Radio Broadcast (May 1929-Apr 1930)

RADIO BROADCAST Fig. 4 of the two plate coils which tends to keep radio-frequency currents out of the B-battery supply circuits. Three-element tubes may be used in the radio-frequency stage with a fair degree of success and Fig. 6 shows the circuit for standard three-element tubes. C3 and C4 are neutralizing condensers. In order to obtain the greatest amplification possible it is essential that the external plate-circuit impedance be kept high. For this reason, greatest efficiency may be obtained by using a tuned plate circuit as shown in the dotted fines of Fig. 5. In this case L3 should be a center-tapped coil tuned by C10 and C4 in parallel, thus making possible a high-impedance load in the plate circuit. The effective resistance at resonance of this W2 L2 combination, —5 — > may be much higher than Hi the impedance of standard r.f. chokes which may be used (untuned) in place of L4 The radio-frequency amplifier couples to the detector circuit through two 25-mmfd. condensers, C5 and C6. These capacities are made small to prevent reaction of the amplifier on the oscillating detector circuit. The detector circuit is similar to the radiofrequency circuit. The potentiometer, R7, is 100,000 ohms and serves to control oscillations by varying the voltage on the plates of the detector tubes. It is bypassed by a 2-mfd. condenser which prevents noise from irregular contacts as experienced with some volumecontrol potentiometers. For ruggedness, selectivity, and elimination of vibration and microphonic noises, the receiver box should be built with heavy aluminum shields. If the proper care is taken microphonic noises, due to mechanical vibration, will be eliminated. Rigid bakelite tube sockets should be used rather than the cushion type. The receiver should be mounted upon a shock-absorbing pad such as a sponge rubber pad about one inch in thickness. The plate circuit of the radio-frequency amplifier may be shielded easily by arranging the two tubes so that each plate connection is near the shield and as short as possible. It will, therefore, not be necessary to use copper cylinders for each tube. If the proper care has been taken in mounting and wiring all the parts, the receiver will have a very low noise level, much lower than found on conventional types of receivers where the detector circuit is connected directly to the antenna system. The condenser C4 is 25 mmfd. which is sufficient to take care of the difference in tuning of the condensers C10 and C12 which may be operated on the same shaft. Construction of Coils COILS for this receiver may be made on Silver-Marshall plug-in forms. The following data are only approximate, but furnish a good starting point. Band Ant. Grid r.f. Plate r.f. Grid det. Plate det. (Meters) (L) (Li) (La) (L4) (L5) 10 3 4 4* 3 4 20 3 8 4 6 6 30 4 15 6 12 6 40 4 17 7 14 6 *One half inch in diameter. Cut off top of coil form so that leads will not be too long. In winding the coils, No. 16 enamel -covered wire should he used for grid coils and No. 22 enamel-covered wire for plate and tickler coils. The two small coupling condensers, C5 and C6, between amplifier and detector may be made from small metal plates, about the size of dimes, and should be arranged so that the capacity of each may be varied slightly. If the a.f. transformers are ungrounded, an unpleasant squeal may be heard in the phones. This may be eliminated by connecting an 0.05mfd. condenser between grid of the last stage and the transformer frame. This will not reduce the signal strength. List of Parts THE following is a complete list of the apparatus employed by the writer in the construction of this receiver. Other parts Fig. 6 may, of course, be substituted with discretion if the parts listed below cannot be obtained. Ci, C12 Two Hammarlund condensers, 11-plate; C>, C3, C8, C9 Four Dubilier condensers, 100-mmfd.; C4 One Hammarlund Junior 9-plate condenser, 25mmfd. ; Cs, Cs Two condensers, 25-mmfd. maximum. (See construction notes) ; C7, Cn Two Parvolt by-pass condensers, 1.0-mfd.; C10 One Hammarlund 7 -plate condenser (not used if Ls is an r.f. choke) ; C13 One Parvolt by-pass condenser, 2.0-mfd.; L, Li, L2, L3, L4, L5 Six coils (see coil table for data); L6, L7 Two Samson r.f. chokes, 125-mh.; Ri, R2 Two Durham grid leaks, 1.0-megohm; Rj, R4 Two Durham grid leaks, 0.5-megohm; Rs One rheostat or fixed resistance, 25-ohm; R6 One rheostat or fixed resistance, 6-ohm; R7 One Carter volume control, 100,000-ohms; Rs One Yaxley potentiometer, 200-ohm; Six Benjamin sockets, four-prong; Three Benjamin sockets, five-prong; Two Thordarson transformers, R200; One Jewell Voltmeter, Pattern-135: One on-off switch; Two National dials. Fig. .5 — Complete schematic diagram of the author's short-wave receiver. SOUND-MOVIE MANUFACTURERS OF INTEREST to workers in the sound motion picture field is the following list of practically all of the prominent manufacturers making acoustic apparatus for use in sound-movie installations: LOUD SPEAKERS Silver-Marshall — Dynamic cone — Silver-Marshall, Inc.. 874 W. Jackson Blvd., Chicago, III. Racon — Exponential horn — Racon Electric Co., Inc 18-24 Washington Place, N. Y. C. Jensen — Dynamic cone — Jensen Radio Mfg. Co., 338 N. Kedzie Ave., Chicago, 111. Peerless — Dynamic cone — United Radio Corp., Rochester, N. Y. DISC REPRODUCER INSTALLATIONS RCA Photophone, Inc. — 411 Fifth Ave., N. Y. C. (Dynamic cones, synchronous and non-synchronous turntables, amplifiers.) Vitaphone — Electrical Research Products, Inc., 250 West 57th Street, N. Y. (Dynamic horn, synchronous and non-synchronous turntables, amplifiers). Duotone — Western Electric Piano Co., 850 Blackhawk St., Chicago, 111. (Dynamic horn, amplifier, nonsynchronous turntables). Motio-Tone — Gates Radio and Supply Co., Quincy. Ind., (Dynamic cone, or horn, turntables, amplifier) Oganovox — National Sound Reproducing Co., 653 Clinton St., Milwaukee, Wise. (Non-synchronous turntable, amplifier, dynamic loud speakers). Good-All Orchestrola — Good-All Electric Mfg. Co., Ogallala, Nebraska. (Dynamic horn, amplifier, nonsynchronous turntables). Duplex-O-Phone — Nelson-Wiggen Piano Co., 1731-45 Belmont Ave., Chicago, 111. (Amplifier and turntables only, for dynamic reproducers). Han-A-Phone — Hanaphone Co. of America — 6010 38th Avenue, Woodside, L. I. Souora — Bristolphone, 50 West 57th Street, N. Y. C. Pacent Beproducer Systems — Pacent Reproducer Corp., 250 West 39th St. N. Y. C. (Dynamic cone, synchronous turntables, amplifiers). Cinetone — S and S Enterprises, 46 Church St., Boston, Mass. (Dynamic cone, non-synchronous turntables, amplifier). Phototnne — Phototone Co.. North Vernon, Indiana. (Cone, synchronous and non-synchronous turntables, amplifier). SOUND— ON— FILM INSTALLATIONS RCA Photophone Inc.— 411 Fifth Avenue, N. Y. C. (Dynamic cones, amplifiers, sound reproducer equipment). Movietone — Electrical Research Products Inc. (Western Electric) 250 West 57th Street. N. Y. C. (Dynamic horns, amplifiers, sound reproducer equipment). De Forest Phonolilm — General Talking Pictures Corp., 218 West. 42d St.. N. Y. C. (Loud speakers, amplifiers, souud-reproducer equipment). • may, 1929 page 14 •