tetrodi

Tetrodi refers to a class of electronic vacuum tubes known as tetrodes. A tetrode is a four-element vacuum tube, consisting of a cathode, a control grid, a screen grid, and a plate (anode). The addition of the screen grid between the control grid and the plate is the defining characteristic that distinguishes it from a triode. This screen grid operates at a constant positive potential relative to the cathode.

The primary purpose of the screen grid is to reduce the capacitance between the control grid and the plate. This reduction in capacitance significantly improves the high-frequency performance of the tube by minimizing unwanted feedback oscillations that can occur in triodes at higher frequencies. This makes tetrodes more suitable for radio frequency amplification circuits.

However, tetrodes also present a challenge known as secondary emission. When electrons from the cathode strike the plate at high speeds, they can dislodge other electrons from the plate material, known as secondary electrons. If the screen grid is at a high positive potential, it can attract these secondary electrons, causing them to flow to the screen grid instead of the plate. This reduces the plate current and can lead to distortion and instability.

To overcome the limitations of secondary emission, the pentode was developed. A pentode is a five-element vacuum tube that adds a suppressor grid between the screen grid and the plate to prevent secondary electrons from reaching the screen grid. Despite this limitation, tetrodes were an important step in vacuum tube development, offering improved high-frequency capabilities over triodes. They found applications in early radio receivers and transmitters.