A type of vaccum 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. See triode
The Tetrode Uses Two Grids
In a tetrode, the feedback and oscillation are controlled by the addition of a screen grid between the control grid and plate.