Radio Broadcast (May 1928-Apr 1929)

174 RADIO BROADCAST ADVERTISER FAHNESTOCK CLIPS RADIO'S GREATEST CONVENIENCE Accepted as standard by the most exacting and c oncientious manuf ac turers There are 48 standard sizes and styles to meet any requirement. The largest manufacturers of radio apparatus depend upon our unexcelled facilities and our ability to supply them with any quantity of clips on short notice. They save you time and expense and add to the convenience of your products. Send us specifications and blue prints to meet special requirements Genuine Fahnestock Clips have our name stamped on the thumb piece. T h e World' s Largest Makers of Clip Terminals FAHNESTOCK, ELECTRIC Ca L.I.City --JSfeujyorRa TORY SHEET THE Radio Broadcast Laboratory Information Sheets are a regular feature of this magazine and have appeared since our June, 1926, issue. They cover a wide range of information of value to the experimenter and to the technical radio man. It is not our purpose always to include new information but to present concise and accurate facts in the most convenient form. The sheets are arranged so that they may be cut from the magazine and preserved for constant reference, and we suggest that each sheet be cut out with a razor blade and pasted on 4" x 6" filing cards, or in a notebook. The cards should be arranged in numerical order. In July, 1927, an index to all sheets appearing up to that time was printed. Last month we printed an index covering the sheets published from August, 1927, to May, 1928, inclusive. All of the 1926 issues of Radio Broadcast are out of print. A complete set of sheets, Nos. I to 88, can be secured from the Circulation Department, Doubleday, Doran & Company, Inc., Garden City, New York, for $1.00. Orders for the next set following can also be sent. Some readers have asked what provision is made to rectify possible errors in these sheets. In the unfortunate event that any serious errors do occur, a new Laboratory Sheet with the old number will appear. — The Editor. No. 201 Radio Broadcast Laboratory Information Sheet Tube Life , 1928 EFFECT OF EXCESSIVE LINE VOLTAGE npHE life obtained from a vacuum tube depends very much upon the filament voltage at which it is operated, for voltages slightly above normal produce a marked decrease in life. This is true of all types of tubes, a. c. or d.c, storage-battery or dry-cell-operated. In a battery-operated receiver we are able to control the filament voltage applied to the tubes quite accurately and normal life is therefore generally obtained from the ordinary types of storage-battery or dry-cell tubes. In an a.c.-operated receiver, however, where the filament voltages are obtained directly from, the power lines, the operator of the receiver has little or no control over the filament voltage applied to the a.c. tubes. Most filament transformers are designed for a line voltage of about 115 but in many communities, rural ones especially, voltages in excess of this are frequently encountered. This higher line voltage of course affects the output voltages of the filament transformer so that the tubes are subjected to a filament voltage above normal. It is suggested that experimenters working on a.c.operated receivers include in the circuit some device which will enable them to control the voltage applied to the filament transformer. In cases where the line voltage is found to vary considerably so that at times it is above normal and at other times normal or below normal, it will be preferable to include in the circuit a variable resistance in the primary side of the filament transformer having a value of about 25 ohms. In those cases where the line voltage is found to be above normal but constant at this value, a fixed resistance may be placed on the primary side of the filament transformer to absorb the excess voltage so that the transformer receives its rated voltage or slightly less, for it has been found that a.c. tubes will generally give satisfactory service on somewhat less than the operating voltage at which they are rated. When remedies for excessive line voltage, such as we have suggested here, are made use of, each case must be treated more or less individually, and when, as is usually the case, the line voltage is not constant, a manually controlled resistance may be essential. These facts have been appreciated by many receiver and parts manufacturers. It is probable that devices will soon be available to home constructors which when placed in the primary side of a transformer will automatically control the voltage actually applied to the receiver, so that the tubes will always receive rated voltage despite fluctuations in the actual line voltage. No. 202 Radio Broadcast Laboratory Information Sheet July, 1928 Farm Lighting Systems AS A SOURCE OF FILAMENT CURRENT '"PHIS Laboratory Sheet is provided in response to requests from several readers for information on how to make use of power from a farm-lighting installation for the operation of the filaments of the tubes in a radio receiving set. Farm-lighting systems are of two types, those using a generator powered by a gas engine in which the energy for the lights is obtained directly from the generator and those systems in which the generator is used to charge a bank of storage batteries which in turn supply energy for lighting. The voltages of these systems are generally either 32 or 110 volts. To make use of this current in the radio receiver it is necessary to reduce the voltage by means of the resistance, R, the value of the resistance depending upon the number of tubes in the set and upon the voltage of the supply, as indicated below. 32 VOLT SYSTEM 110 VOLT SYSTEM OF TUBES R WATTS R WATTS RECEIVER IN OHMS IN R IN OHMS IN R 1 51 15 190 57 2 35 ' 22 130 84 3 27 30 100 105 4 21 37 80 135 5 18 43 65 160 6 15 50 58 90 7 13 57 50 210 8 12 66 45 240 Two circuits are given, circuit B being the easier to use, but sometimes with this arrangement there may be some hum audible in the loud speaker. In such a case it is necessary to use circuit A. With circuit B it is simply necessary to connect the resistance R in series with a 20-ohm resistor and connect the plus and minus A terminals to the corresponding terminals in the radio receiver. Using circuit B the same changes must be made but in addition the B minus and C plus leads are removed from where they connect on the receiver and are connected instead to the center point of the 20-ohm resistor. When this arrangement is used the C voltages should all be increased by minus 3 volts to compensate the positive bias produced with the C plus and B minus leads connected to the center tap.