier with their necessary accessories.
These equipments are supplied with tubes, tube sockets, condensers, etc.
for the power output requested. They may be operated at reduced power
by simple adjustments. The power rating may be changed by the substi-
tution of the proper tubes, tube sockets and other components.
[1]

GENERAL DESCRIPTION
Amplifier Input Circuit
The radio-frequency input voltage is obtained from the tuned. input
transformer (L1B) coupled to the output of the associated radio trans-
mitter. The input energy is dissipated in resistances (R1B and R2B)
which are in tapped sections to permit adjustment to the value required
to produce the necessary radio-frequency grid voltage across the tuned
circuit.
Amplifier Tubes
The amplifier employs two radiation-cooled vacuum tubes (V1B and
V2B) operating as class "B" in a balanced push-pull circuit, the tubes
being biased to approximate cut-off.
Amplifier Output Circuit
The plates of the tubes operate into a tuned output transformer con-
taining a thermoammeter (M5B) to facilitate tuning and neutralizing.
Harmonic Suppression and Antenna Coupling
Harmonic suppression and antenna coupling is accomplished by the
network consisting of L3B, C11B, C12B and L4B which couples the trans-
mitter to the antenna and suppresses the radio-frequency harmonics.
Meters (M6B and M7B) necessary to facilitate tuning are provided in
this circuit. A resistance (R8B) of 100 ohms is included, which may be
connected by means of a link (D14B) to serve as a load for testing at
times when it is not permissible or desirable to energize the antenna.
Monitoring Circuit
An audio output transformer (T4B) connected in the high voltage
return lead provides ample audio power for monitoring purposes.
Power Circuits
The filaments of all tubes operate on alternating current. The filament
transformer primary voltage is measured by a voltmeter (M1B) and is
maintained at 200 volts by adjustment of the variable auto-transformer
(T6B).
Grid bias voltage is supplied by a single full-wave mercury vapor
rectifier tube (V9B) . The output of this rectifier is passed through a two-
section filter and dissipated in an adjustable resistance (R10B).
Plate power for the amplifier tubes is supplied by a three-phase full-
wave mercury vapor rectifier employing six tubes (V3B to V8B, incl.) .
The output of this rectifier is filtered by a single-section filter.
[2]

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Power Control and Protection Circuits
The power control circuits consist of a combination of relays arranged
to energize the various power circuits of the amplifier in the correct
sequence and to provide the necessary delay before applying plate voltage
to the rectifier tubes. The amplifier is completely energized automatically
by the operation of the "Master Control" switch located on the No. 12B
Radio Transmitter, if the "Main Power Disconnect" (D1B) and the
"Filaments" (D3B) switches are closed, and when the "High Voltage"
switches on both the transmitter and amplifier units are "On." For semi-
automatic starting, the "High Voltage" switches are left in the "Off"
position when the "Master Control" switch is closed. After the control
circuits have functioned, the starting operation may be completed by
closing the "High Voltage" switches. (See "Control Circuit Sequence
Chart," Figure 2).
The protection circuits (of the No. 71B Amplifier) consist of the fol-
lowing relays and fuses to protect the equipment and personnel.
The door switch relay (S3B) removes bias and plate voltages whenever
a door switch (D7B) is operated either in the No. 12B Radio Transmitter
or the No. 71B Amplifier.
The grid bias marginal relay (S5B) removes the plate voltage when
the bias voltage is below a proper value and prevents application of the
plate voltage when the bias voltage is low.
The overload relay (S6B) removes the plate voltage when an overload
beyond the safe capacity of the rectifier occurs. The current demanded
by modulation peaks and supplied by the filter condenser does not pass
through this relay and cause unnecessary operation.
The surge relay (S7B) and the filter resistance relay (S8B) are pro-
vided to short out a resistance, used to limit the condenser charging cur-
rent, after the condenser has assumed its full charge. It is necessary to
limit this charging current to prevent excessive loads on the rectifier tubes
and false operation of the overload relay.
Fuses are provided in the 220-volt a-c. circuits and in the rectifier out-
put circuits as additional protection to the apparatus against circuit
defects.
INSTALLATION
General
The installation of the No. 71B Amplifier in conjunction with the No.
12B Radio Transmitter, being a typical case, is herein dealt with in detail.
Installation in conjunction with a transmitter other than the No. 12B
should be facilitated by these instructions.
The No. 302C, 303C, or 304C Radio Transmitting Equipments should
be installed in accordance with the installation drawings furnished. The
[3]

transmitter and amplifier units should be installed in a light, well ven-
tilated room and so situated as to provide easy access to the antenna and
ground connections. At least three feet of clearance should be allowed on
all sides and top of the units. If forced ventilation of the transmitter
room is employed, care should be exercised in arranging this ventilation
so that dust and dirt are not blown or drawn into the equipment.
Two hardwood bases should be prepared for the units in accordance
with the installation drawing, and if practicable, all ground leads, power
and audio conduits should be in place before the equipment is set up. The
No. 12B Radio Transmitter should be placed on one of the hardwood bases
and the back and two sides of the unit should be removed by removing the
screws which fasten them to the frame. The right side of the transmitter
unit is removed permanently and side guide pins at the top of the trans-
mitter frame should be removed to permit the proper alignment of the
transmitter and amplifier units.
Before the No. 71B Amplifier is set in place on its hardwood base,
remove the knock-outs in the junction box in the No. 12B Radio Trans-
mitter. This junction box is located in the lower front right-hand corner
of the transmitter unit (see view "C-C" on installation drawing ESX-
601752). Remove the back and the right side from the amplifier unit by
removing the screws which fasten them to the frame. The left side of the
unit should not be removed. When the amplifier unit is in place and care-
fully aligned with the transmitter unit, a 11/2 inch Chase nipple and lock-
nut should join the junction box in the transmitter unit to the amplifier
unit.
Ground System
A typical interior ground system layout is shown on the installation
drawing (ESR-601783). All metal structures such as building frame,
water and steam pipes, conduits, roofs and stacks should be bonded to the
interior ground system. All ground connections should be as short and
direct as possible and all joints should be soldered or welded. Where in-
stallations are made in tall buildings the information given on ESR-601783
and associated drawings may not apply entirely, in which case additional
engineering information should be requested.
The main ground terminal of the transmitter unit is located at the
extreme lower right-hand corner in front of the junction box. It consists
of a cadmium plated copper strip 43/4 inches long which is fastened to the
main transmitter frame by means of two bolts located at the ends of the
strip. The main ground terminal of the amplifier unit is located at the
extreme lower left-hand corner of the cabinet in front of the terminal
strip. It is similar in construction to that of the transmitter unit and is
fastened to the main frame of the amplifier by means of bolts. The ground
connection for each unit should consist of a 4 inch by 1/64 inch copper
strip, one end of which should be soldered to the removable strip which is
then bolted to the ground terminal. The other end is soldered to the main
ground system. It is suggested that the lugs which are bolted to the
[4]

ground terminal in the transmitter unit be temporarily removed while
soldering the ground strip. All ground connections should be as short and
direct as possible.
Antenna
The No. 71B Amplifier can be operated with an antenna of any resist-
ance and reactance, but where antennas of less than 12 or more than 90
ohms are encountered, additional engineering information should be
requested. The antenna lead-in from the grounding switch is connected
to terminal 27 located on top of the amplifier unit. Copper tubing is recom-
mended for this purpose.
Power and Control Connections
A 1 -inch conduit should be installed from the service entrance to the
junction box in the transmitter unit and three No. 8 B&S gauge BRC
wires should be pulled through from the 220-volt, 3-phase, 50- or 60-cycle
power supply. These wires connect to terminals 1, 2 and 3 of the amplifier
terminal block. Connect terminals 2 and 3 of the amplifier to terminals 1
and 2, respectively, of the transmitter terminal block. Using No. 14 B&S
gauge BRC wire connect terminals 4, 5, 6 and 7 of the transmitter unit
to terminals 4, 5, 6 and 7, respectively, of the amplifier unit.
Radio-Frequency Input Connections
In the No. 12B Radio Transmitter open link switch D28A and remove
condensers C21A from their mounting posts. Connect the ground strap
of the transmitter to terminal 26 of the amplifier. Connect the ungrounded
post justvacated by condensers C21A to terminal 25 of the amplifier. When
these connections are complete, link switch D2A may be used to connect
resistance R12A into the circuit : that is, when this link is between "3"
and "4" the resistance is in circuit, and when between "2" and "3" the
resistance is out of circuit.
Audio-Frequency Connections
The speech input and monitoring output leads should be run in con-
duit and brought out near the terminal blocks in the transmitter unit.
The speech input leads connect to terminals 15 and 16 on the transmitter
terminal block and the monitoring output leads connect to the spare
terminals 13 and 14 on the transmitter terminal block. Terminals 13 and
14 then should be connected to terminals 9 and 10, respectively, of the
amplifier unit. A No. 19 B&S gauge twisted pair, rubber and lead covered
cable per KS-6531 should be used for all audio leads. Both ends of the
lead cable sheath should be bonded to the ground system with a No. 16
B&S gauge bare copper wire and all conduits should be soldered or welded
to the ground system. In grounding the lead cable sheaths, terminal 10
of the transmitter unit or terminal 11 of the amplifier unit may be used.
[51

Transformer Connections
The heaviest transformer (T5B) is shipped separately and must be
mounted in its place in the lower compartment. Each transformer ter-
minal is marked and each wire is correspondingly tagged. Remove the
metal tags or slide them back upon the wires to avoid a chance of short
circuiting, and connect numbered wires to the correspondingly numbered
transformer terminals.
Rectifier transformers (T3B and T5B) are equipped with taps marked
with the primary voltage. The line voltage should be measured at regular
intervals during an operating day, the average taken, and the connections
to the transformer taps made on that marked terminal which is nearest to
the average voltage.
Miscellaneous Installation Notes
When the harmonic suppression coil is fitted with an internal supple-
mentary coil it is shipped with fiber wedges to protect it from breakage.
Remove these wedges before energizing the amplifier.
The equipment is shipped with condensers, meters, and other parts, all
of which are appropriate for the frequency, power and antenna impedance
specified in the order.
Change of Transmitter Nameplates
Certain apparatus designation nameplates on the No. 12B Radio Trans-
mitter are changed when the transmitter is operated in conjunction with
the No. 71B Amplifier. The new nameplates and mounting screws are
included with the equipment and can be attached readily with a screw-
driver. The nameplates to be substituted are listed below :
Original New
Apparatus Nameplate Nameplate
Designation Designation Designation
M3A "Antenna Current" "OutputCurrent"
C19A "Antenna Tuning" "Output Tuning"
The correct equipment nameplate also will be included and should be
mounted in the designated place in the center of the front panel of the
No. 12B Radio Transmitter. The panel is drilled and tapped, and the
proper screws are provided for this purpose.
PRELIMINARY ADJUSTMENTS OF POWER CIRCUITS
Power Supply Circuits
Open main switch D1B and install fuses in accordance with the follow-
ing table which gives the fuse ratings in amperes.
[ 6 ]

R.F. Output Power FIB F2B F3B
1000 watts 5 2 15
500 watts 4 2 10
250 watts 3 2 6
Install five 6-watt 120-volt (candelabra base) lamps in the panel lamp
sockets.
Preheating Rectifier Tubes
Install six No. 249B Vacuum Tubes* (mercury vapor rectifiers) in the
sockets (VS3B to VS8B, inclusive) and one No. 301A Vacuum Tube*
(mercury vapor rectifier) in its socket (VS9B). Preheat these tubes in
the following manner.
Set the "High Voltage" switch (D8B) on "Off" and temporarily re-
move bias rectifier fuses (F2.1B and F2.2B) . Apply filament voltage to
all rectifier tubes by closing the main switch (D1B), the "Filaments"
switch (D3B), and the main switch on the transmitter unit. Adjust the
"Filaments Volts" meter (M1B) to 200 volts by means of the variable
auto-transformer (T6B) designated "Filament Volts." This places 2.5 ±
0.1 volts on the filaments of the plate rectifier tubes (V3B to V8B, incl.)
and 5.0 ± 0.1 volts on the filament of the grid rectifier tube (V9B) . After
15 minutes or more the preheat is complete and the switches may be opened
and fuses replaced.
Delay Relay
During the preheating operation just described remove the cover from
the delay relay (S2B) and verify its timing by comparison with the second
hand of a watch. The time of operation should be at least 15 seconds from
the instant "Filaments" switch D3B is closed. If this time is not obtained
set and lock the calibrating disc within the relay to "15" or "16" (seconds)
and verify the setting.
If the equipment is being operated upon a 50-cycle power supply the
calibrating disc should be set at five-sixths of the desired number of
seconds, for example, the correct setting for fifteen seconds is 12. Any
setting made should be verified by timing with a watch.
A delay of 15 seconds assumes a room temperature of at least 60 de-
grees Fahrenheit. If room temperatures below 60 degrees but above 50
degrees are encountered, greater delay up to the full range of the relay
must be used. If below 50 degrees, the tubes should be preheated for a
greater length of time and the high voltage applied manually by means
of the "High Voltage" switch (D8B). At 32 degrees this time should be
at least 15 minutes.
*The filaments of new mercury vapor tubes should be heated at least 15 minutes
before the high voltage is applied. This preheating removes any particles of mercury
adhering to the sides or elements of the tubes after shipment or handling, thus mini-
mizing the possibilty of flash-overs. (See section on "Maintenance.")
[7]

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Grid Bias Load Resistances
Connect an ammeter suitable for measuring 0.32 ampere direct current
in series with the grid filter retardation coil (L21B). This may be done
by unsoldering the wire from the terminal lug marked "1," soldering the
negative ammeter lead to the lug, and the positive lead to the loose wire.
Energize the grid bias rectifier by closing the main switch (D1B) and
the "Filaments" switch (D3B) as in the preceding paragraph. Do not
close the "High Voltage" switch (D8B) .
The load resistances (R10.2B and R10.3B) are connected in parallel
and must be adjusted together so they will have approximately equal re-
sistances. Adjust these resistances by means of the lower slider clamps
which short out the unused end turns so that the load current (including
the current taken by the grid bias marginal relay in the operated condi-
tion) read on the ammeter is 0.32 ampere when the a-c. line voltage is at
its average value.
The remaining sliders provide a potentiometer adjustment of the bias
voltage on the tubes (V1B and V2B) and should be set for 275 volts meas-
ured between the grid terminals on sockets (VS1B and VS2B) and ground,
pending final adjustment. In the case of the 250-watt equipment, the bias
voltage shall be 250 volts. Having completed this adjustment the rectifier
should be shut off, the meter removed, and the wire resoldered to lug "1"
on the retardation coil (L21B).
Door Switch and Filament Relays
The filament relay (S4B) should operate promptly when the "Fila-
ments" switch (D3B) is closed. Door switch relay (S3B) , should operate
and release positively whenever any door in either the transmitter or the
amplifier is closed or opened. Check each door switch several times for
positive operation.
Do not remove the strap from across the resistance (R5B) and con-
denser (C32B) in series with the door switch relay (S3B). This apparatus
is provided only for use when the amplifier is operated with a transmitter
other than the No. 12B which may have a 110-volt door switch circuit.
Plate Rectifier Contactor, Surge
and Filter Resistance Relays
Close the main switch (D1B), "Filaments" switch (D3B), and all
doors. Allow time for the delay relay (S2B) to operate. Observe the plate
rectifier contactor (S1B) and the surge relay (S7B) for positive operation
and the filter resistance relay (S8B) for somewhat delayed operation when
the "High Voltage" switch (D8B) is closed. The surge relay (S7B) oper-
ates momentarily and drops back to permit the operation of (S8B) .
Grid Bias Marginal Relay
The grid bias marginal relay (S5B) is adjusted by means of slider
clamps on a resistance (R16B) in series with the relay winding. This
[9]

relay (as well as all others) is adjusted when it leaves the factory and
should not require readjustment. However, if the sliders have been moved
it will be necessary to readjust it as follows:
Temporarily connect an adjustable resistance of 200 ohms maximum,
capable of carrying 1/2 ampere in series with the primary of the bias
rectifier plate transformer (T3B) and use this to adjust the voltage out-
put of the rectifier. Connect a d-c. voltmeter with a 0-300 scale across the
entire load resistance to measure this voltage. Set the d-c. voltage at 230
and adjust the resistance (R16B) to a value such that the relay will just
pull up when the a-c. voltage is applied. Reduce the d-c. voltage to 200
and adjust that portion of the resistance (R16B) between the "break"
contacts until the relay drops out at this voltage. Care must be taken not
to attempt to make this marginal adjustment so close to the "operate"
(230 volt) point that the relay will act as a buzzer at some critical voltage.
The voltages specified in this paragraph may be lowered for those installa-
tions which are subject to line voltage variations greater than ± 10 per
cent from normal.
Overload Relay
The overload relay (S6B) is properly adjusted at the factory and should
not require readjustment. Adjustment of its operating point in the cir-
cuit is controlled by a shunt resistance (R9B) which is set in the follow-
ing manner.
This adjustment must be made with the cover of the relay (S6B) in
place. Open the main switch (D1B). Connect the positive terminal of a
six-volt battery to the transmitter ground, the negative terminal to a 12-
ohm rheostat, and the other terminal of the rheostat to the cap terminal
of the rectifier tube designated "V8B." Adjust the current indicated on
"Power Amp. Plate Current" meter by means of the rheostat to the nor-
mal value indicated in the following table and allow the current to flow
(i.e., "soak") for at least one minute.
Output Rating
1000 watts
500 watts
250 watts
Normal Current Overload Current
1.00 amperes
0.50 amperes
0.25 amperes
1.50 ± 0.15 amperes
1.00 ± 0.10 amperes
0.50 ± 0.10 amperes
The shunting resistance (R9B) shall be adjusted so that the overload
relay (S6B) operates on the overload value when the current is increased
from the normal value. In the case of the 250 watt amplifier it is not
possible to use the shunting resistance (R9B), therefore in this case re-
move the strap wire at one end of the resistance and test the relay for
operation between the limits indicated in the table.
Upon completion of the above tests, close the main switch (D1B), open
"High Voltage" switch (D8B) and energize the grid bias rectifier by clos-
ing "Filaments" switch (D3B). Operate the overload relay (S6B) manu-
[10]

0.010
0.0090
0.0080
0.0070
0.0060
0.0050
0.0045
0.0040
0.0035
0.0030
0.0025
á 0.0020
OcrU_îZ- 0.0015
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0.00080
0.00070
0.00060
0.00050
0.00045
0.00040
0.00035
0.00030
0.00025
ANTENNA
RESISTANCE
IN OHMS
10
90
0.00020500 600 700 800 9001000 1500 2000 2500
FREQUENCY IN KILOCYCLES PER SECOND
Fig. 3-Frequency Kilocycles
3000 4000 5000

ally, using a slender wooden rod through a perforation in the cabinet and
note that the relay holds up after operation and is released by the "Over-
load Reset" key (K1B). This must be done with the relay cover off.
General
Remove all testing equipment, replace relay cover, energize the ampli-
fier and observe for the sequence of operations indicated on the "Control
Circuit Sequence Chart" Figure 2. Note that the plate voltmeter (M3B)
indicates somewhat more than 3000 volts, that there is no appreciable
current on the plate current meter (M4B) and that the panel lamps
operate.
PRELIMINARY ADJUSTMENTS OF RADIO
FREQUENCY CIRCUITS
General
Refer to the attached tuning chart and set up the specified number of
turns on coils L1B, L2B and L3B. Verify that the fixed condensers listed
in the table have been installed and connected. Small differences in tube
characteristics and manufacturing variations in the coils and condensers
may render the tuning chart inexact, therefore during the following tuning
operations a change of a turn or two may be anticipated.
Adjustment of Antenna Impedance
If the antenna has positive reactance, it will be necessary to install an
antenna series condenser (C13B) so that the total antenna reactance will
be only enough positive to tune properly with the required value of cou-
pling capacity (C12B). Space, temporarily closed by a link (D11B), has
been provided for this capacity (C13B). Specifically if the antenna has
more than j20 ohms positive reactance, the antenna series condenser
(C13B) shall be in accordance with the following table, wherein Ra is the
resistance component of the antenna impedance. The necessary Cornell-
Dubilier condenser may be ordered from the nearest Branch House of the
Graybar Electric Company. For antennas of more than 90 ohms resist-
ance, additional engineering information should be requested.
Frequency Capacity
Kilocycles Mfd. Ra Between
20 and 90 Ohms
500 to 900 0.00120 PL-242-50
910 to 1800 0.00060 PL-265-50
1800 to 3000 0.00035 PL-288-50
Installation of Amplifier Tubes
Install the amplifier tubes in the sockets, taking care to see that they
are not subject to any mechanical strains. In the case of the 250-watt
equipment, two adapter assemblies for use in connection with the stand-
ard sockets, are provided and should be installed before the tubes are
fitted to them.
[12]

Verify the proper installation of the tubes by lighting the filaments
(close "Master Control" on transmitter, the main switch D1B and "Fila-
ments" switch D3B) setting the "Filaments Volts" voltmeter (M1B) to
200 and reading 10.0 ± 0.1 volts across the filament terminals or pins on
the tube with an accurate portable voltmeter. Poor socket contacts will
cause low filament voltage measured across the tube terminals.
Energize the amplifier as previously described and adjust the plate
currents of the two tubes to equality, ás indicated upon the "Balance"
meter (M2B), by adjusting the potentiometer slider on either load resistor
(R10B) of the grid bias rectifier. One tube (V1B or V2B) should be biased
to 275 volts, the other adjusted to balance. In the case of the 250 watt
equipment the bias voltage shall be 250 volts balanced as above.
The "Power Amp. Plate Current" meter (M4B) readings should be
approximately as given in the table below.
Type of Output Bias Plate
Tubes Rating Volts Amperes
279A 1000 -275 0.3 ± 0.1
251A 500 -275 0.3 ± 0.1
270A 250 -250 0.075 ± 0.025
TUNING ADJUSTMENTS
Preliminary
These instructions describe the tuning of the No. 71B Amplifier in
connection with the No. 12B Transmitter as a typical case. It is assumed
that the transmitter is tuned and adjusted in accordance with instructions
contained in Bulletin No. 727. This preliminary tuning of the transmitter
shall be done with the "Input Coupling" control (L1B) of the amplifier
set on zero and the 100-ohm resistance (R12A) in series with the other
elements of the link circuit, i. e., "D2A" shall connect "3" and "4" as de-
scribed (page 5) under "Radio Frequency Input Connections." If con-
denser C19A shows a tendency to flashover, reduce the turns on L13A
and increase the capacity (C19A) . Note :-"Antenna Tuning" condenser
(C19A) and "Antenna Current" meter (M3A) referred to in Bulletin No.
727 have been redesignated as "Output Tuning" and "Output Current"
respectively.
Amplifier Input Circuit
With the amplifier "Input Coupling" control (L1B) set on zero, tune
the output of the No. 12B transmitter until the full output power (100
watts) is dissipated in the 100-ohm resistance (R12A). Record all meter
readings and dial settings on the No. 12B Transmitter, reduce the "R.F.
Output" control (R20A) to zero and remove the 100-ohm resistance
(R12A) by means of the link switch (D2A) described (p. 5) under
"Radio Frequency Input Connections."
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Energize the No. 6071B Amplifier by operating the main switch (D1B),
and the "Filaments" switch (D3B) . Leave the "High Voltage" switch
(D8B) open. Increase the "R.F. Cont:ol" until a convenient reading (1/2
ampere or more) is obtained upon the "Output Current" meter (M3A),
and increase the "Input Coupling" control (LIB) to five or ten divisions.
Tune the amplifier input circuit with the "Input Tuning" control (C1B)
until a distinct minimum is obtained on the "Output Current" meter (M3A)
of the transmitter. During this tuning the "Balance Adjust" control (C2B)
may be left on 50. When the input circuit has been tuned increase the
"Input Coupling" control (LIB) only until the readings of the No. 12B
Transmitter read as recorded in the preceding paragraph.
Neutralizing and Amplifier Output Tuning
Reduce the "R.F. Output" (R20A) to zero, connect the sensitive ther-
mocouple (TC1B) by operating the four-pole knife switch (D4B) to the
"Neut." position, set the "Neutralizing Adjust" control so that the neutral-
izing condenser plates (C4B and C5B) are about one-third engaged and
set the "Output Coupling" (L2B) control on zero. Operation of the knife
switch also opens the high voltage control circuit to prevent application of
the plate power while the sensitive couple is in the circuit.
Increase the "R.F. Output" slightly and adjust the amplifier "Output
Tuning" (C10B) for a maximum reading on the "Power Amp. Output
Current" meter (M5B) , adjusting the "R.F. Output" control carefully so
that the meter (M5B) reading does not exceed full scale. Using the
"Neutralizing Adjust" control (C4B and C5B) reduce the meter (M5B)
reading to zero. Increase the "R.F. Output" control gradually to its maxi-
mum and at the same time carefully put a fine adjustment on the "Neutral-
izing Adjust" control so that the meter (M5B) reading is maintained at
zero or very nearly so. When the "R.F. Output" control is on maximum
and the meter (M5B) reads sensibly zero, the amplifier is properly neu-
tralized.
Reduce the "R.F. Output" to zero, and operate the four-pole knife
switch (D4B) to the "Operate" position to place the thermocouple TC2B
in the circuit. Leave the "Output Coupling" (L2B) on zero. Apply plate
voltage to the amplifier by operating the "High Voltage" switch (D8B)
and increase the "R.F. Output" until mid-scale deflection is obtained on
the "Power Amp. Output Current" meter (M5B). Check the "Output
Tuning" (C10B) for minimum on the "Power Amp. Plate Current" meter
(M4B) .
Coupling Circuit Tuning
Connect the 100-ohm load resistance (R8B) into the coupling circuit
by means of link switch D14B and disconnect the antenna circuit by open-
ing the link switch D12B. Set the "Output Coupling" control (L2B) to
about five divisions and adjust the "Coupling Circuit Tuning" (L3B) for
a definite minimum on the "Power Amp. Output Current" meter (M5B),
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