Radio Broadcast (Nov. 1925-Apr 1926)

334 RADIO BROADCAST JANUARY, 1926 A circuit which would perform satisfactorily in city and country on dry cells or with a storage battery, without wasting B batteries, which would give more than ordinarily good quality of reproduction on a loud speaker over comparatively long distances, which was easy to build and easy to operate after it was built, and, last, but not least a circuit for which the parts could be procured in any town of any size in any part of the world; that was our notion of what the " Universal" should be. We believe we have found it. It is not a new circuit, by any manner of means. Fundamentally it was used in slightly modified forms in such popular receivers as the Teledyne, the Browning Drake, The Roberts, Radio Broadcast's Four -Tube, Three -Tube, and Two -Tube Knockout Receivers. HammarlundRoberts, Radio Broadcast's Aristocrat, the Samson T. C. Receiver and the Silver Knockout. But since the appearance of most of these receivers in Radio Broadcast, improvements have been made in the design of many of the integral parts and this improvement is particularly evident in the matter of tubes. In order to show how various parts may be used in this circuit with satisfaction, we are illustrating with this article, a receiver employing just about the same circuit and sold in kit form by the Samson Company of Cambridge, Massachusetts, and another built to our design by the American Mechanical Laboratories of Brooklyn, New York. Other variations on the same theme may be seen by looking over the article by Allan T. Hanscom in our October, 1925, number and the descrip Of"0 FIG. I I A panel view of the Universal. Symmetrical layout has been one of the prime considerations in the construction of this receiver. tion of Radio Broadcast's "Aristocrat," by the present writer, in our November number. We are very anxious to have the fact understood that intelligent substitution of parts for those we have used will not detract from the performance of the receiver. It is impossible for us to list all those which can be used, but we wish to show no favoritism in the matter of recommending parts for the Radio Broadcast Universal Receiver. So, then, as an example of what may be done, let us first consider the circuit and then the components of a single manufacturer which may be used to advantage in it. From a study of Fig. 1 it will be seen that there are two coil assemblies, one, a primary and secondary in the antenna circuit (L, and L2; the latter tuned by the variable condenser Cz) the other, a tuned radio-frequency transformer of the auto transformer type and a tickler coil, which is fixed in position but adjusted electrically by means of the 500 to 50,000-ohm resistance, R5 shunted across it. These coils are indicated in the diagram by L3 and L4. By properly using the windings already provided on the General Radio Company's coils, No. 277D, both these coil units are instantly provided. No changes whatever need be made as the coils are of solenoid type with two windings on a single form. Two such forms are necessary. On each form there is a small and a large winding. The small ones are used for L and L4 while the large ones are used for L2 and L3. The tap indicated on L3 is easily provided by picking up a turn of the large coil, and scraping clean. It is merely necessary to solder the proper wire to it to carry out the correct circuit arrangement. In Lj this tap is made 39 turns from the grid end and the tap on L2 is made in the exact center of the coil. These coils may be used with .00035 mfd. variable condensers to cover the broadcast frequency range and the results obtained in our laboratory tests of the completed receiver indicate that they will go well below the lowest and well above the highest frequencies transmitted by the broadcasting stations now on the air. Let us now consider the remainder of the parts used for storage battery operation and once having done that we will study the few changes necessary for using the same circuit arrangement with dry cell tubes. parts used in r. b. lab model of the universal receiver THE parts employed are: 1 Panel, 7X 18, 1 Wood sub-base 7 x 1 cut as shown in Fig. 3 and for simplicity of mounting and wiring we recommend the use of wood not more than \ inch thick, 2 sub-base supports, made as shown in Fig. 4, from \ x j\ inch brass strip (the approximate length of this strip required for the re FIG. 12 A base view of the receiver. Note that the coil units are in line with and at right angles to each other, this is absolutely necessary for obtaining proper neutralization