Radio Broadcast (May 1928-Apr 1929)

NOVEMBER, 1928 NEW APPARATUS 53 I ranges being manufactured cover all values of current, voltage, and wattage ordinarily dealt ; with in radio work. An Adjustable Resistor Network for BPower Units X74 Device: Truvolt Divider. A compact unit consisting of a network of resistances mounted in a small nicely finished container and designed for use in conjunction with all types of B-power units delivering voltages not in excess of 220. It permits one to obtain readily output voltages of various values between o and 180 and to also obtain two values of C voltage. It can be used with any type of rectifier or filter circuit. Manufacturer: Electrad, Inc. Price: $12.50. Application: The problem of obtaining from the output of a B-power unit the correct voltage for application to any given receiver is a serious one. Fixed resistances across the output of a B-power unit have the disadvantage that the voltage which will be delivered from the various taps varies considerably according to the amount of current drawn from these taps. The use of variable resistances for each tap has the disadvantage that the range of voltage which can be delivered from each resistance is very wide and therefore rather difficult to adjust accurately. A compromise between these two is probably the best. We therefore make use in the Truvolt Divider of the combination of fixed and variable resistances so that the voltage from each tap is approximately fixed but can be varied over a range sufficiently wide to adapt the unit to practically any receiver. This might be called a universal divider since it can be used interchangeably with any receiver and any power unit and can be adjusted to supply the correct voltages to the set. The unit will find application not only in the construction of new power units but in conjunction with old power units which perhaps deliver incorrect voltages or have not sufficient voltage taps. Also by using the Truvolt Divider we can obtain from a power unit that ordinarily delivers only B voltages two values of C voltages as well as the B voltages. The Truvolt Divider is designed to supply four B voltages and two C voltages. The voltage obtained from the Bi tap is the maximum voltage from the B-power unit. Tap B2 supplies an average voltage of 135 but this voltage can be adjusted to any value between 1 10 and 160 volts. Tap B3 supplies an average of 90 volts which can be adjusted between the limits of 65 and 110 FIG. 2. Method of connecting Truvolt divider in a B-power unit volts. Tap B4 is the 45-voIt tap capable of supplying a range of voltages from 20 to 55. Tap B5 is the negative B. Tap C6 is the low voltage C terminal and it will supply a grid bias from minus 1 to minus 20 volts. For higher C biases tap C7 is used. It supplies voltages from minus 20 to minus 40. The connection of this device to a typical B power unit is indicated in Fig. 2. Because the resistances used in this divider are of a type which can be accurately calibrated it is possible by the use of either tables or curves to adjust the various controls to give the proper voltages without the use of a voltmeter. If a voltmeter is available, it of course affords a simple, rapid, and certain method of adjusting the unit. The voltmeter is shunted between that particular tap which is being adjusted and the negative B and the control varied until the correct voltage is supplied. This test is, of course, made with the receiver connected to the unit. Many experimenters desiring to use this device will probably not have available a high-resistance voltmeter; for them it would be helpful to have the booklet supplied with the divider, which describes in detail how to adjust the resistors for any given receiver. This booklet, which is called the Truvolt Divider Manual, is not only very helpful in adjusting this device, but also contains a great deal of excellent information in connection with the output circuits of a B-power unit. Useful Tester for Radio Sets X 75 Device: Set Tester. This tester is designed to aid in servicing all types of a.c and d.c. -operated receivers. The following paragraphs indicate some of the tests which may be made. In balancing the various tuning condensers in a single-control receiver the tester is very useful. This test is made by placing a tube in one of the sockets of the tester and inserting a plug into the power-tube socket of the receiver, thus converting the tester into an oscillator. When these connections are made, and the set is turned on, it will be found that the milliammeter in the tester will jump as the set is tuned to resonance with the oscillations produced by the tester. Each individual tuning condenser is then adjusted to resonance, as indicated by the movement of the meter. The tester will measure the normal emission, and the emission when oscillating, of all types of tubes. In addition it will indicate the plate voltage, grid voltage, and filament voltage of each tube of a set, both underload condition and with no load. Another important application is the detection of open, short, -40 V. RADIO TEST SET MADE BY SUPREME INSTRUMENT CORP. and high-resistance joints in any parts of the circuit. The instrument contains a complete set of tools with space for spare tubes. The supplies include a rachet screw driver, soldering iron, hook-up wire, various adapters, test leads, etc. The instruments in the tester are: a doublerange milliammeter, 0-50 and 0-100; a triplerange voltmeter, 0-10, 0-50, and 0-250, and a triple-range a.c. voltmeter, 0-3, 0-18 and 0-150. The photograph illustrates the Model iooa instrument priced at $108.50. Manufacturer: Supreme Instrument Corporation. Application: Indicated above. It should prove invaluable to service men. Hook-up Wire for Various Uses X76 Device: Celatsite Hook-up Wire. The following kinds can be obtained. 1. Celatsite Flexible Wire. A flexible hook-up wire consisting of tinned stranded copper wire covered with a non-inflammable insulation. Available in the following colors and wire sizes. No. 22 — $ .70 per 25-ft. No. 20 — .75 " No. 18— .80 " No. 16 — 1. 00 " " No. 14 — 1. 00 " " Colors: Black, Yellow, Red, Green, Brown, Slate, Blue, White, Maroon. 2. Celatsite twisted wire for a.c. filament circuits. One strand of Red and one strand of Black No. 18 Flexible Celatsite twisted together. The two colors are used so that, if desired, the same sides of all filaments can be maintained at the same relative potential. Packed in 25 ft. coils. List price: $1.75 per carton. 3. Solid Celatsite wire. Bus-bar made of tinned No. 14 copper wire and covered with a non-inflammable insulation. Strips easily for soldering. Available in same colors as the flexible wire. Price; $.09 per 30-inch length. Manufacturer: Acme Wire Company. Application: The above material has been used in the Laboratory for some time in constructing receivers and power units and has been quite satisfactory.