Radio Broadcast (May 1928-Apr 1929)

204 RADIO BROADCAST AUGUST. 1928 FIG. 2 brass and steel. This feature of quiet bearings may be possessed by other types on the market as well. In building this type of set, the parts should be placed just about as shown, so that a short direct ground line can be run in from the antenna choke, along the back of the variable condensers and over to the a.f. transformer frames, which must be grounded. All wiring should be short, direct and well soldered, and care should be taken to avoid the possibility of "closed loops" of wiring which would pick up energy and possibly cause irregular regeneration control. The apparently unnecessary bypass condensers, as across the a.f. C battery, C2, and from the second a.f. tube plate to ground, C6, should be used; their purpose is to cut stray r.f. currents from the audio amplifier, all in the interest of smooth operation. The parts used in the model, which was rebuilt several times to make sure that it would go together the same way and with the same results in spite of the minor variations bound to occur in home assembly, are listed at the end of the article. The panel is not a necessity, and can be left off to cut cost, if appearance is not an object. Everything else is quite important to smooth performance, though fixed condensers, sockets, and such parts might be substituted if on hand. Unless you are only interested in code reception, do not substitute for r.f. chokes, coil form size and variable condensers unless willing to "smooth up" your own particular set's operation by the "cut and try" scheme of adding bypass condensers, r.f. chokes, and resistors at needed points. The coils are all wound on the same type of Silver-Marshall form, with No. 34 d. c. c. wire for the ticklers, and No. 22 enamelled wire for the secondaries (except the 104.0-204.5 meters coil, which used No. 24 d. c. c.) All secondaries have turns so spaced that the windings cover the full 1 5" of form space. The windings are so connected that the top or start of the secondary terminates in the G post of a standard 5-prong tube socket and the bottom or end in the right hand F post (the F post nearest the P or plate post). The slotted tickler, wound in the same direction, starts at the "F" post nearest the "C" or cathode post and ends at the "P" post. The number of turns necessary to cover the four bands from 17 to 240 meters are given below, using a 0.00014-mfd. condenser and a 0.00035 mfd. tickler condenser. COIL DATA M 'avelength Type Range Secondary TicMcr Meters "T" 17 5—32 . 1 65 turns 5! turns "U" 3° 7—59.0 1 35 turns 5J turns "V" 57 2 — 110.0 255 turns 9j turns 'W" 104 0 — 204 5 492 turns 1 5 j turns OPERATION THE tuning curves for a particular set of four coils are given (Fig. 3) as an aid in finding stations when the set is first operated, and it will be seen that the amateur wave bands fall well away from the ends of the condenser scale, so that with good vernier dials no difficulty is had in tuning amateur code signals. To duplicate the curves given, it may be necessay to trim coils a bit once they are wound, but this is easily done, or coils simply rewound on the small Bakelite forms. Coils of fewer or greater numbers of turns for other wave bands can be quickly wound for the "tube base four" as the set might well be named. The operation of the set is simple, almost any antenna from fifteen to fifty feet giving quite good results; even a long broadcast antenna does not seem to destroy the sweet control of the set. Any good storage battery, nine volts of C battery and 135 volts of B battery (or as low as 90 will do) are all that is necessary for operating power. Socket-power units are generally noisy on short waves and are not to be recommended. If a Bsupply unit is used, a 45-volt battery should be supplied for the detector plate voltage. This will cut down the noise appreciably. Two 112A audio tubes, a 201 a detector, a 222 screen grid r.f. tube, and phones or loud speaker are also necessary. LIST OF PARTS THE coils are the only special parts employed in this receiver, and the data for them is given above. Parts of similar characteristics may be substituted for all the other apparatus mentioned in the list below. Ci — 1 Condenser, .00015 rnfd. C-2 — 2 Condensers, \ mfd. C3 — 1 Sangamo condenser, .005-mfd. C4 — 1 .S-M condenser, .00014 mfd., type 317 C5 — 1 S-M condenser, .00035 mfd., type 3 1 6-A C6 — 1 Condenser, .002 mfd. Li, L3 — 2 S-M short-wave chokes, No. 227 Li — 1 S-M short-wave choke, No. 275 Ri — 2 Yaxley resistors, 10 ohms R2 — 1 Yaxley midget rheostat with switch, 20 ohms R3 ■ — 1 Lynch resistor, 5 to 10 megohms Pm — 1 Lynch resistor, jV to J megohm R5 — 1 Yaxley resistor, 2 ohms T — 2 S-M audio transformers, type 240 2 National vernier dials 4 S-M blank coil forms, type 130 4 S-M tube sockets, type 411 1 S-M five-prong socket, type 512 2 Lynch resistor mounts 9 Fahnestock clips 1 8" x 17" x \" wood base 1 7" x 18" x |" micarta panel Screws, nuts, hook-up wire, solder, lugs, etc. If factory wound coils are preferred instead of winding coils at home, one each of SilverMarshall 131 T, 131 V, 131 U and 131 W coils may be obtained. The amount of wire needed for a set of home-made coils is given here. 2 lb. No. 22 plain enameled wire I lb. No. 24 double cotton covered wire 3 lb. No. 34 double cotton covered wire T U V W 0 10 20 30 40 50 .60 70 80 90 101) DIAL DIVISIONS FIG. 3 These tuning curves were obtained with coils mentioned in the lists of parts. Home-made coils can he made which will give similar results