Q.Describe the basic construction of a triode valve, name its three electrodes, and state briefly the role of the control grid.

A triode is a vacuum diode with a third electrode, a fine open wire mesh (the control grid), inserted between the cathode and the plate, much closer to the cathode. The grid is normally held slightly negative, partly repelling the emitted electrons. Because it sits so close to the cathode, even a small change in grid voltage strongly changes the field right at the cathode surface, and so strongly changes how many electrons get past it to the distant plate -- a small grid-voltage swing produces a much larger plate-current swing, the triode's basic amplifying action.

Name the three electrodes, then explain why the (nearby) grid has a disproportionate effect on plate current compared with a small grid-voltage change.

• Describing the grid as a solid blocking plate -- it is an open mesh so electrons can pass through it.
• Missing that the AMPLIFYING effect comes specifically from the grid's closeness to the cathode, not merely from its presence.