Control-grid meaning

An electrode used in triodes , tetrodes , and pentodes , placed between the cathode and the plate (usually very close to the cathode). Electrons emitted by the cathode must pass through the control grid to reach the positively charged plate and create current flow through the tube, but a negative voltage on the intervening control grid impedes that flow; the more negative the control grid, the smaller the flow. Small variations in the voltage of the grid thus have a great effect on the current flow.