A type of vacuum tube currently used in high-end audio amplifiers. A tetrode is like a triode with the addition of a “screen grid” between the control grid and the plate (anode). The screen grid is an electrostatic shield typically biased with a fixed voltage that isolates the control grid and protects it from voltage potential changes on the plate.
Dynatrons are special types of tetrodes that utilize a screen grid voltage higher than the plate voltage. This causes the plate current to decrease as plate voltage increases. This characteristic, known as negative resistance, is useful in oscillator circuits.