Radio Broadcast (May 1928-Apr 1929)

146 RADIO BROADCAST JULY, 1928 Complete A Power Unit in Single Case FIG. [. THE CIRCUIT OF AN A B C-SUPPLY AND POWER AMPLIFIER A power-amplifier and B-supply have been combined with an A-power unit in this unit so that the combination not only gives the constructor a stage of power amplification but also supplies to the receiver all the necessary A and B-voltages. The various pieces of apparatus in this drawing have been lettered to correspond to the lettering on the picture diagram and photograph and list of parts in this article made up such a set — evidently its name alone, or possibly the vague impressions of a.c. tubes, together with their novelty, etc., etc. has been the cause of building up the present runaway market. It is true that with a.c. tubes plus the proper power supply modern receivers can readily be built which operate without the use of a single battery. The a.c. source of B and C voltage has been practically solved for two years or more because of socket-power devices but it is the appearance of the various substitutes for the A battery that have suddenly built the recent excessive demand for "electrified sets." It does not make a particle of difference what sort of tubes one employs because each is a three-dimensioned device and thus calls for an A potential for heating the filament, a B voltage for its plate circuit, and in most cases an additional C voltage for biasing its grid. Many experimenters have worked on the problem for many years in effort to produce a tube which would operate satisfactory when its filament was energized by alternating current instead of d.c. It has always been a major problem, and even to-day is far from being solved, regardless of what the average broadcast listener may think. The trouble, and there has been much of it too, during our short experience with a.c. tube sets is that set owners simply demanded such receivers a little early; this growth should have been gradual, and not the overnight demands such as we have all seen. Radio haste always makes waste. 201 -a and 199 types of receiving tubes require direct current for heating their filaments and may be considered standard because their efficiency and operating characteristics have been generally accepted through almost five years of usage. These are excellent tubes and give faithful service for better than 1000 hours of life — a feature in which a.c. tubes in most cases have fallen down badly. [The rumored short life of a.c. type tubes may be due more to excessive line voltage which causes the tubes to be supplied with filament voltages above normal than to defects in the tube itself. It is interesting to note that at least one maker of a.c. filament transformers has reduced the rated output voltage from 2.5 volts to 2. 25. — Editor.] It is important, too, to note that 201-A and 199 type tubes cost less than the present a.c. varieties. A COMPLETELY ELECTRIFIED 201-A RECEIVER PROM the above remarks it is quite apparent 1 that the only real difference as far as the user is concerned, between a.c. and 201-A or 199 type tubes lies in the method of energizing their filaments — one employs so-called "raw" a.c. house lighting current and the other pure direct current. In the last analysis it is the A storage or dry battery which is the real bone of contention. How the use of these batteries, and also other substitutes such as electrolytic rectifiers, chargers and trickle-chargers, can be avoided is described in this article. This unit consists of a unique dry or "electronic" full-wave rectifier for converting the house current into a pulsating direct current. The latter is then filtered in quite the usual way by means of heavy choke coils and a condenser network. The A-filter condenser, like the rectifier, is also of novel design. The condenser is bone dry and has a capacity of approximately 1500 to 2500 mfd. stored in a space of but 2 x 2 x 7 inches. For a description of this unit, which is the Knapp A-power unit the reader is referred to the March, 1928, issue of Radio Broadcast. In view of the excellent results obtained through the use of the latter equipment, and also considering (1) its low first cost, (2) cheapness of operation, (3) complete elimination of both storage battery and charger, and (4) faithful radio service at all times, it was decided to incorporate the Knapp unit together with a B and C power unit and also a one-stage audio amplifier as an integral assembly. The power unit and amplifier were mounted in the smallest possible space so that it could operate efficiently any type of receiver, such as the Atwater Kent, Crosley, Freshman, Fada, Grebe, or homeconstructed receivers such as the BrowningDrake, Universal, etc. In its final form this unit as shown in the illustrations occupies a space of 12 x 17 inches. It will supply A power for all receivers up to 10 tubes, and also B voltages up to 180 or 200 volts, in addition to a C bias voltage of 40 for its own amplifier stage. NOT A RECEIVER WIRE OR TUBE CHANGED NOT a wire of the receiver proper need be changed. Simply connect cable leads from the receiver which ran to the storage and B batteries over to this device. And by means of a Thordarson plug connector fitting into the old last audio stage tube socket, the latter becomes automatically replaced with a modern and upto-date last stage audio amplifier. This feature is most important to users of factory-made sets. Many of these do not permit operation of a last-stage power tube, either because the output tube does not employ a C battery or because the latter is in common to both audio stages, a system found in many Atwater Kent and Crosley models. ASSEMBLING THE ELECTRIFIED PACK A KNAPP A-Power unit may be assembled from a kit of parts or it can be purchased fully assembled; the latter arrangement is probably the better to follow for those who are not familiar with or do not care to make up the kit. ' Since the Knapp steel casing measures 8J x 12 inches, the last dimension determines one side of the general mounting board. The writer found that a standard 12 x 17-inch drafting board, which can be obtained from any stationer, is strong, and saves all wood-working time. For the B-supply section of the pack a simple and efficient type of B power circuit was adopted. In view of the well-designed transformer and choke coil arrangement put out by Thordarson their standard R-171 Power Compact was employed. The steel housing of the R-171 unit encloses the transformer, choke coils and buffer condensers for the Raytheon type bh rectifier tubes. It is important that a well built and sturdy filter condenser block be installed. A standard Dubilier type PL-574 power block condenser was used. By means of a Thordarson type R-508-1 wirewound resistor, B voltages of 45, 90 and 135 volts become available for the radio-frequency, detector, and audio circuits of the receiver. This resistor also supplies a 40-voIt C bias potential for the cx-371-A power tube. A Dubilier type 907 l-mfd. by-pass condenser, Ce, is connected