Radio Broadcast (May 1928-Apr 1929)

RADIO BROADCAST Schematic diagram of the new S-M. sixtube a. c. -operated screen-grid receiver 6 +45 RedMaroon in, r 2.25 V.A.C. +45 to 135 BGreen and Black BlackBlack Red Green + 135 Maroon Red aid in obtaining maximum results in the way of sensitivity and selectivity. Actual measurements on a single r.f. stage show a gain of about 14 at 550 kc. and about 28 at 1500 kc. Each r.f. stage employs one 224-type a.c. screen-grid tube, together with an r.f. transformer consisting of a secondary of 98f turns of No. 29 enamelled wire wound upon a threaded mould ed-bakelite form \\" in diameter and 1|" long, with a primary consisting of 35 turns of No. 38 enamelled wire upon a \\" tube located at the filament end of the secondary. From the antenna coupling system a voltage gain of about 60 is obtained. From the picture, the r.f. amplifier section is seen to consist of a large antenna coil tuned by a single .00035-mfd. condenser and provided with a tapped primary and with a 75mmfd. antenna series condenser for selectivity control. This coupler feeds the grid circuit of the first screen-grid r.f. amplifier which, in turn, feeds into three almost identical shielded r.f. circuits, each housed in a small copper can. Each of these stages employs the small r.f. transformer described above which is tuned by a section (equipped with individual compensator) of the three-gang die-cast condenser. In the two left-hand shields are the second and third screen-grid r.f. amplifiers, and in the righthand shield the 227-type detector. By-pass condensers are contained in each stage shield to localize r.f. current paths. Volume control for the receiver is affected by means of a 3000-ohm potentiometer arranged to control the screen-grid potential of the three 224-type r.f. amplifier tubes. The heaters of screen-grid and detector tubes are operated in parallel and are fed from a 2.5-volt winding of the transformer contained in the power supply for the receiver, and which furnishes A, B, and C power to the entire set. The audio amplifier employs the wellknown Clough audio system in two stages with a 227-type tube in the first stage, and a 245-type tube in the output stage. In its frequency-versus-amplification curve, as given on this page, it will be seen that it is extremely satisfactory over the frequency range involved in reproduction of music and speech. 200 z100 o P 60 | 40 . — i Q_ I 20 30 50 100 500 1000 FREQUENCY, C.P.S. Fig. 2 5,000 10,000/ No output transformer is supplied in the receiver, though space is left for the inclusion of such a device. This omission is justified on the ground that the builder will generally employ one of the better types of dynamic loud speakers, and such loud speakers are ordinarily equipped with output transformers. Should this not be the case, and should a magnetic loud speaker, or other type not equipped with output transformer, be used with the set, an output-coupling device, such as a transformer or choke and condenser filter, must be connected between set and loud speaker to prevent the high plate current of the 245-type power tube from damaging the loud-speaker windings. The undistorted power output of 1.6 watts is sufficient to provide adequate fidelity and sufficient volume. Inasmuch as complete constructional data for this receiver may be had from the manufacturer offering the kit, space will not be taken to present it here. Constructional pamphlets may be had upon application directly to this magazine. The parts required for the construction of the receiver are as follows: List of Paris Ci One S-M condenser, 0. 00035-mfd., type 320r; C2, Ca, C4 One S-M three-gang condenser, 0. 00035-mfd., type 323 ; Cs One S-M midget condenser, 0.000075-mfd., type 342 b; C6, Cis Two Potter condensers, 1-mfd.. type 104; C7-C12, Cic Seven Sprague condensers, 0.25-mfd.; C13, One Polymet grid condenser, 0.00015-mfd.; C14 One Polymet by-pass condenser, 0.002-mfd.; Li One S-M antenna coil, type 140; L2, L3, Li Three S-M plug-in r.f. transformers, type 132a; Si S7, S9 Eight S-M tube sockets, five-prong, type 512; Ss One S-M tube socket, four-prong, type 511; Ti One S-M a.f. transformer, first-stage, type 255; T2 One S-M a.f. transformer, second-stage, type 256; Ji, J2 Two Yaxley tip jacks, insulated, type 420; Ri One Yaxley midget potentiometer, 3000-ohm, type 53000; P12 One Yaxley resistor, 150-ohm; R.6 One Carter sub-base rheostat, type a6; F17 One Polymet grid leak, 2-megohm; Rs One Durham resistor, 0.15-megohm; R9 One Yaxley resistor, 1500-ohm; Rio One Yaxley resistor, center-tapped, type 840c; R11 One Ohmite resistor, 1500-ohm; One S-M universal pierced chassis, type 701 ; One S-M dual-control escutcheon, type 809; Two S-M vernier drum dials (one left and one right), type 806; Three S-M copper stage shields, type 132a; Three moulded binding posts; Miscellaneous hardware, battery cable, hook-up wire, etc. Chassis views of the S-M Screen-Grid Six receiver with and without sliields in place • april, 1929 . . . page 410 «