Schematic diagram of a pentode.
A pentode is an electronic device having five active electrodes. The term most commonly applies to a three-grid vacuum tube, which was invented by the Dutchman Bernhard D.H. Tellegen in 1926. (Please see reference.)
Advantages over the tetrode
A tetrode could supply sufficient power to a speaker or transmitter, and offered a larger amplification factor than the earlier triode.
However, the positively charged screen grid can collect the secondary electrons emitted from the anode, which can cause increased current toward the screen grid, and cause the anode current to decrease with increasing anode voltage over part of the Ia/Va characteristic. Tellegen introduced an additional electrode, called the suppressor grid, which solved the problem of secondary emission. It does this by being held at a low potential, usually either grounded or connected to the cathode. The secondary emission still occurs, but the electrons can no longer reach the screen grid, since they have less energy than the primary electrons and hence cannot pass the grounded suppressor grid. Therefore these secondary electrons are re-collected by the anode.
Usage
Pentode valves were first used in consumer-type radio receivers. A well-known pentode type, the EF50, was designed before the start of the World War II, and was extensively used in radar sets and other military electronic equipment. The pentode contributed to the electronic preponderance of the Allies. After WW II, pentodes were widely used in TV receivers, particularly the successor to the EF50, the EF80. Vacuum tubes were replaced by transistors during the 1960s. However, they continue to be used in certain applications, including high-power radio transmitters and (because of their well-known valve sound) in high-end and professional audio applications, microphone preamplifiers and electric guitar amplifiers. 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.
See also
References
G. Holst and B.D.H. Tellegen, "Means for amplifying electrical oscillations", US Patent 1945040, January 1934.
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