Radio Broadcast (Nov. 1925-Apr 1926)

Record Details:

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V The Use Different Type of Tube GL vwww Filament Resistance Can be, Mow to Decide the Proper Size for Each THE vacuum tube, that indispensable unit of the radio receiver, which transforms unintelligible radio signals into sounds that we can hear has been dealt with at length admirably by Keith Henney in the December, 1925, and February, 1926, issues of Radio Broadcast. In these articles, Mr. Henney dwelt upon the selection, use, and function of the vacuum tube in radio circuits and described in detail the parts played by the three elements of the tube, the plate, grid, and filament. It is the last named, which is taken as the subject for this article. We shall try to show how this filament performs its task efficiently by the use of suitable control devices which adjust or regulate the current and voltage applied to it. The filament is the thing that lights up when the A battery is applied to its terminals and emits electrons at a given rate. The grid is the regulator or shutter which stops or lets flow the electronic stream to the plate, the third element. In this way, feeble radio impulses impressed on the grid release a stronger impulse in the plate and its attendant circuits. Manufacturers of the early tubes found it necessary to employ a control in the filament circuit of the tube so that the tube might be adjusted to its most efficient point of operation. Naturally they could not use a six-volt filament energized by a six-volt battery because the regulation so necessary would not be obtained. The five-volt filament, energized by the six volt storage battery, allowed for an adjustment FIG. I A simple circuit comprising a source of voltage (battery) a resistance (R) and a means for reading the current flowing This arrangement is comparable to a tube circuit where the resistance R is represented by the tube filament and the external control device R=rl + r2 + T3 Orifall r values are the same R=r* x N Where N = Number of resistance units r1 r2 r3 o R FIG. 3 Resistances in series retard to a greater extent the flow of current in a circuit than where only one resistance unit is used. The total resistance of a series-resistance circuit is equal to the sum of all the resistances employed. In a circuit of this kind it is possible to employ two 3-volt tubes energized from a 6-volt source presumably from zero to six volts by means of a variable resistance. Soft tubes, as they were known not so long ago, required critical filament adjustment and to obtain this end, a rheostat had to be used. Sometimes the best operating voltage was found to be five but most times not. Each tube had its own peculiarities. Now, the manufacturers have advanced the design and manufacture of the tubes to such a point that the filament adjustment is not critical. Keith Henney has pointed out in Radio Broadcast that with the present tubes, a decrease in filament voltage below five is usually accompanied by a falling off in signal tone quality. Also, a slight increase above the rated filament voltage always causes a surprising decrease in filament life. To prevent this it is obvious that the old six-volt storage battery, borrowed from the automobile days, must still be retained so that a regulation of one volt in the rheostat may be had to maintain the filament at five volts. When the battery is newly charged, more resistance of the rheostat will be in the circuit to maintain it at five volts. However, when the battery voltage drops off then this resistance is cut out of the circuit to compensate for the drop in voltage at the battery terminals. Theoretically this is true but in actual practise, the battery maintains its full charge over the major portion e>f a single charge life. Toward the end it does drop in voltage but its energy has been expended to the point where the rheostat is useful, in maintaining the tube voltage at 5 only for a few hours. Then the battery may be considered in need of recharging. Some tubes require 1.1 volts, others 3 volts and still others 5 volts. Therefore, it is essential that we adjust each of these filaments economically and efficiently. To make this clearer, it would not be strict economy or efficiency to control a 1. 1 -volt tube filament with a 60-ohm rheostat where only 1 .6 ohms are required, only unless the battery voltage was much higher than 15. Even this is an unusual case. SELECTING THE PROPER RESISTANCE POR the radio set constructor then, there * arises a problem in selecting the proper size of rheostat for the tube or tubes he is going to use. To understand what is happening in a circuit where voltage, resistance, and current are present, it is necessary to review the law governing the use and application of resistances in a circuit. Ohm"s Law says that where a pressure of one volt is exerted in a circuit whose resistance is one ohm, then one ampere of current will flow. Now if the resistance is reduced to one half, the voltage FIG. 2 When several resis tances, such as tube filaments are connected in {-vwwwwww^ parallel as is the case in the majority of receivers the total resistance of the circuit offered to the flow of current is less than were only one resistance unit used in the circuit because several paths are provided for the flow of current oA/VVWVWVVVmj KVvVvvVvVvVW -L+J_ + _!_ r1 r* r3