Pentode

The term most commonly applies to a three-grid amplifying vacuum tube or thermionic valve that was invented by Gilles Holst and Bernhard D.H. Tellegen in 1926.

The screen-grid tube was limited in performance as an amplifier due to secondary emission of electrons from the plate.

During the first quarter of the 21st century, a few pentode tubes have been in production for high power radio frequency applications, musical instrument amplifiers (especially guitars), home audio and niche markets.

The simple tetrode or screen-grid tube offered a larger amplification factor, more power and a higher frequency capability than the earlier triode.

The primary electrons from the cathode have a higher kinetic energy, so they can still pass through the suppressor grid and reach the anode.

A well-known pentode type, the EF50, was designed before the start of World War II, and was extensively used in radar sets and other military electronic equipment.

Large stockpiles in countries of the former Soviet Union have provided a continuing supply of such devices, some designed for other purposes but adapted to audio use, such as the GU-50 transmitter tube.