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Gated Valves

    
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It is convenient for many purposes to be able to control the output of a valve from two different sources. If sufficient negative voltage is applied to the suppressor grid of a pentode valve the anode current can be cut off. The effect of cutting off the anode current is to cause a considerable increase in screen current; care must then be taken to avoid exceeding the safe screen dissipation. If the suppressor grid is biased negatively to cut off the anode current and if a positive pulse is then applied to it, an input fed to the control grid will affect the anode current only during the pulse, as if a gate were opened, the pulse being called a gating pulse.

Because it is difficult to control accurately the magnitude of a pulse, it may happen that the suppressor grid is pulsed positively into the region where it will take current and 'rob' the anode. To avoid such a possibility a diode is usually connected between this grid and cathode to act as a low-impedance shunt and thereby prevent a significant positive voltage. In certain valves specially intended for this purpose, a diode is built on the suppressor grid, which is then known as a dioded suppressor grid, and the pitch of the turns in the suppressor grid is made finer so that the anode current is cut off by a voltage of the same order as that of the control grid.

Instead of a suppressor grid, specially designed beam plates may be employed (as in the beam tetrode) in conjunction with aligned control and screen grids, and the resultant valve is a gated-beam valve.

Examples of the use of these valves are television synchronizing circuits, radar, and volume compression and expansion.

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