Consider a case in which the grid potential is maintained at some high potential, say 30o volts and the anode potential is gradually increased from zero. At first the anode current in creases at the expense of the grid current, but, as the proportion of secondary electrons emitted by the anode increases, the collec tion of these secondary electrons by the grid more than counter balances this effect and eventually the anode current falls with increasing anode potential.
A typical characteristic curve is shown in fig. r i where it will be seen that between A and B the node current falls with in creasing anode potential, the dif ferential internal resistance of the tube being negative. The system, therefore, instead of dis sipating energy, may be used as a source of energy and so used for maintaining or amplifying a current in a circuit (e.g., an oscillatory circuit in a transmitter as in fig. 12) which dissipates energy. When used in this way the tube is called a dynatron, a name given to it by A. W. Hull, who first called attention to the marked effects of secondary emis sion for high electrode potentials.
The Four-electrode Valve or Tetrode.—Tubes with two grids, and thus four-electrodes in all, are of two types. In both types the grids are situated between the filament and anode. In the first type the grid nearer the filament (known as the inner or space-charge grid) is maintained at a constant positive potential with respect to the filament, while the outer grid performs the normal functions of the grid in a three-electrode tube. The posi tive charge on the inner grid, even when the potential of the latter is low, is sufficient to neutralize the effect of space charge near the filament. Thus practically the whole of the emission current leaves the filament and passes through the interstices of the inner grid. For such conditions we can regard the inner grid surface as the cathode or "filament," the outer grid and anode performing their normal functions. The chief advantage of this type of tube is that only low anode potentials need be used.
In the second type of tetrode (the shielded grid type) the inner grid plays the part of the normal one, and the outer grid, which is usually in the form of a wire net or mesh, is maintained at a con stant potential, usually a little lower than the anode potential.
The function of the outer grid is to shield the inner grid electro statically from the anode, and so prevent undesired reaction between the grid and anode circuits when the tube is used in a high-frequency amplifier.
The shielding grid has, however, the further effect of shielding the filament from the anode so that variations of anode potential cause, under normal working conditions, inappreciable variations of the current leaving the filament. The result is that this type of tube has a large amplification factor and a large internal resistance. The shielding grid is usually maintained at a lower potential than that of the anode—a shielding grid potential of 8c) volts, together with an anode potential of 120 volts, being com monly used. To increase the shielding effect between the anode circuit and the other electrodes it is usual to complete the shield with a metal screen as shown in fig. 13 where a screened grid high frequency stage is added to an ordinary receiver.
For good-quality loud speaker reproduction a mean anode current of about i 5 milliamps. (m.a.) is required, and it must be possible to vary the current substantially 12 m.a. above and below this value. For this to take place, as a result of small grid potential variations, the mutual conductance of the valve should be high. This condition is fulfilled in the case of the pentode which is therefore specially suitable for the output or loud speaker valve in a wire less receiver.