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How the Vacuum Tube Works
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given a varying voltage with respect to the filament, and we shall see that this voltage has a great effect on the flow of electrons. If the voltage is sufficiently negative, all the electrons will be repelled from the grid, and none will get through to strike the plate. This is the starting point "A" in Fig. 12. As the grid becomes, less negative (ABC), it offers less and less resistance to the electrons, and more of them flow past it to the plate. When the grid becomes positive (CDF), the flow of electrons is increased still further until at last a point F is reached where the current can increase no more, because the filament is giving off the maximum number for that particular temperature and filament-plate voltage.
The first part of the characteristic is curved (A to B); the importance of this we shall see later. The second part BCDE is practically a
A L
Grid ft /am en f Vo/fage (£Gf)
FIG. 12
The flow of electrons from the filament to plate is controlled by the relative charge on the grid as shown in this graph
straight line. This means (i) that each change in grid voltage causes a large change in plate current; and (2) that the current change is exactly proportional to the voltage change. The significance of (i) is that the voltage change of the grid requires very little current, and hence very little power; the current change in the plate circuit, on the other hand, may vary the power in its circuit through a wide range. Exact proportionality (2) means that if the input voltage is a voice wave, the output current will be a perfect copy, "but on a much larger scale. . In other words, the device will amplify without distorting.
MAKING ONE TUBE WORK AS TWO
IF NOW we connect the "input" terminals G'H' of a vacuum-tube amplifier set (Fig. 13), to the terminals GH of Fig. 11, we shall greatly increase the loudness of the signals
22 fO 80 VO/fS
FIG. 13
Altering the circuit GH in Fig. 1 1 by adding this amplifier
arrangement permits much better reception and results
in a circuit represented by Fig. 14
which were heard when we connected the telephone receivers directly to GH. A diagram of part of this "hook-up" is shown in Fig. 14, in which it will be seen that the filaments of the two tubes are connected, and that the plate of Tube i is connected to the grid of Tube 2. It is evident that the filament and grid of a threeelectrode tube can operate the same as the filament and plate of a Fleming valve, i. e., that the filament and grid will rectify incoming high-frequency currents just as do the filament and plate. So we can eliminate the Fleming valve and rectify and amplify in the same threeelement tube. This circuit is shown in Fig. 15, and is familiar to most radio amateurs.
Let us now review briefly what happens in this circuit. The high-frequency voltage across the secondary S varies in proportion to the voice currents at the transmitter. This voltage, rectified by the filament-grid circuit, produces • a small direct current through the tube, the high resistance L and the secondary S. This current will of course produce a proportional voltage drop across L, by Ohm's
Tube 2
FIG. 14
A Fleming Valve detector with a threeelement tube used as an amplifier