Radio Broadcast (May 1928-Apr 1929)

New Apparatus PRODUCTS of radio manufacturers whether new or old are always interesting to our readers. These pages, a feature of Radio Broadcast, explain and illustrate products which have been selected for publication because of their special interest to our readers. This information is prepared by the Technical Staff and is in a form which we believe will be most useful. We have, wherever possible, suggested special uses for the device mentioned. It is of course not possible to include all the information about each device which is available. Each description bears a serial number and if you desire additional information direct from the manufactuter concerned, please address a letter to the Service Department, Radio Broadcast, Garden City, New York, referring to the serial numbers of the devices which interest you, and we shall see that your request is promptly handled. — The Editor. Shielded Wire Prevents Stray Coupling X6o Device: Shielded Hook-up Wire. This hook-up wire is a rubber-covered No. 18 with an additional braid of metal over the rubber insulation. When this metal braid is grounded it forms an effective shield around the wire. The metal braid is sufficiently flexible so that the wire may be readily bent into any desired form. It has all the conveniences of a flexible hook-up wire with the additional ad I vantage that it is shielded. It J is available in rolls of 100 feet. | Manufacturer: Belden Manu | facturing Company. Price: \ $3.50 per 100' roll. Application: In constructing | high gain r.f. amplifiers, espe | cially those using type 222 | screen-grid tubes, it is absolute | ly essential that no coupling of any sort exist between the input and output circuits of the tube. Frequently it is possible that the comparatively short leads connecting one tube to the next will produce sufficient capacitative coupling to some other part of the circuit to FIG. 2 cause the amplifier to oscillate. Coupling of this type can be eliminated by the use of shielded wire. The circuit diagram in Fig. 2 indicates at what point in the circuit of a stage of r. f. amplification it might be wise to use shielded conductor. For Smoother Volume Control X6i Device: Variable High Resistances. For volume control. Many of us when operating a volume control have noticed that frequently the volume does not vary uniformly as the control is turned, fuming the control a given distance at a certain point will produce a given change in volume and then turning it twice as far will produce a very much greater change in volume. It would be desirable to make use of a volume control resistor of a characteristic such that the changes in volume were more nearly proportional to the movements of the control. Resistances of this type have been designed to give uniform variation of volume and can be 28 26 24 22 20 18 16 14 12 10 DIAL DIVISIONS FIG. I obtained in sizes ranging from 2000 to 10,000 ohms. The curves of such resistances are given in Fig. 1. The solid curve indicates that of an ordinary resistance in which the variation in the resistance is directly proportional to the dial setting. The dot-dash curve is that of the new type resistance. It will be seen that the resistance does not vary directly with the dial setting but varies slowly at first and then more rapidly, which results in smoother volume control. The catalog numbers and prices of these new type resistances are given below. No. 1896, 2000 ohms — $2.25 No. 1897, 5000 ohms — 2.25 No. 1898, 10,000 ohms — 2.25 Manufacturer: Herbert H. Frost, Inc. Application: As indicated above these units are designed for use as volume controls in radio receivers, and because of their special characteristics have certain advantages over other types of resistances. All of these resistors are made with three terminals, like a potentiometer, and can be used as a potentiometer type of control or, if desired, by connecting to the center terminal and one of the outside terminals, the unit can be used as a two terminal resistor. The direction of rotation of the knob to decrease or increase the resistance will depend upon which outside terminal is connected. A Safeguard for A. C. Tubes X62 Device: Centralab Radio Control Box. This is a device designed for use in conjunction with light-socket operated receivers. It consists of a small metal box in which is placed a variable resistance. The power lead on the box is plugged into the light socket and then the power lead from the receiver is plugged into the receptacle on the box. This places the variable resistance in CENTRALAB CONTROL BOX 372 series with the line. If the line voltage is excessive, i. e., greater than that on which the set is designed to operate, the control on the box can be adjusted so that part or all of the resistance is in series with the line and the excess voltage will then be absorbed. Manufacturer: Central Radio Laboratories. Price: $3.00. Application: Excessive line voltages have evidently caused considerable trouble with a.c operated receivers. If a.c. tubes, or any tubes for that matter, are subjected to excessive filament voltage, their life is materially shortened. Excessive line voltage supplies excessive filament voltages to tubes in a set, and it is essential, therefore, that some device be used to permit compensation for line voltages greater than about no, the voltage on which most receivers are designed to operate. A device of this sort, of course, requires manual control if the line voltage varies during the day and night. Also there is no means of determining when the voltage applied to the set is 110. Probably the best method of operation, therefore, is to gradually increase the resistance (which reduces the voltage applied to the receiver) until an effect on reception is noted and then to decrease slightly the resistance. Devices can be obtained which will automatically regulate the line voltage so that the receiver is always supplied with exactly 1 10 volts. Such devices are expensive, costing perhaps five times as much as this device. For a simple unit that will permit longer life from a.c. tubes the power control box is to be recommended. A Complete PushPull Amplifier X63 Device: Push-Pull Amplifier, Model P-P-2. A complete single-stage push-pull amplifier designed for the use of two 171 type tubes. This device does not contain any power supply, but must be operated in conjunction with batteries or A and B power units. Manufactured by Harold Power, Inc. Price: $38.00, with tubes. Application: This amplifier is designed for use in conjunction with a radio receiver in those cases where more power output is desired than can be obtained without distortion from the power tube incorporated in the receiver. This amplifier can deliver at least 1400 milliwatts of power without overloading — about ten times as much as can be obtained, for example, from a 1 12A type tube. A Plug Which Can't Be Broken X64 Device: Soft Rubber Plug. Ruggedly constructed of solid soft rubber. Shaped to form a convenient grip for the fingers when pushing it in or pulling it out of a socket. Only sold attached to Belden cords which come in 10-, 20-, and 50foot lengths. Manufactured by the Belden Manufacturing Company. Price: $2.75,. Application: This plug can be thrown around, dropped, or stepped on without fear of breaking it. Judging from the number of ordinary hard rubber plugs that have been broken in Radio Broadcast Laboratory we venture a guess that we ought to have about 600 of these! A 30-Henry Choke for Filter Circuits X65 Device: Samson Filter Choke Coil, Type 312. A choke coil designed for use in the filter circuits