Radio Broadcast (Nov 1924-Apr 1925)

Record Details:

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86o Radio Broadcast life to one fourth. Hence, no matter how conscientious the manufacturer, he cannot make a current tap suitable to all conditions, unless it is equipped with a filament voltmeter and means of regulating the voltage supplied to the rectifier tube filaments. A device which works perfectly under laboratory conditions may in your service completely belie all life and silence tests. In order to increase output, many current taps are so designed that even with normal voltage the filaments are considerably overloaded and operation on increased voltages, which obtain late at night, make tube life a matter of hours instead of hundreds or thousands of hours. OVERLOADING FILAMENTS IS EXPENSIVE THERE is a basis which permits the elimination of certain current taps without further investigation by reason of the fact that their tube equipment is such that it is under no circumstances economical. A current tap equipped with uv-201-A tubes has a definite limit to economical output. To quote John F. Rider, a radio writer in "The LaboratoryScrap Book," New York Sun Radio Section, Oct. 18, 1924: The 201-A tube has often been recommended for the rectifying medium in B battery eliminators. Unfortunately, however, while it does function as a rectifier and may be utilized as such, its current output is so limited that its application to this type of work is very poor — that is, if one is desirous of utilizing the unit in conjunction with a multi-tube receiver. In order that the B battery eliminator may be a success, it is necessary that its current output be at least twice the total possible current drain of the tubes used in the receiver with the maximum plate voltage applied. Having both equal is impractical, as a strong signal will increase the drain beyond the normal value, and the moment the drain is in excess of the supply the voltage of the latter will undergo a considerable drop. The current output of the average 201-A tube when used as a rectifier and arranged so as to supply about 120 volts is about 20 to 22 milliamperes. With several tubes which were tested the maximum rectified current was only 18 milliamperes. In addition, it was necessary to maintain the filament brilliancy above the value specified by the manufacturer, thus greatly decreasing the life of the tube. To attempt to use a rectifying arrangement of this type with a receiver that draws 30 or more milliamperes is out of the question, hence it is necessary somehow to increase the current output of the B battery eliminator. It may appear at first sight that the use of two tubes (as is done on practically all devices) may double the output. But indeed this is not the case, for one tube functions during one half of the cycle and the other tube during the other half. A five-tube set, for instance, may draw as high as 25 milliamperes, and consequently a supply of 18 milliamperes, which is the maximum output, even with considerable overloading, that uv-201-A tubes can be expected to deliver, it is quite out of. the question that satisfactory results may be had. The total output must be at least 30 milliamperes, or else there will be distortion in the received signal. Another type of tube which is sometimes used by experimenters, is the Tungar rectifier, familiar through its use in battery charging devices. This device is designed to supply a heavy current at low voltages. It is not a high vacuum tube but filled with a gas that is easily ionized so that it can become a conductor of considerable current. Consequently when it is used with excessive voltages it has the unstable characteristics of gas tubes and the effect on the life of the filament is disastrous. "5" tubes do not have any of these disadvantages. However, they are designed for use with radio transmitters to deliver high voltages and small currents. A current tap made with s tubes is likely to overload the receiving set unless extraordinary precautions are used. A very high voltage is necessary to make these tubes work and it is not wise for the novice to wear headphones so intimately associated with an s tube requiring 750 volts as plate potential. Damage to the set may be repaired but ear drums are irreplaceable. ADAPTABILITY OF THE CURRENT TAP DEVICE SO FAR, we have seen that a current tap device must be suited to power line conditions and that the tubes must be able to supply the necessary output. These conditions can be met through the selection of a current tap device of the proper characteristics. The third condition — adaptability to your particular set — can be met in the same way. But the user must keep in mind the fact that the same device will not work with the same satisfaction with all other receivers. Inasmuch as there is a comparatively large investment involved in the purchase of a current tap, it is essential it give service over a period of years. For instance, an economical five-tube receiver used 500 hours a year can be supplied with B batteries for five years for S75.00 if heavy duty batteries be used. A current tap costing $50.00 suited to the load so