Gates BC1G Instruction Manual

GATES

INURUCTIONS FOR InaLLING AND OPERATING
TH2 GATES TYEE BC1G, 1000/250 WATT
BROADCAST TRAI\UIIITTER
Gates Radio Company
I.B. # 888 0800 001 Quincy, Illinois

MODULATION TRANSFORMER INSTRUCTIONS
Please read these instructions before attempting to test
the modulation transformer in this transmitter.
The modulation transformer employed in this transmitter
may be of a type which will indicate unequal resistance
in the primary windings. An ohmmeter check of the wind-
ings may indicate that the transformer is defective;
whereas in reality, this is a normal reading and the
modulation transformer is performing normally.
In order to properly check this transformer outside of
the transmitter circuit, merely apply 117 volts, 60 cycle
a.c. to the secondary winding. Check the voltage on each
half of the primary winding. If the transformer is oper-
ating normally, then these voltages s'r-viicl be approximately
equal.
Gates Radio Company
Quincy, Illinois

SAFETY NOTICE
This equipment employs voltages which are dangerous and .
may prove fatal if contacted by operating personnel.
Extreme caution should be exercised when working with the
equipment.
KEEP AWAY FROH LIVE CIRCUITS
Observe safety regulations. Do not change tubes or make
adjustments inside equipment with hiLh voltages on.
Do not depend on door interlocks or switches for protection.
No reliance should be placed on the interlock switches for
removing high operating voltages.
SWITCH TO SAFETY
SAFETY FIRST: . 1hen workind on the transmitter, disconnect
the primary power at the building wall switch.
WARRANTY
The Gates warranty, gladly supplied in detail on request,
generously covers all materials when returned to the Gates
factory for inspection, transportation paid. Certain moving
parts and tubes are guaranteed usually on an hourly bnsis and
that of the manufacturer's guarante. This warranty does not
extend to free service in the field, but this service is wail-
able at a modest cost, where required.
BC1G

TABLE OF CONTENTS-
Pàe
SAFETY NOTICE
WARRANTY
TABLE OF CONTENTS
SPECIFICATIONS
ILLUSTRATION, BC- 1G FRONT VIEW - PHOTO 800 0437 001
SECTION I - GENERAL DESCRIPTION
1.1 Mechanical Construction
1.1.1 Transmitter Controls
1.2 Incidental Information
1.2.1 Tube Handling
1.2.2 Transmitter Building Temperatures
1.2.3 Grounding
1.2.4 Antenna Coupling
SFCTION II - THEORY OF OPERATION
2.1 M5422 Oscillator Unit
2.1.1 Tube Complement, M5422 Oscillator Unit
2.1.2 Type of Oscillator Circuit
2.1.3 Type of Crystal Used
2.1.4 First IPA, 12BY7A
2.1.5 M5422 Oscillator Tuning Procedure
2.2 BC- 1G Transmitter Description
2.2.1 - Tute.___Lineup
2.2.2 BC- 1G llrfayrem .- 1-t-t- er - - CIr cu i: t ry
2.3 Power Supplies
2.3e1 Bias Supply
2.5.2 600/625 . yolt, Low Voltage Supply
2.3.3 2800 Volt Supply
2.3.3.1 High Voltage Supply Operation at
250 Watts Output
2.3.3.2 BC- 1G Power Change Facility
2.4 Attachment of Remote Control in BC- 1G
SECTION III - INSTALLATION
3.1
3.2
3 .3
3.3.1
).3.2
3.3.3
3.3.4
3.3.5
3.3.6
3.3.7
Installation Hints
Transmitter Inspection
Transmitter Connections
Primary Power
Transmitter Ground
Audio Input
Modulation Monitor
Frequency Monitor
RF Output
Remote Control ( if used)
-1- BC- 1G

3.3.7.1
3.3.7.2
3.3.7.3
3.3.7.5
3.3.7.6
3.3.7.7
3.3.7.8
3.3.8
3.4
Detailed Instructions for Rerrate-Control
Connections
Plate Rheostat Motor Assembly
'Remote Power Change
Reset, Plate Off
Remote. Plate Voltage Indication
'Remote Plate- Current Indication
Remote Tower Light Indication
Remote Antenna Current Metering
Studio Procedure, for Remote Control
Operation
Crystal Installation
SECTION IV - TUNE-UP PROCEDURE, 1000 WATT CARRIER
4. 1
4.1.1
4.1.2
4.2
4.3
4.4
4.5
4.5.1
Preliminary Tune-up Checks
(Transmitter locally controlled)
Checking Filament Voltage, Bias Supply
and Low Voltage Supply
Checking Power Amplifier Tuning
Components
Neutralizing the Power Amplifier
Power Amplifier Tuning
Modulator Adjustment
Modulation Monitor Connections
Method of Adjustment, Coil L43
and Resistor R34
SECTION V - TUNE-UP PROCEDURE, 250 WATTS
SECTION VI - HELPFUL OPERATIONAL INFORMATION
6.1 Frequency Adjustment
6. 9 Transmitter Cleanliness
6.3 Relay Maintenance
6.4 Care of Printed iring Boards
6.5 Cabinet Ventilating Fan
6.6 - Teet-quipment revinsroeen Esc
5.7 D.C. Resistance-tle - a-ectrem - entallod•
AM- 30469E - Gates 478 0084 000
6.8
Reprints
ary
a Replacing Components on the Printed
Chassis. ( Gates has a printed sheet furnished
on request, no charge)
b) HGlpful General Information
SECTION VII - PARTS LIST
SECTION VIII - TRANSMITTER PHOTOGRAPHS
800 0437 002 -
800 0437 003 -
800 0437 004 -
800 0437 005 -
800 0437 006 -
800 0437 007 -
800 0437 008 -
800 0437 009 -
Front Door Open, Screen in Place
Front Door Open, Screen removed
Front Door Open, Screen removed, Panel & Shelf
Inside View, Top Portion, Transmitter
Inside View, Bottom Portion, Transmitter
Rear View, Back Removed, showing Rectifiers
Rear View, Back Removed, showing Panel & Shelf
Side View, Side Cover Off
-2- BC- 1G

SECTION IX - CIRCUIT DIAGRAMS AND CHARTS
A-30584 Typical Curves, Frequency Stability,
Vacuum Crystal
821 3816 001 Schematic, Oscillator Unit, M5422
813 7774 001 Typical Voltage Chart
813 7711 001 Tuning Chart
813 7626 001 Wiring Diagram, Modulator Selector Switch
813 7928 001 Supplemental Relay Connections, KlA
813 7961 001 Supplemental Relay KlA Location
813 7914 001 Wiring Diagram, Remote Plate Voltage Control
837 8485 001 Wiring Diagram, Studio Unit, RDC-10C
837 9369 001 Wiring Diagram, Transmitter Unit, RDC-10C
826 9003 001 Interconnections using RDC-10C Transmitter
Unit
813 7963 001 Running Sheets, Panel & Shelf
813 7962 001 Running Sheets, Cabinet
826 8808 001 Control Circuitry
813 7924 001 Base Layout
813 7529 001 Diagram, Primary Relay Kl
813 7628 001 Wiring Diagram, Multimeter S2 Connections
813 8885 001 Contact Identification
852 5878 001 Overall Schematic
-3- BC- 1G Tranauitter

SPECIFICATIONS
BC- 1G, 1000/250 WATT TRN.31` , IITTER
Talf, FOLLOWING SPECIFICA . 2IONS A.; .E TYPICAL -
RATED POWER OUTPU2:
1000/250 watts. Capable of 1100/275
watts, if necessary, to overcome
possible losses in directional arrays.
FREQUENCY RANGE: 2000 Kc to 540 Kc
PRIMARY PUER INPUT: 230 volts, 3 wire, solid neutral, single
phase, 50 to 60 cycles. Approximately
3850 watts consumed at 100% tone modu-J..-
tion, at 1000 cycles.
FREQUENCY STABILITY: + 5 cycles within temperature range of
- 50 to 122°F.
ELEVATION: 6500 feet.
VENTILATION NECESSARY: Provision should be made to allow 1500
CPM of clean, outside air under all
circumstances
AUDIO INPUT:
INPUT AUDIO IMPEDANCE:
FREQUENCY RESPONSE:
DI0TORTION:
CARRIER SIM:
RF OUTPUT IMPEDANCE:
DUMMY ANnNNA:
16 DB, + 2 7Y 1 k, for 100% modulation for
both output powers.
As supplied, 600 ohms, w.ich will also
serve to match 500 ohms satisfactorily.
Input may be connected for 150/250 ohms,
if desired.
+1.5 DB, 30 to 12,000 cycles.
Rated at 3% from 50 to 10,000 cycles,
at 95% modulation.
60 DB, or better, below 100% modulation.
37, or less, between 0 and 100%
modulation.
Jill match resistive loads from 50 to
70 ohms.
51.5 ohms, inbuilt
TUBES USED: 2) 12BY7A, Osc. & let IPA
6) 807, Audio & 2nd IPA
4) 833A, Power Amp. & Modulators
Silicon rectifiers are used in bias,
intermediate voltage and high voltage
supplies.

SECTION I
GENERAL DESCRIPTION
It is the purpose of this instruction book to thoroughly explain
in a clear, concise manner, the workings of the Gates BC- 1G Broad-
cast Transmitter, as well as installation and operational infor-
mation. The pictures show clearly all components within the
cabinet, these parts are adequately marked for easy reference
back to the parts list and to the written text. This instruction
book is a manual for installational information and for future
reference during servicing of the transmitter.
1.1 MECHANICAL CONSTRUCTION
The BC- 1G Transmitter is completely self-contained in one attrac-
tive steel cabinet, measuring 78 n high, 37° wide and 29" deep,
with a full front door with its " shadow moulding' covering practi-
cally the complete front. This door is hinged from the left side,
it requires 33" floor space to swing. Four large meters are loca-
ted on a panel mounted across the top of the cabinet. Most of the
controls are mounted behind and hidden by this door, the exception
being the filament start/stop, reset, plate off, low power and
high power combination switch and neon indicators. These switches
are mounted on the right hand cabinet corner post, protruding
through an opening in the door, when it is closed.
The heavy power components are mounted on the base of the cabinet.
The low powered audio and radio frequency stages are built upon a
"panel and shelf" assembly, along with the control circuitry and
the bias supply. Mounted on the shelf portion of this assembly
is the multi- winding filament transformer used to energize all
tube filaments in the silicon powered transmitter. ( If tube
rectifiers are used, two additional rectifier filament trans-
formers must be used). At the top of this panel assembly is
the four sets of filament connectors which secure the P.A. and
modulator 833A tubes. The two tubes to the front of the trans-
mitter, V40 and V41, are the RF amplifiers, the two toward the
rear, V42 and 11 1 1.3, are the modulators.
This complete " panel and shelf" assembly is hinged to the right
rear cabinet corner post, and held securely by three captive,
slotted head screws at the front corner post. This feature allows
this panel to be loosened, then swung inward on its hinges, to
provide access to the complete panel without removing the right
hand side of the cabinet. This is of great advantage if the
transmitter is located in a position necessitating other equip-
ment to be placed directly against the right hand side of the
-transmitter.
All tuning controls are available from the front of the trans-
mitter. ( Large front cabinet door must, of course, be opened).
An interlocked perforated metal screen is mounted over the front
opening of the transmitter, which gives the utmost physical pro-
tection to the operating personnel. This screen is easily re-
movable from the cabinet, allowing full access to the inside of
-1- BC- 1G

dUle cabinet from the front.
One exhaust fan is located above the 833A power tubes, in the top
of the cabinet, to draw the heated air up and out during operatior...
Two disposable air filters are located in the lower portion of the
back cabinet cover, through which cool air is drawn into the
transmitter.
The dummy antenna asselnbly is mounted on the left cabinet wall,
toward the top, as viewed from the front.
Both the back and right hand side of this transmitter cabinet is
removable for servicing, if required.
1.1.1 TRANSMITTER COUTROLS
All transmitter tuning controls are available from the front of
the transmitter. The small vertical panel, which is an integral
part of the " panel and shelf" assembly located on the right side
of the cabinet, behind the front door, has the following controls:
1. The crystal selector switch, Sl, and the two crystal
trimmer capaoitors, Cl and 02. ( The M5422 Crystal
Oscillator Unit is mounted directly behind this panel.
Its controls, Sl, Cl and 02, protrude through a small
aperture in this panel, and thus, are available from the
front).
2. The R2 driver tank tuning ca - oacitor, 04.
3. Multimeter switch, 32,
4. Modulator cathode current selector sitch, Si.
5. Modulator bias controls, R1 and R2.
The following controls are located on the right hand corner post
section of the cabinet:
1. Filament rheostat, R43.
2. Plate rheostat, R41.
3, Filament On/Off, 341. ( Red pushbutton).
4. Reset, 342. ( Red pushbutton),
5. i'late stop, 343. ( White pushbutton).
66 250 latt carrier, $ 44 6 ( Amber pushbutton).
7. 1000 ', Iatt carrier, 345. ( lhite pushbutton).
8. Local/Remote toggle switch, 340.
-2- BC- 1G

A complemeet of four- large ineters-are_mounted on - a panAl at - the
top of the cabinet. From left to right, they are:
1. Multimeter, M40
2. Modulator cathode current, M43.
3. P.A. plate current, M42.
4. P.A. plate volts, M41.
The line meter, M44, is mounted on the power amplifier panel,
and is visible when the front cabinet door is open. Also, on
this I.A. panel we find the power amplifier tuning control, L40,
the power amplifier load control, L42, and the neutralizing
adjustment, C40. This latter facility is a screw driver adjust-
ment made through a small opening in the panel, it is, of course,
a seldom manipulated control.
1.2 INCIDENTAL INFORMATION ( TUBE HANDLING, TRANSMITTER
MIDING TEMPERATURE, ETC.
It is well to mention several areas that make for better and
more profitable operation.
1.2.1 TUBE HANDLING
The Gates BC- 1G Transmitter uses 833A power tubes in the power
amplifier and modulator stages.
_These_are of the single wire or thread filament type, as com-
pared to other tubes which _ may have the filament ( heater) con-
tained in a tube, which is commonly called the cathode assembly.
Tubes having single wire, or thread type filaments, supported
by springs ( such as the 833A) require more than normal care in
handling. These filament wires are easily broken by sudden,
heavy vibration. At all times handle the tubes with care, until
they are safely inserted in the tube sockets of the transmitter.
At this point, more care must be exercised in this type of power
tube, as the filament prongs are also the means by which the
tubes are secured. Make sure the filament connections have some
"give" so that no undue strain is placed on the glass- to- metal
filament prongs. As the glass envelope will expand a bit during
operation, the two securing filament connectors must be free to
move themselves.
Take care when making the grid and plate connections to the tube,
do not put any undue strain on these connections during tube in-
stallation. Of course, the connections to the grid and plate
should be flexible to allow for expansion of the tube. For
shipping or storing it is advisable to use the packing material
and carton that the tube was shipped in from the tube manufacturer.
Following these reasonable precautions, there should be no trouble
in handling these tubes.
-3- BC- 1G

1.2.2 TRANSMITTER BUILDING TiMPIATURES,
If this transmitter is to be unattended ( operated by remote con-
trol) care should be taken that winter temperatures inside the
transmitter building do not go below 50°F. Mercury vapor tubes.
(if used) will arc back at low temperatures, often causing severe
damage either to themselves or other expensive components. Pro-
tective relays and fan motors may also become sluggish under
extremely cold conditions. Failure to provide adequate winter
minimum building temperatures will void the guarantee.
1.2.3 GROUYDriG
The grounding of the transmitter installation is of major import-
ance. Remember, it is a part of your radiating system. It can
be safely assumed that the better the complete ground system, the
more efficient will be the radiating system. A lack of complete
grounding of the transmitting and audio equipment may cause trouble
from stray RF getting into the audio, and may cause unstable trans-
mitter performance, etc. It is wise to bond all electrical con-
duit, water piping, metal building framework to the overall
ground system. If these suggestions aie followed, there will be
less trouble over the years, as the ground system ages.
1.2.4 ANT_NNA COUPLING
Antenna coupling equipment not involved in these instructions is
a very important part of the entire successful operation. The
instructions supplied with the antenna coupler will aid in its
adjustment. As all radiating towers must be measure& electric-
ally by an approved - engineer, he could check and advise on the
tune-up of the antenna coupler. If your operation is directional
the engineer will, of course, tune the entire directional system,
which includes the antenna coupling equipment.

SECTION II
THEORY OF OPERATIE
This section of the instruction book will include the theory of
operation of the M5422 Oscillator Unit when combined as an in-
tegral part of this 1000/250 watt Transmitter. A general des-
cription of the complete, overall transmitter operation will be
given.
2.1 M5422 OSCILLA . 20à UNIT
This oscillator unit physically is inches wide, including
mounting flanges, 6 inches high, and 6g inches deep, including
connector plug. The unit is mounted by means of its flanged
bottom on the aluminum vertical portion of the ° panel and shelf",
Its controls extend out through a cutout in the front vertical
panel. The oscillator shield cover held in place by one thumb
screw, can be re,.loved by unfastening and sliding horizontally
away from the oscillator chassis. Connections to the M5422
oscillator unit are made by a 8 position female plug, Pl, at the
rear.
2.1.1 TUBE COMPL2hENT OP TH2 M5422 OSCILLATOR UNIT
This oscillator unit uses a 12BY7A oscillator tube, V1, driving
another 12BY7A, V2, the first IPA.
2.1.2 TYPE OF OSCILLit20.'i CIRCUIT
The 12BY7A oscillator tube operates in a crystal controlled grid
plate circuit, also often referred to as a grounded plate Colpitts
circuit. Excitation is controlled by the proper ratio of tl/é two
capacitor values of C3 and 04.
2.1.3 TYPE OP CRYSTAL UD
This oscillator unit has facilities for two vadulim and glass
enclosed crystal assemblies, each crystal car{be selected for
use by means of the rotary switch, Si. ( One crystal is needed
for operation, the second, if used, would be a spare).
These crystals are mounted in octal based, glass envelopes which
have been pumped to a high vacuum. These plug into octal sockets
in the oscillator unit. The crystals are of the low temperature
co- efficient type, there is no need for crystal heater ovens for
normal operation.
Frequency trimmer capacitors, Cl and 02, are tunable from the
front - these capacitors are connected in shunt with the crystals
and afford a slight frequency adjustment which can be used during
initial tune-up. Also, ageing of the crjstals could cause a
slight frequency change during day to day operation. This change
can be compensated for by re- adjustment of these capacitors.
-5- BC- 1G

2.1,4 12BY7A FIRST IPA
This tuned first IPA stage is lightly capacitively coupled to the
oscillator. Its output circuit L3 and 09, is used on frequencies
from 1600 Kc to 800 Kc; from 800 Kc to 540 Kc a padder capacitor,
Cll, 100 mmfd. mica is connected in parallel with capacitor, C9,
The output of this stage is capacitively coupled to the grid cir-
cuit of the two 807 second IPA tubes, through C10 in the oscilla-
tor unit and C8 in the 807 stage. Adequate drive of from 2 to 5
ma., depending upon operating frequency is provided for the two
807's. Approximately 180 to 210 volts DC is applied to the oscil-
lator unit, being supplied by the 625 volt power supply through
dropping resistor, R5.
Drive voltage for operation of a Frequency Monitor, such as the
Gates 114990, is provided. The monitor drive output is obtained
from the plate circuit of the 1st IPA stage. A small coupling
capacitor, 012, is used.
2.1.5 M5422 OSCILLATOR TUNING ± ROCEDURE
The following tuning instructions should be followed when placing
the M5422 oscillator in operation. If this procedure is not
followed, it is possible to tune the oscillator to the second
harmonic of the crystal rather than the fundamental.
Information that follows was obtained with the M5422 oscillator
connected to its proper UP load and 30 feet of RG62/U cable con-
nected to the monitor terminal 6 with shield to ground, or
terminal P. RG62/U cable runs 13.5 mmfd. per foot, or a total
of approximately 400 mmfd. effective capacity on the 30 foot
lengths. Shorter lengths of cable on frequencies above 600 Kc
will effect the tuning of the unit. More tuning capacity ( 09)
or more turns of the slug in L3 may be required for resonance.
Shorter lengths of monitor cable on frequencies from 600 Kc to
540 Kc may prevent the unit from tuning to resonance. If this is
the case, capacity should be added across the cable to make up
the difference in effective capacity. Longer lengths of cable
would mean less capacity or less inductance needed for resonance
in this frequency range. It is recomended tnat the proper length
of RG62/U be used whenever possible.
Frequencies from 1600 Kc to 800 Kc
1. NO . ADDING needed in this frequency range,'
2. Make sure that slug of L3 is screwed all the
way out.
From 1600 Kc to approximately 1100 Kc, tune 09 for dip in plate
curr nt or peak in grid current of following stage.
If C9 does not tune through resonance, screw in slug on L3 a
turn at a time, until resonance is obtained with C9. 800 Mc is
tuned with 09 near maximum capacity and slug of L3 screwed in
7 turns.
-6- BC- 1G

If above procedure is not followed, it will be posatble for
crystals from approximately 900 hO to 800 4Qtia---turie to their
second harmonic, if slug in L3 has not - been screwed dawn to'
approximately 7 turns for 800 Kc.
Frequencies from 540 Kc to 800 Kc
1. The padder capacitor Cil, 100 mmfd. located on
bottom of L3 must be connected in the circuit.
2. The slug of L3 should be screwed down 14 turns.
Frequencies from 540 Kc to approximately 600 Kc can be resonated
with capacitor 09. If complete resonance cannot be obtained on
09, screw the slug of L3 back out a turn at a time until reso-
nance is obtained by turning 09. At 800 Zc resonance will be
with 09 near minimum capacity and the slug of L3 screwed out
approximately 7 turns from the starting point, 14 turns down.
CAUTION - If above procedure is not followed and
padder not connected, it will be possible
to tune crystals from 540 Kc to 800 Kc to
their second harmonic.
!‘fter resonance has been obtained, the crystal may be set to
oxact frequency by using the frequency monitor. Set the slots
of the trimmer capacitors, Cl and 02, located on the front of
the unit, at right an G les to the plane of the trimmer mounting
screws. qith the crystal selector switch turned to W1 crystal,
the frequency should be very close to zero; if not, adjust the
trimmer FREQ. j1 until frequency is zero or to point desired for
operation. Turn crystal selector switch to 2 position and re-
peat above.aperation with trimmer FREQ. 7,2.
The tuning of these condensers will not effect tile resonate tuning
of the unit and capacitor 09 will have very little, if any, effect
on the trimmer adjustments.
2.2 BC- 1G TRANSMITTR DEJCRIPTION
The following information will briefly describe this transmitter,
giving tube line-up and circuitry of the audio and RP sections
along with the various power supplies used.
2.2.1 TUBE LINE-UP
As mentioned previously the M5422 Oscillator Unit uses a 12BY7A
oscillator and a 12BY7A first IPA. This stage drives a pair of
807's second IPA, which in turn supplies tile driving power for
a pair of 833A tubes operating in parallel as the modulated
Class " C" power amplifier.
The audio system uses a pair of push-pull 807's. as the audio
input amplifier, these driving another pair of 807's operating
as a cathode follower stage which in turn drives the two Class " B"
833A modulator tubes, these tubes in turn high-level plate modu-
late the 833A's in the 2,i7 power aniplifier.
-7- BC- 1G

The bias supply - uses
si 1 trcm - rectifiers.
The intermediate voltage supply makes use of silicon rectifiers
in- a- full- wave center tapped configuration.
Silicon units are used in the high voltage, full- wave center
tapped rectifier.
2.2.2 BC- 1G TRAN,JMITTER CIRCUITRY
The BC- 1G transmitter uses the M5422 oscillator unit to drive the
two 807's operating in parallel as the RF driver stage.-- This
stage operates with approximately 600/625 volts on the plate of
the tubes, 400 volts on the screens, and 60 to 65 volts negative
on the grids. Forty-five ( 45) volts of this bias is fixed, being
supplied from the small bias power supply, this voltage is suffi-
cient to limit the plate dissipation to an allowable value in the
event that grid excitation is lost. In normal operation, the
cathode current of this 807 RF driver stage will run from 150 to
200 ma total for - both tubes, varyng so7levihat with operating fre-
quency and loading. This current is indicated on the multimeter
when the multimeter switch is set in the ° RF Driver Cath." posi-
tion. With this same selector switch set in the " RF Driver Grid"
position, grid current to the 807 ill' driver stage will be indi-
cated. This will be on the order of 2 to 5 mils.
The plate and screen voltages of the 807 RF driver are modulated
slightly, this feature tends to increase the UF drive to the modu-
lated power amplifier on peaks of the modulation cycle, this
improves the distortion figure of the transmitter.
The RF driver stage is capacitively tuned by the 250 imfd. vari-
able capacity, 04. Below 1150 Kc a padding capacitor must be
connected in parallel with C4.
The power amplifier of the transmitter uses two 833A tubes con-
nected in parallel. The output circuit of this PA stage can be
said to be made up of an " L.' and two itTn networks, which effec-
tively transform the operating tube impedance down to the 50/70
ohms found at the line terminal of the transmitter. This network
also does a commendable. job in reducing to a minimum the trans-
mission of harmonics which might be generated in the transmitter.
Power amplifier coils 140 and 142 are of the continuously variable
type and are used to tune the power amplifier to resonance, in the
case of 1,40, and to vary the loading by means of L42. Other than
the neutralizing capacitor 040, there are no variable air dielec-
tric capacitors used in the power amplifier of this transmitter.
This adds greatly to its
Grid drive to the amplifier should be at least 100 ma. for good
operation. This will be indicated by the multimeter when the
multimeter selector switch is in the " Power Amp. Grid" position.
Higher grid drive up to 150 ma. is acceptable, but this drive
will vary slightly, depending upon the transmitter frequency.
-8- BC- 1G

The-tranmitter - wfll match 50/70-chm-nhyl need loads, delivering
full power ou - 4ut with power amplifier plate efficiency of 70»
or better. Other load impedances are available on special order.
Audio wise, the Gates BC- 1G transmitter is novel in many respects.
The audio input/audio driver assembly is made up basically of
components mounted on a printed wiring board. Tais assembly is
. located on the panel and shelf section of the transmitter and in-
cludes the two 807 audio input tubes, the two 807 cathode follower
audio driver tubes, along with the balance coatrol, 113, condensers
and resistors for these two stages. The audio system is push-
pull in operation for all stages. The cathode follower audio
driver tubes, V3 and V4, are biased by voltage controlled by tàe
potentiometers, R2 and R1, located on the small aluminum front
panel. These controls indirectly adjust the operating bias on
the modulators by varying the operating constants of the cathode
followers, this causes a bias voltage change on the modulators by
having a voltage drop occur across the high resistance cathode
resistors, 1111 and 1112, of the cathode followers. A very smooth
modulator bias change can be attained in this manner, making it
possible to adjust the modulators for correct operating conditions.
There is no metering of the plate current of the 807 cathode
followers, V3 and V4, it is believed that if proper modulator
operation is had, then the 807 cathode followers are operating
satisfactorily.
High level Class " B" modulation is used in the BC- 1G, a pair of
833A tubes providing the means. The grids of the modulators are
excited by the two 807 cathode follower audio driver. The output
of the modulators is coupled to the Class ° C" amplifier by means
of the capacitor 045, and the reactor L47. The secondary of the
modulation transformer T41 does not curry any power amplifier DC.
Feedback from the plates of the modulators back to the audio input
tube grids has been provided. A small feedback ladder printed
wiring board is located on the panel and shelf assembly direct1y
above the modulation transformer, T41. By means of a resistor/
capacitor divider network out- of- phase voltage is fed back to
the audio input. The transmitter makes use of approximately 12
to 14 DB of feedback measured at 1000 cycles and 90% modulation.
This feedback helps to reduce the noise and also improves tae
distortion figures.
The power amplifier and the modulator plate circuits are protected
against abnormally high overload currents by means of relays, K6
and K7. These are located on the top shelf of the " panel and
shelf" assembly, adjacent to the multi- winding filament trans-
former, T3.
The overload relays, K6 and K7, have their coils shunted by 20
ohm semi- variable resistors. By adjustment of the slider tap.,
the relay pull- in point can be selected. These resistors have
been set at the factory for normal operation. K6, the modulator
overload will pull in at a modulator total - plate current of
approximately 600 Ma. ( Normal plate current for voice and music
programming, hitting 100 will be around 400 ma total).
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PA overload K7 is set for approximately 700 1.1a pull- in ( normal
PA plate current will range from 525 to 600 1, 1a, dependin g on
PA transmitter .efficiency). These relays may pull-in-prmatave
during sine wave audio modulation at the 100» level. In this event
the adjustments can be made to allow for this type of operation.
If the current in either circuit exceeds the value for which its
relay was set, the relay will energize, causing its normally
closed contacts to open, which in turn opens the coil circuit
of auxiliary relay, K9. This causes the contacts of relay K9
to. eat- urn to their normally open position; thus, opening the
coil circuit of the high or low power contactor ( whichever had
been in use), this reoves primary voltage from 240, the high
voltage power transformer.
2.3 POWER 3UPPIinS
The Gates BC- 1G, 1000/250 watt, Transmitter makes use of three
separate power supplies. These use full wave, C.T. rectifier
and filter assemblies. Each of the three silicon supplies used
in the transmitter will be fully described in the following
paragraphs.
2.3.1 BL'5 JUPPLY
This supply is made up of a plate transformer, Ti, working in
conjunction with the bias rectifier, a silicon rectifier consist-
ing of 10 diodes, 400 volts, PDT, filter choke Li, filter capaci-
tor, C3, and associated resistors and potentiometers. The bias
potentiometers, R1 and 112, indirectly vary the modulator bias by
controlling the cathode follower bias and, thus, the current flow
through the cathode follower resistors, Rll and 2.12. There is
applied a negative 280 volts between these resistors and ground.
An opposing voltage of approximately 210 volts is developed by
current flow through Rll and 1112; thus, putting the difference
(about 60 to 70 volts) on the grifa3 of the modulators. This bias
supply also supplies 45 volts of fixed bias to the two 807's in
the RF driver stage. " This voltage is obtained by a tap on bias
resistor, R12. This bias supply is energized at tue time that
the filament start button, 41, is depressed.
2.3.2 600/625 VOLT LOI VOL2,GE : 3UPILY
This supply uses 14 diode units of 600 volt PIV rating working
as a full wave C.2. rectifier, with a choke input filter system.
Choke 1,46 is rated at 10 hy., capacitor C47 is a 10 mfd unit.
This supply develops aparoximaely 600/625 volts which is applied
to the two 807 HF driver tubes. The same voltage is dropped to
around 575 volts through series resistor, 114, and applied to the
two audio stages. The 115422 oscillator unit derives its plate
potential from this same power supply, the voltage being dropped
to approximately 195 volts by means of series resistor, R5.
This supply has a time delay relay, K8, connected in its primary,
which delays the application of this low voltage for approximately
10 seconds after the filament voltage has been applied. This
-10- BC- 1G

supply also has its primary in series - with door interlock - switches
346 - and 348. If either the front protective screen or the buck
cabinet cover is not securely in place this supply will be in-
operative.
2.3.3 2800 VOLT 3- UPPLY
High voltage for the power amplifier and modulator is developed
by two silicon rectifier assemblies, each consisting of 30 diode
units of 600 volt PIV rating, working as a full wave C.T. recti-
fier. This supply is capable of delivering slightly over one
ampere DC. The main power transformer, T40, the filter choke,
L45, and filter capacitor, 048, are located in the bottom section
of the transmitter cabinet. This high voltage supply is inter-
locked with the front panel grill and the interlock switch, 347.
As mentioned previously, this transmitter has a metal grill work
covering the front of the unit, this protects the operating per-
sonnel from the dangerous high voltages which are present inside
the transmitter cabinet. The lower edge of the protective grill
is secured by two quick operating ON/OFF fasteners. when this
grill is in place, its lower edge operates the safety door inter-
lock switches, S46 and 347.
2.3.3.1 HIGH VOLTAGE SUPPLY OPERATION AT 250 UTT3 OUTPUT
The BC- 1G 1000/250 watt Transmitter can operate at 250 watts,
this is made possible by reducing the primary voltage applied to
the high voltage transformer, T40. For 1000 watt operation this
primary voltage is approximately 230 volts, for 250 watt carrier
output this primary voltage is dropped to 115 volts, this develops
around 1350 volts through the supply which is applied to the power
amplifier and modulators.
2.3.3.2 BC- 1G POLER CdA7GE FACILITY
The operation of the BC- 1G Transmitter at either 1000 watt or
250 watt carrier level is accomplished by the operation of two
power contactors, ( K2 or k3) and one auxiliary relay, K9. The
relay and contactor operating sequence is as follows:
For 250 watt carrier - Filament OFF/OH pushbutton switch, 341, is
depressed. All filaments are energized, and after another 10
seconds, time delay relay, K8 has closed. gith both front screen
and rear cabinet cover in place, the neon indicating lamps of
"Pilamentn switch 341, the red " Reset' button 342 and the white
"Plate Off" button 343 will be illuminated. A check of the multi.
meter switch positions will show all multimetered circuits indi-
cating correctly, the multimeter switch can be left in the P.A.
grid current position. Relaj, K9, must now be locked in, this
action will provide a 230 volt AC source for either high power
contactor, K2, or low power contactor, K3. Press the red " Reset
button 342; this operation will comiJ.ete the auxiliary relay, K9,
coil circuit, causing it to lock in. A pair of normally open
contacts A and B close, this makes 230 volts AC available for
contactors K2 or K3. rhen the red " Reset" button was pressed,
the neon lamp indication of both the " Reset" and the " Plate Off'
-11- BC- 1G