Stromberg-Carlson 255 Engineering Data

Engineering Data
Stromberg-Carlson No. 255 Radio Receivers
STROMBERG-CARLSON TELEPHONE MANUFACTURING COMPANY
Rochester, New York
ELECTRICAL SPECIFICATIONS
Type of Circuit_
Tuning Ranges_
Number and Types of Tubes-
Input Voltage Rating_
Power Frequency Rating-
Input Power Rating_
Frequency of Intermediate Amplifier
_Superheterodyne with Automatic Frequency Control
A—530 to 1600 Kc.; B—1600 to 4800 Kc.; C—4800 to 11,000 Kc.
D—11,000 to 22,000 Kc.; E—22.000 to 60,000 Kc.
2 No. 6K7,1 No. 6A8, 2 No. 6J7,1 No. 6B8,1 No. 6H6,1 No. 6F5
1 No. 6C5, 2 No. 6L6,1 No. 6G5,1 No. 5U4G
_105 to 125 Volts A. C.
_25 to 60 Cycles and 50 to 60 Cycles
_145 Watts
_465 Kilocycles
APPARATUS SPECIFICATIONS
No. 255-L _50 to 60 Cycles; P-27633 Chassis; P-27504 Loud Speaker
No. 255-LB_25 to 60 Cycles; P-27634 Chassis; P-27504 Loud Speaker
CIRCUIT DESCRIPTION
The No. 255 Receiver is a 13 tube "Adjustable High Fidelity" receiver and is equipped with an Automatic
Frequency Control (A. F. C.) Flash Tuning Circuit. This circuit is operative only in the standard broadcast
range, and the mechanical arrangement is such that seven favorite broadcast stations may be set up on the
A. F. C. Flash Tuning Unit (local and other stations that give the best daytime and evening service should be
selected). There are five tuning ranges in this receiver. The Short-Wave Foreign spectrum is divided into two
ranges in this receiver; this arrangement gives "spread-band" tuning which enables the operator to tune the re¬
ceiver to stations located in these ranges with far greater ease.
With the A. F. C. knob (located on the front of the panel, rotated to the "On" position, any one of these
favorite stations may be easily selected; as the station selector knob is rotated, each lamp located behind one of
the call letters of the station setup in the Flash Tuner Unit will flash on whenever the receiver is tuned to the
frequency of one of these stations. Between any of these stations no noise will be heard from the receiver while
operating the Flash Tuning System. To properly set-up the A. F. C. Flash Tuning System read the section, "In¬
structions for Setting Up the A. F. C. Flash Tuning System" appearing on page 7 of these instructions.
Maximum selectivity between adjacent stations located in the Standard Broadcast Range is obtained by
the use of an additional tuned radio frequency ("Bi-resonator")*'circuit. When any of the other ranges are in
operation, this additional tuned circuit is cut out of the receiver circuit. When either the Aircraft or Short
Wave ranges are in operation the receiver functions as a three gang tuning capacitor receiver and when the
Ultra-Short Wave range is in operation the receiver functions as a two gang tuning capacitor receiver. When
reception conditions warrant, the fidelity of this receiver can be increased by rotating the "Tone-Fidelity"
switch control knob in a clockwise rotation from the normal position of this control. High fidelity reproduction
is obtained in two steps from the normal position of this control. The maximum undistorted power output of
this receiver is 15 watts when working into the loud speaker supplied with this receiver (P-27504 Loud Speaker).
This speaker has a voice coil impedance of 1% ohms and its field coil resistance, when cold, is 505 ohms.
These receivers are also equipped with the Stromberg-Carlson Acoustical Labyrinth, a device which ex¬
tends the bass response and provides reproduction only from the front of the cabinet, eliminating all cabinet
resonance. Sound diffusing vanes are also employed in front of the loud speaker opening which distribute the
higher pitched tones, thereby providing excellent reproduction in all parts of the room by spreading out these
directional frequencies. These receivers are also provided with a low level bass frequency compensating circuit,
which in conjunction with the volume control circuit, operates so that balanced reproduction is obtained for any
setting of the volume control. A "Bass" control is also provided to increase the response at the lower (bass)
audio frequencies if desired when operating the receiver at medium or low volume levels.
A metal guard frame is furnished on these receivers to prevent damage to the chassis components and also to
facilitate ease of servicing should this become necessary. Do not turn the chassis over on its guard frame with¬
out first removing the tuning indicator unit which is secured to the metal guard frame. To remove the tuning indi¬
cator unit from the guard frame, first unscrew the knurled screw which holds the tuning indicator's clamp to
the metal guard frame, which will then allow the tuning indicator unit to be removed from the guard frame.
The various tubes are used in these receivers as follows: One No. 6K7 tube is used in the R. F. Amplifier,
and the other No. 6K7 tube is used in the I. F. Amplifier. The No. 6A8 tube is used as a Demodulator tube. One
No. 6J7 tube is used as the Oscillator tube, and the other No. 6J7 tube is used in the Oscillator Control Circuit for
Automatic Frequency Control. The No. 6B8 tube is used as an I. F. Amplifier, Demodulator, and A. V. C. tube.
The No. 6H6 tube is the Discriminator tube for the Automatic Frequency Control circuit. The No. 6F5 tube is
used as the Audio Amplifier tube and the No. 6C5 tube is used as the Phase Inverter tube of the Audio Amplifier.
The two No. 6L6 tubes are used in the Audio Power Output stage. The No. 5U4G tube is the Rectifier tube of the
Power Supply Unit, and the No. 6G5 tube is used for indicating resonance in the Tuning Indicator System,
P-28458 Form 2229 Issue 1 Printed in U. S. A.