Assembled Vacuum Tube Receiving Sets.--You can buy a receiving set with a vacuum tube detector from the very simplest type, which is described in this chapter, to those that are provided with regenerative circuits and amplifying tubes or both, which we shall describe in later chapters, from dealers in electrical apparatus generally. While one of these sets costs more than you can assemble a set for yourself, still, especially in the beginning, it is a good plan to buy an assembled one for it is fitted with a panel on which the adjusting knobs of the rheostat, tuning coil and condenser are mounted and this makes it possible to operate it as soon as you get it home and without the slightest trouble on your part.
You can, however, buy all the various parts separately and mount them yourself. If you want the receptor simply for receiving then it is a good scheme to have all of the parts mounted in a box or enclosed case, but if you want it for experimental purposes then the parts should be mounted on a base or a panel so that all of the connections are in sight and accessible.
A Simple Vacuum Tube Receiving Set.--For this set you should use: (1) a loose coupled tuning coil, (2) a variable condenser, (3) a vacuum tube detector, (4) an A or storage battery giving 6 volts, (5) a B or dry cell battery giving 22-1/2 volts, (6) a rheostat for varying the storage battery current, and (7) a pair of 2,000-ohm head telephone receivers. The loose coupled tuning coil, the variable condenser and the telephone receivers are the same as those described in [Chapter III].
The Vacuum Tube Detector. With Two Electrodes.--A vacuum tube in its simplest form consists of a glass bulb like an incandescent lamp in which a wire filament and a metal plate are sealed as shown in Fig. 37, The air is then pumped out of the tube and a vacuum left or after it is exhausted it is filled with nitrogen, which cannot burn.
When the vacuum tube is used as a detector, the wire filament is heated red-hot and the metal plate is charged with positive electricity though it remains cold. The wire filament is formed into a loop like that of an incandescent lamp and its outside ends are connected with a 6-volt storage battery, which is called the A battery; then the + or positive terminal of a 22-1/2 volt dry cell battery, called the B battery, is connected to the metal plate while the - or negative terminal of the battery is connected to one of the terminals of the wire filament. The diagram, Fig. 37, simply shows how the two electrode vacuum tube, the A or dry battery, and the B or storage battery are connected up.
Three Electrode Vacuum Tube Detector.--The three electrode vacuum tube detector shown at A in Fig. 38, is much more sensitive than the two electrode tube and has, in consequence, all but supplanted it. In this more recent type of vacuum tube the third electrode, or grid, as it is called, is placed between the wire filament and the metal plate and this allows the current to be increased or decreased at will to a very considerable extent.
The way the three electrode vacuum tube detector is connected with the batteries is shown at B. The plate, the A or dry cell battery and one terminal of the filament are connected in series--that is, one after the other, and the ends of the filament are connected to the B or storage battery. In assembling a receiving set you must, of course, have a socket for the vacuum tube. A vacuum tube detector costs from $5.00 to $6.00.
The Dry Cell and Storage Batteries.--The reason that a storage battery is used for heating the filament of the vacuum tube detector is because the current delivered is constant, whereas when a dry cell battery is used the current soon falls off and, hence, the heat of the filament gradually grows less. The smallest A or 6 volt storage battery on the market has a capacity of 20 to 40 ampere hours, weighs 13 pounds and costs about $10.00. It is shown at A in Fig. 39. The B or dry cell battery for the vacuum tube plate circuit that gives 22-1/2 volts can be bought already assembled in sealed boxes. The small size is fitted with a pair of terminals while the larger size is provided with taps so that the voltage required by the plate can be adjusted as the proper operation of the tube requires careful regulation of the plate voltage. A dry cell battery for a plate circuit is shown at B.