Radio Broadcast (May 1928-Apr 1929)

SEPTEMBER, 1928 A SIX-TUBE SCREEN-GRID RECEIVER 283 BP3, and the hole for the battery cable. To the center rear of the chassis is the audio amplifier consisting of two transformers, T, and T2, and two tubes, S7 and Ss, with space left either for two large type transformers, or an output transformer in addition to the small types specified. To the right front is the three gang die-cast condenser, Q, Q, and C4, tuning the three shielded r.f. circuits housed in the copper shields, SHi, SH2, SH3, just behind this condenser. The two condenser assemblies are tuned by the drum dials, Di, and D2, visible through the windows of the front control escutcheon. Below the vernier knobs of the dials are, to the right, the volume control potentiometer, Ri, to which is attached the on-off switch, S\V, and to the left is the selectivity condenser, C5, in series with the antenna. The positions and uses of the various bypass condensers and resistors are evident from a study of Fig. 7. Just behind one of the copper stage shields is located a rheostat, R6, used to compensate variations in A-battery voltage. It is intended to be adjusted by a screw-driver. The receiver requires for operation three cx322 tubes, one CX-301A (or preferably cx-H2a) detector tube and one CX-301A (or preferably cx-i I2-a) first a.f. amplifier tube. Any power output tube such as cx-ii2a, CX-371A, cx-310, or cx-350 may be used, provided suitable A and B supply is available. In the circuit shown, a CX-112A or CX-371.A output tube may be used at will, though with the latter the addition of an output transformer is desirable to protect speaker windings. Either batteries or standard light socket power units may be used to operate the receiver. The plate current consumption is about 30 mA with 1 12A output tube, or 40 mA with 371 output tube. Hence a B-power device should prove more economical for operating the receiver, especially if a 371 tube is used. Some may want to operate the set from the light socket through the use of a.c. tubes. In such cases heater type a.c. screen-grid tubes should be used in the r.f. stages and ordinary three electrode heater type 227 tubes in the detector and first audio stages. The output tube may be any type, with its filament operated from a.c. Complete circuit diagrams for a.c. operation can be obtained though Radio Broadcast or directly from the manufacturer. For a.c. operation some changes in the circuit and parts required for the construction of the receiver will be necessary. FIG. 6. UNDER THE SUB-PANEL LIST OF PARTS THE complete kit for this receiver may be ob■*• tained from Silver-Marshall, Inc., including a drilled chassis and all the necessary hardware. The special parts used in the circuit are the antenna coil, Li, the r.f. transformers, L2, L3, and L4, and the audio transformers, Ti and T2. The specifications of the antenna and r.f. coils are given in the article, and may be made at home. It is not advisable to substitute for the audio transformers, since the advantages of the Clough system of audio amplification, as described in the article, will be lost. The remainder of the parts in the list below are of standard design, and other parts electrically and mechanically equivalent may be substituted. B — 1 S-.M Universal pierced base chassis, No. 701 E — 1 S-M dual control escutcheon, No. 809 Di — 1 S-M vernier drum dial (left). No. 806L D2 — 1 S-M vernier drum dial (right), No. 806R Ci — 1 S-M 0.00035-rhfd. Universal condenser, No. 320R Q, C3, Ci — 1 S-M 0.00035-mfd. 3-gang condenser, No. 323 Q, — 1 S-M 0.000075-mfd. midget condenser, No. 342 B Ri — 1 Yaxley 3,000-ohm midget potentiometer, type 53000 S\V — 1 Yaxley switch attachment, type 500 Ji, J2, — 2 Yaxley insulated tip jacks, type 420 Ri, R>, R3 — 3 Carter lo-ohm resistors type RU10 R5 — 1 Carter ii-ohm resistor, type Hii Re — I Carter 6-ohm sub-base rheostat, type A6 C6 — 1 Potter i-mfd. bypass condenser, No. 104 C7, C8, C9, C10, Cu, Ci2— -6 !-mfd. midget bypass condensers C13 — I 0.00015-mfd. grid condenser with clips Cu — 1 0.0002-mfd. bypass condenser R-, — 1 2-megohm grid leak Rs — 1 Durham 0.15-megohm resistor with leads SHi, SH2, SH3 — 3 S-M copper stage shields, No. 638 Li — 1 S-M. antenna coil, No. 140 L2, L3, Li — 3 S-M plug-in r.f. transformers, No. 132A Si, S>, S3, — 3 S-M 5-prong tube sockets, No. 512 S4, S3, S6, ST, Ss — 5 S-M tube sockets. No. 5 1 1 Sg — 1 Naald cushioned tube socket, type 481 XS Ti — 1 S-M first stage a.f. transformer, No. 255 T2 — 1 S-M second stage a.f. transformer, No. 256 BPi, BP2, BP3 — 3 Moulded binding posts consisting of 3S2 screw, nut, and moulded top. 1 S-M 708 8-Iead, 5-foot connection cable 1 — S-M 818 hook-up wire (25 ft. to carton) 1 Set hardware (obtainable from Silver-Marshall) Note: Condenser C5, which is unlettered in the above diagram, appears to right oj binding post BPt. 0 FIG. 7. THE CIRCUIT DIAGRAM This circuit uses a combined on-off switch and volume control, and when using dry B batteries, about 12 mA, flows with the switch off. Hence it is necessary with B-battery operation to place a single-pole switch in the negative B lead. 6 AB-C + BlackGreen o 45 to 135 BlackRed 6 -t-135 C-Amp Maroon Green o » 180 Red