The grid, plate and cathode of a triode form an electrostatic system, each electrode acting as one plate of a small capacitor. The capacitances are those existing between the grid and plate, plate and cathode, and grid and cathode. These capacitances are known an interlectrode capacitance. Generally, the capacitance between the grid and plate is the most importance. This capacitance may act to produce undesire coupling between the input circuit - the circuit between the grid and cathode - and the output circuit - the circuit between the plate and cathode. This coupling is undesirable as it may cause instability and unsatisfactory
performance of the amplifier.
In order to decrease the capacitance between the grid and the plate it was mounted an additional electrode, or screen and in this way since the valve had now four inner electrical elements it was called as a tetrode.
The screen was mounted between the grid and the plate and acted as an electrostatic shield and thus reducing the grid-to-plate capacitance.
However, in the screen-grid valves, it was noticed an undesirable electrical phenomenon kown as secondary emission. In reality it was an emission caused by the bombardment of an electrode by the electrons from the original cathode emission. In this type of valve, the proximity of the positive screen to the plate it arises a strong attraction to these secondary electrons stream in such away the plate voltages swings lower than the screen voltage.
Several types of tetrodes made in the USA between 1925 - 1933:
a)type 124
b)type 35
c)type 36
Tetrode made by Philips circa 1928 type A442 used in RF stages.