A beam tetrode, sometimes called a beam power tube, is a type of vacuum tube or thermionic valve that has two grids and forms the electron stream from the cathode into multiple partially collimated beams to produce a low potential space charge region between the anode and screen grid to return anode secondary emission electrons to the anode when the anode potential is less than that of the screen grid. Beam tetrodes are usually used for power amplification, from audio frequency to radio frequency. The beam tetrode produces greater output power than a triode or pentode with the same anode supply voltage. The first beam tetrode marketed was the Marconi N40, introduced in 1935. Beam tetrodes manufactured and used in the 21st century include the 4CX250B, KT66 and variants of the 6L6.
History
In amplifier circuits, the useful anode voltage - anode current region of operation of the conventional tetrode tube was limited by the detrimental effect of secondary emission from the anode at anode potentials less than that of the screen grid. The detrimental effect of anode secondary emission was solved by Philips/Mullard with the introduction of a suppressor grid, which resulted in the pentode design. Since Philips held a patent on this design, other manufacturers were keen to produce pentode type tubes without infringing the patent. In the UK, three EMI engineers (Isaac Shoenberg, Cabot Bull and Sidney Rodda) filed a patent on an alternative design in 1933. Their design had the following features (compared to the normal pentode): The apertures of the control and screen grids were aligned, by winding the grids with the same pitch (the grids of the pentode used different pitches). Greater distance between the screen grid and the anode than an ordinary tetrode or pentode. An auxiliary electrode structure at or near cathode potential and substantially outside of the electron stream, to establish a low electrostatic potential region between the screen grid and anode, limit the included angle of the beam and prevent anode secondary electrons outside of the beam region from reaching the screen (the pentode has a suppressor grid in the electron stream).
The design is today known as the beam tetrode but historically was also known as a kinkless tetrode, since it had the same number of grids as the conventional tetrode but without the negative resistance kink in the anode current vs anode voltage characteristic curves of a true tetrode. Some authors, notably outside the United Kingdom, argue that the beam plates constitute a fifth electrode.
The EMI design had the following advantages over the pentode:
The design produced more output power than a similar power pentode. The transconductance was higher than a similar power pentode. The plate resistance was lower than a similar power pentode. The screen grid current was about 5–10% of the anode current compared with about 20% for the pentode, thus the beam tetrode was more power-efficient. The design produced less third-harmonic distortion in class A operation than a comparable power pentode.
The new tube was introduced at the Physical and Optical Societies' Exhibition in January 1935 as the Marconi N40. Around one thousand of the N40 output tetrodes were produced, but MOV (Marconi-Osram Valve) company, under the joint ownership of EMI and GEC, considered the design too difficult to manufacture due to the need for good alignment of the grid wires. As MOV had a design-share agreement with RCA of America, the design was passed to that company. RCA had the resources to produce a workable design, which resulted in the 6L6. Not long after, the beam tetrode appeared in a variety of offerings, including the 6V6 in December 1936, the MOV KT66 in 1937 and the KT88 in 1956, designed specifically for audio and highly prized by collectors today.
After the Phillips patent on the suppressor grid had expired, many beam tetrodes were referred to as "beam power pentodes". In addition, there were some examples of beam tetrodes designed to work in place of pentodes. The ubiquitous EL34, although manufactured by Mullard/Phillips and other European manufacturers as a true pentode, was also produced by other manufacturers (namely GE, Sylvania, and MOV) as a beam tetrode instead. The 6CA7 as manufactured by Sylvania and GE is a beam tetrode drop-in replacement for an EL34, and the KT77 is a similar design to the 6CA7 made by MOV.