Western electric 61 a schematic

Page 1

Wemm Hecn-ic Company
No.6 | -A AMPLIFIER

No.247-A ND 2477A
VACUUM TUBE VDCULMAYUBE

vb Vc,

PLATE CURRENT 62 PLATE CURRENT
JACK.TUBE Vl \J \leACKIu'BE v2

GAIN CONTROL KEV

AMPLIFIER
No. 61-A

[nitrmfz'om for Use

The No. 61-A Amplifier is primarily designed for use in Western Electric
Speech Input Equipments for radio telephone broadcasting or similar service.
It may be used in tandem with, and ahead of, a No. 60-A Amplifier where more
gain is required than can be obtained from the No. (ifi-A Amplifier alone. It
may also be used as a booster amplifier at points where amplification with low
output levels is desired.

The No. 61-A Amplifier shown above and in Figure 1 is a two stage trans-
former coupled vacuum tube amplifier and has a maximum gain of approxi-
mately 58 db. The gain can be reduced 25 db by means of a two position, key
operated, potentiometer. A No. 247-A Vacuum Tube is used in each stage. The
No. 61-A Amplifier is designed to operate between impedances of 200 ohms.

The No. 61-A Amplifier requires a filament supply of 3.2 amperes at 2 volts
and a plate supply of approximately 5 milliamperes at 350 volts. The grid bias
for the vacuum tubes is obtained from the potential drop across resistances
located in the cathode circuits.

The component parts of the No. 61-A Amplifier are mounted on a panel
19 inches wide and 51/4. inches high designed for relay rack mounting. The
vacuum tubes are mounted in recesses in the panel and together with the jacks
and gain control key are accessible from the front. The other apparatus is
mounted on the rear of the panel and is protected by a removable dust cover.

747,

Page 2

Western Hecnic Company
No.6 I -A AMPLIFIER

SECOND STAGE PLATE CIRCUIT?
BYF'ASS CONDENSER

SECOND STAGE PLATE CIRCUIT(RIOS

FIRST STAGE

COND STACE INRUT rcz LATE C'RCU'T

AMPLIFICATION
R2 CONTROL
RESISTANCE
I
I

SE _
FILTER RESISTANCE @ TRANsFORMER COBNDFEfilSEISER
SECOND STAGE VACUUM H.251 STAGE
TUBE SOCKET VACUUM FIRST STAGE
INPUT
OUTPUT TRANSFORMER

TRANSFORMER . I

SECOND STAGE
GRID CIRCUIT
FILTER RESISTANCE

SECOND STAGE
GRID BIAS
RESISTANC E

SECOND STAGE PLATE CIRCUIT('C'§)
BLOCKING CONDENSER
TERMINAL
SECOND STAGE GRID <.IRCIJIT(C4J . STHIF
5' PASS CONDENSER C, FIRST STAGE GRID CIRCUIT
SECOND STAGE FLATE (fig) BVTPAss CONDENSER
FEED RESISTANCE I I @FIRST STACE FLATE CIRCUIT

FILTER RESISTANCE
FIRST STAGE GRID CIRCUITO_I R3

FILTER RESISTANCE
FIRST STAGE GRID/SJ

FIRST STAGE PLATE

, FEED RESISTANCE
FIRST STAGE PLATE CIRCUIT
5' RES'STNCEI L} BLOCKING CONDENSER
AMPLIFICATION

CONTROL RESISTANCEIEQ

Fig. leNo. 61-A Amplifier-Rear Vicit'

DESCRIPTION

The schematic circuit diagram of the N0. 61-A Amplifier is shown in Figure
2 and the wiring diagram in Figure 3. The signal is applied to the grid of the
first stage vacuum tube V1 through the input transformer T1, which is designed
to work from a ZOO-ohm circuit directly into the grid of a vacuum tube. The
first stage is coupled to the second through the interstage transformer T2. The
plate current is supplied to the vacuum tube V1 through the resistance BS The
condenser C3 located in the circuit between the primary of the transformer and
the cathode of the vacuum tube prevents direct current from flowing through
the transformer winding.

The secondary of the transformer T2 is connected to the grid of the second
stage vacuum tube V2. The key operated gain control potentiometer is con-
nected across the secondary of this transformer and allows the gain of the
amplifier to be reduced by 25 db. This potentiometer consists of the resistances
R1 and R2 and is operated by the key K1. When the key is in the HIGH posi-
tion, terminal 4 of the transformer is connected through the contacts of the key
to the grid of the vacuum tube and the potentiometer is removed from the cir-
cuit. When the key is in the Low position, terminal 4 of the transformer is
connected through the contacts of the key to the free end of the potentiometer

48