The Triode Valve

Construction. A triode valve adds a THIRD electrode to the basic vacuum-diode structure of Section 9.3: a fine, open wire mesh or spiral called the CONTROL GRID, placed in the vacuum envelope between the heated cathode and the plate, much closer to the cathode than to the plate. Being an open mesh rather than a solid sheet, the grid does not block the flow of electrons from cathode to plate; it merely sits in their path, close enough to strongly influence the electric field the emitted electrons experience as they leave the cathode.

The amplifying action, in outline. The control grid is normally held at a small NEGATIVE voltage relative to the cathode (via a small grid-bias battery), which tends to repel the emitted electrons and reduce the plate current below what it would be with no grid present at all. Because the grid sits so close to the cathode -- much closer than the distant plate -- even a SMALL change in the grid's voltage has a disproportionately LARGE effect on the electric field right at the cathode surface, and hence on how many electrons succeed in getting past the grid and reaching the plate. A small swing in grid voltage therefore produces a comparatively large swing in plate current -- and, since the plate current flows through an external load resistor connected to the high-tension supply, a large swing in the VOLTAGE developed across that load resistor as well. A small AC signal applied to the grid can thus appear, greatly magnified, as a voltage swing across the plate load -- the basic amplifying action of a triode, historically the first practical amplifying device in the whole of electronics, decades before the transistor amplifier of Section 9.22. …