tétrode

tétrode, also spelled tetrode, is a vacuum tube with four active electrodes: a heated cathode, a control grid, a screen grid, and a plate (anode). The arrangement allows amplification by modulating the flow of electrons from the cathode to the plate. The screen grid sits between the control grid and the plate and is typically supplied with a positive voltage. Its purpose is to shield the control grid from plate voltage variations and to reduce the capacitance between grids and plate, improving gain and frequency response.

Operation of a tetrode relies on small changes at the control grid to produce larger changes in plate current. The screen grid stabilizes the electron stream and reduces feedback through inter-electrode capacitances, enabling higher gain than a triode of comparable construction.

A notable drawback of the tetrode design is the tetrode effect, a nonlinear phenomenon caused by interactions involving secondary emission from the plate and grid structures, which can limit gain at higher plate voltages. This issue contributed to the development of the pentode, which adds a fifth electrode, the suppressor grid, between the screen grid and plate to mitigate these effects. Some later tetrodes were designed as beam tetrodes, which use beam-forming electrodes to reduce space-charge effects and further improve efficiency.

Applications and history: tétrodes were widely used in early radio receivers and audio amplifiers, valued for their higher gain and bandwidth relative to triodes. They were gradually superseded in many applications by pentodes and, later, by transistors. Nonetheless, tetrodes remain of interest in certain high-power RF equipment and in vintage vacuum-tube gear, where their characteristics are preserved for specialty use or historical preservation.