Radio Broadcast (May 1928-Apr 1929)

Compactness Is Symbolized in the Interesting Screen-Grid Receiver Described Below A Five-Tube Screen-Grid Receiver By James Milieu CONTRARY to the impression created in the minds of most radio set constructors and experimenters at this time, the ux-222 type of four-element tube is far more than just a plaything for the engineers of some of the large radio research laboratories. Rumors of some of the difficulties encountered in the highly experimental applications of the new tube should not be misconstrued by the home constructor to mean that he too may become entangled in involved engineering problems when he attempts to build a simple set with a ux-222 type tube as a radio-frequency amplifier. In fact, not only can the home constructor build such a receiver with less trouble than one in which a ux-201-A is employed as the radiofrequency amplifier tube, but also he will at the same time obtain markedly superior performance. ► For a good many years now the BrowningDrake receiver has been recognized as one in which simplicity of construction, and excellence of performance, have been combined in a most satisfactory manner. The use of the screen-grid tube as the radio-frequency amplifier in this receiver not only improves its general performance but also simplifies its construction and operation. Fig. 1 shows a diagram of the screen-grid tube Browning-Drake receiver described here. Without resorting to the use of shielding or neutralization, the receiver may be made exceedingly stable. Even the operation of the detector circuit is improved because all advantage can be taken of the regeneration control without throwing the r. f. tube into undesired oscillation. Because the r.f. tube is not on the point of oscillation most of the time, the tone quality obtainable from the receiver is also improved. But the real pleasure to be derived from the operation of such a receiver comes from the way in which it will break through the locals in even such congested areas as New York and Boston and bring in distant stations with volume and quality. When the original model of the receiver shown in the illustrations was hooked up in the writer's laboratory outside of Boston, a wire about 3 feet long was employed as an antenna. The dial was tuned to approximately the setting that should bring in the nearest local station — wbet. Sure enough, in came a dance orchestra with vol t<@f— ~ ^^^ISV A COMPLETE receiver using a screen-grid tube in a Browning-Drake circuit and an impedance-resistance amplifier is described in this article by Mr. Millen. The interesting points are: The antenna or r.f. stage is kept in resonance with the detector circuit by means of an inductance trimmer; (which in reality converts the antenna coil into a variometer) it is not necessary to neutralize or shield the amplifier which uses the screen-grid tube; the plate voltage is choke-fed to the r.f. amplifier. This receiver, with a ten-foot antenna wire, proved to be a DX-getter in the Laboratory. During an average evening it was possible to receive Chicago stations and wsm with about the same selectivity and sensitivity as an average commercial receiver of five or six tubes. Considerable selectivity was gained by the use of such a small antenna directly connected to the grid of the r.f. amplifier. A longer antenna connected through a small fixed condenser added little to the efficiency of the receiver. Originally designed in the laboratory of the National Company, the receiver was subsequently tested by Radio Broadcast, and was found eminently satisfactory. — The Editor. <yge)i' — — — — ^— ^— . — l(§)»p ume and quality about right for wbet, considering the short antenna used. And then, with the end of the musical selections, came the announcement— wmbb, Chicago! By the time the local stations began to sign off for the evening, we had had about everything but the Coast. But even a more pleasant surprise was when wjz was brought in through the local wnac (only 10 kc. separation) with practically no cross talk. The secret of good selectivity with this type of receiver seems to be in the use of a very short antenna, the use of a regenerative detector, and the use of the slot wound high-impedance transformer primary. Certainly the transformer coupling, along with the proper location of coils and condensers, is responsible for the extremely stable operation. The grounded metal drum tuning control placed as it is between the two tuning units offers considerable shielding. This drum, which is of the vernier type, is also of considerable aid to easy tuning. The tuning unit employed is similar to the regular National Browning-Drake unit, except that the inductance of the slot wound primary of the r. f. transformer has been increased and the neutralizing winding has been omitted. An inductive or variometer type of antenna compensator or trimmer has been included to keep the r.f. stage accurately in tune. The 15-ohm cartridge in the negative lead of the ux-222 filament circuit serves the double purpose of dropping the voltage down to the proper value and also of supplying the grid bias potential for this tube. In the other lead is a 20-ohm rheostat serving as a volume control. It will be noticed that a radio-frequency filter circuit, composed of an r.f. choke and a i-mfd. condenser, is used in the screen-grid lead. As a result of such an arrangement no trouble will be had in operating the receiver from a B power device. With some such device the condenser alone is sufficient, but in most instances the addition of the choke is well worth while. The r. f. choke for the detector plate circuit 20