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Valve Technology - A Practical Guide A series of articles from 1993 by Graham Dixey C.Eng., MIEE republished by kind permission of Maplin Magazine. The Triode Valve The diode valve is essentially a rectifier, turning AC into DC, whether these be large mains voltages or relatively small signal voltages, as in the detection of modulated radio signals, for example. True, it can perform other useful functions as well, but one thing it cannot do is amplify a signal, For this we need to develop the basic device further. We mentioned earlier Lee de Forests work with a diode, in which he had inserted a wire grid between cathode and anode in order to control the anode current. This was the first triode valve – triode obviously meaning three electrodes, although he actually called it an audion. The construction of a modem triode is shown in (a) below, together with its circuit symbol, (b). (a) Construction of a triode valve, (b) circuit symbol for a triode valve. Based on what we have already seen for the diode, it has an indirectly heated cathode and an outer anode electrode. The third electrode is known as the control grid. It is quite open in form, so that there are relatively vast areas for the electrons to pass through on their way to the anode. It is, quite literally, in the vast majority of cases, no more than a single spiral of wire. In use, the control grid is taken to a potential that is negative with respect to that of the cathode. As a result, there is a negative potential gradient between the cathode and the control grid (tending to repel electrons), and a positive potential gradient from grid to anode, so that electrons that do get through the grid are accelerated to the anode where they are collected. If the negative potential of the control grid is fairly small, then most electrons emitted by the cathode have sufficient energy to counter the repelling force of the grid and make it to the anode; a small percentage are turned back to the cathode so that, o