Radio Broadcast (May 1928-Apr 1929)

OCTOBER, 1928 A NINE-TUBE SCREEN-GRID SUPER 335 Cable W FIG. 2. THE SCHEMATIC DIAGRAM tubes. A similar potentiometer, R2, at the lower right of the control panel escutcheon carries an on-off switch attachment and serves to vary the screen potential of the two intermediate amplifiers. The r.f. amplifier employed is very similar to that found in the 6-tube receiver described upon pages 281-283 of the September issue of Radio Broadcast, except that in this super-heterodyne a dummy input tube is utilized to simplify tuning (since there is amplification and sensitivity to spare) and a screen-grid first detector is employed. The 222 tube is used here for a number of reasons, among them the fact that the effective selectivity of the first detector circuit is somewhat greater than would be the case were a 201 a to be used with the same circuit arrangement. The receiver is mounted upon a steel chassis 2 1 1" long, 9rt" wide, and f" deep. At the left end (as in Fig. 4) is the first r.f. tube. Its grid circuit is connected across an antenna resistance, Ri2, attached beneath the chassis to ground and antenna binding posts BPxand BP2. This tube feeds the two tuned r.f. amplifier stages utilizing tubes S2 and S3, while S3 feeds the detector stage including tube St. The three tuned circuits are identical and employ r.f. transformers having secondaries consisting of 985 turns of No. 29 enameled wire wound upon a threaded bakelite tube in a space \\" in diameter and \\" long. These secondaries are tuned by sections TCi,TQ,TC3 of the 0.0003 5-mfd. die-cast gang condenser (each section is equipped with an individual trimmer). The r.f. transformer primaries are wound with 35 turns No. 38 wire upon a 1 1" tube placed at the filament end of the secondary. Individual 10-ohm filament resistors are used for each of the first four screen-grid tubes and the screen-grid and plate circuits are bypassed by i-mfd. condensers to provide very short r.f. paths and thus prevent disturbing interstage coupling. In preliminary tests it has been found that if the plate lead of the detector tube, S4, is removed from the P post of the first intermediate-frequency transformer and instead is connected to post No. 2 of the first-stage audio transformer, that the t.r.f .receiver resulting will provide very satisfactory distance reception over a range of several hundred to one thousand miles or more, and selectivity which in itself is considerably greater than that obtained from many ready-made receivers. This arrangement may be resorted to for preliminary testing of the receiver. (While a screen-grid detector tube would not work into an ordinary audio transformer at all satisfactorily, the characteristics of the Clough audio system are such that a screen-grid detector may feed directly into a transformer of this type with a quite satisfactory resultant frequency characteristic.) The oscillator coil, L4, has a grid winding equivalent to the r.f. transformer secondaries and in addition a coupling coil consisting of 35 turns of No. 34 d.c.c. wire on a ij" tube and a tickler coil wound in a slot at the bottom of the form consisting of 35 turns of No. 34 d.s.c. wire. The grid winding of the oscillator is shunted by the 0.00035-mfd. condenser, TC4, controlled by drum dial D2. A small r.f. choke coil, L5, is placed directly under the oscillator coil base with its axis at a right angle to the axis of the oscillator FIG. 3. WITH THE SHIELDS REMOVED Screen-grid tubes are used in the three r.f. stages, the first detector stage, and the two i.f. stages. A cx-joiA tube is used for an oscillator , and 1 izh tubes in the second detector and first audio sockets. coil. (The oscillator and t.r.f. coils are wound upon moulded S-M plug-in forms which fit any standard 5-prong tube socket.) In working upon the intermediate amplifier, a number of different transformer designs were evolved and tested, and in the course of this work the possibilities of standard i.f. transformers now on the market were investigated. The S-M 210 transformers which are normally broadband iron core types were found to give excellent results with screen-grid tubes and due to their comparatively small primaries, functioned as rather sharply tuned transformers, giving an amplification of 65 per stage at 65 kc. and a band width of about 10 kc. at forty per cent, of maximum amplification — a quite satisfactory characteristic for an intermediate amplifier. The intermediate amplifier tubes are S6 and S7, the second detector S8, and the three i.f. transformers are Ti, T2, T3. The second detector employs a grid condenser and leak for rectification, while the first detector has really no means provided to effect rectification. While this may seem peculiar, it was observed in operating tests that due to the effect of the impressed oscillator voltage, very satisfactory detection was obtained even though none of the usual precautions were employed in the circuit to insure it. This being the case, it was thought best to take advantage of this propitious condition and discard the usual adjuncts to rectification, such as grid condenser and leak or C battery, which seemed only likely to add complications without any resulting gain to the receiver. In actual operation this 9-tube super-heterodyne operating into a power amplifier stage has allowed reception of weak outof-town stations within 10 kc. of any Chicago local station when the receiver was operated in residential districts. In comparative tests against other receivers, it was found that this super-heterodyne would in every case show a considerably higher noise level