Thermionic valves

A beam tetrode employs principles not found in other types of valve: the electron stream from the cathode is focused ('beamed') towards the anode. The control grid and the screen grid are made with the same winding pitch, and they are assembled so that the turns in each grid are in optical alignment (see drawing). The effect of the grid and sereen turns being line is to reduce the screen current compared with a non-beam construction. For example, in a pentode of ordinary construction the sereen current is about 20% or the anode current, whereas in a beam valve the figure is 5-10%.