Radio Broadcast (May 1928-Apr 1929)

JMnke Lighting Lines Safe For A.C. Tubes KASSON HOWE A MAN who lives in New York City paid over five hundred dollars for an a.c. radio set. It was one of the best known makes, and he looked forward with more than a little anticipation to the pleasure and entertainment it would give him. He bought a high priced set because he wanted real tone and power, single dial control, a.c. operation, and most important of all, freedom from those minor troubles sometimes associated with radio. The set had that perfection of construction and attention to detail which gave every assurance of long and uninterrupted service. Imagine his amazement and disappointment when two sets of a.c. tubes blew out during the first month of operation. It seemed incredible. Like many other a.c. set owners, he denounced the new type of tube as a costly failure, and criticised the manufacturer and dealer from whom he had purchased the set in no gentle terms. To some degree he was justified, for certainly the user cannot be blamed for tube failure, no matter what the cause, unless he has actually tampered with the receiver. Probably the dealer who sold the set had to stand most of the gaff; it would, therefore, seem advisable for dealers to become familiar with the method described in this article of compensating excessive line voltage, and seriously consider the installation of such devices when the receiver is first placed in operation at the customer's home. This entails some additional expense, but will probably result in fewer calls for service and fewer dissatisfied customers. One can't, unfortunately, bring dead a.c. tubes back to life, but there are means of preventing trouble from short tube life before it ever starts. The work involved is simple, and can be done by anyone. Consider that a.c. tubes cost anywhere from $3.00 to $6.00 apiece, dependingupon the type, and it will be seen that one set of tubes blown out or with life shortened means considerable expense. And what is more to the point, it is needless expense. When an initial check on incoming line voltage shows a figure above •fa The use of a fixed resistor to prevent a.c. filament overload from high line voltage is valuable when the line voltage is uniformly too high. This method does not give real regulation, inasmuch as it cannot he varied to meet irregular line voltage variations, hut it does provide insurance, when the proper value of resistance is selected, against maximum line voltage overloads. It has the advantages of inexpensiveness, ease of installation, and the fact that once installed the resistor needs no further adjustment. Where the line voltage is variable, more accurate adjustment is required, and here a power rheostat may he used. This article gives the necessary data for determining line voltage variations, choosing the correct value of resistors or rheostats, and making the installation. — The Editor. =0$ that specified by the manufacturer as the safe maximum limit for the operation of the set, it is little short of sheer extravagance to neglect the fact and place the receiver in operation without doing something to reduce the line voltage. An accurate a.c. voltmeter, with range from o to 150 volts, is the only apparatus needed to make the initial test. For practically all factorybuilt a.c. sets sold to-day, any figure above 1 1 5 volts should be considered excessive unless TABLE I FIXED RESISTORS FOR LINE VOLTAGE CONTROL Current in Amperes These Resistors Will Reduce the Voltage Across the Receiver from the Voltages Shown to 110 Volts Drawn by Receiver 2.5 Ohms 3.5 Ohms 5 Ohms 7 Ohms 10 Ohms 12.5 Ohms 15 Ohms 22 Ohms 31 Ohms 45 Ohms 62 Ohms line line line line line line line line line line line voltage voltage voltage voltage voltage voltage voltage voltage voltage voltage voltage 0.25 112.5 113.0 114.3 116.3 119.2 122.7 0.50 112.7 114.0 115. 1 116. 1 119.0 122. 7 128.7 136.0 0.75 113.0 114.2 116. 1 117.5 119.2 123.6 129.4 138.1 1.00 112.7 114.0 115.6 118.2 120.2 122.2 128.2 136.0 1.25 112.5 113.5 115.0 117. 1 120.2 122.7 125.2 133.1 1.50 113.0 114.1 116.1 118.5 122.2 125.5 128. 7 1.75 113.5 114.8 117. 1 120.0 124.3 128.2 131.9 2.00 114.0 115.6 118.2 121.5 126.5 130.8 2.25 114.5 116.5 119.1 123.0 128.7 2.50 115.1 117. 1 120.2 124.3 130.9 otherwise stated by the manufacturer. If the reading is above 1 1 5 on the a.c. voltmeter, it will certainly be cheap insurance for the a.c. tubes in the set to compensate excessive line voltage. A 10 per cent, increase in filament voltage may cause as much as a 50 per cent, decrease in tube life. This means, obviously, that the tube budget over the course of a year may be double what it would be if proper precautions were taken. The extra 5-volt range from 115 to 120 volts should be regarded as dangerous and worthy of attention from the a.c. set owner. Probably a majority of a.c. sets in New York City operate at a figure somewhere within these limits. The trouble that has been experienced with short a.c. tube life is proof enough in itself that any reading of line voltage above 1 1 5 calls for attention if one wants to be absolutely safe. Where line voltage on accurate measurement is over 120 volts there can be absolutely no question of the need for the correct value of resistor to compensate the extra voltage, which is dangerous beyond a shadow of doubt. HOW TO GO ABOUT THE JOB THE measurement of line voltage, as mentioned above, should be done with a high grade a.c. voltmeter. The other bit of needed information is the primary current rating of the receiver in amperes, which will in practically all cases be found upon the name plate inside the cabinet, on the power unit, or on the name plate of the power supply transformer which supplies A, B, and C voltages to the receiver. If you fail to find this after careful inspection. write to the manufacturer of the set, asking him for the primary current rating of the particular model in question. Having the accurate line voltage figure and the current rating of the receiver, it is now an easy matter to select the correct value of resistor to reduce line voltage. Table 1 lists the current in amperes drawn by various kinds of re 269