How do you spell Hannukah?
Any electron tube with the basic structure and functionality of a triode, but including two extra electrodes, a screen and a suppressor grid. The screen helps the tube respond well at high frequencies (as in a tetrode), while a negatively charged suppressor grid adjacent to the plate prevents secondary emission of electrons from the plate, increasing the efficiency of the tube. See more at suppressor.
vacuum-type electron tube with five electrodes. Besides the cathode filament, anode plate, and control grid of the triode and the added screen grid of the tetrode, there is still another grid (suppressor grid) placed between the screen grid and the anode plate and maintained at cathode potential. Thus, any electrons emitted from the plate surface by secondary emission are repelled back to the plate. The pentode can be used for almost all purposes for which vacuum tubes are used, including amplification, mixing, oscillation, and pulse generation, and in circuits for timing, control, and counting. Today pentode tubes are primarily used in audio amplifiers, because they generate less noise than solid-state components.