Beam tetrode

A beam tetrode, sometimes called a "beam power tube", is a type of tetrode vacuum tube (or 'valve') with auxiliary beam-focusing plates designed to augment power-handling capability and help reduce unwanted emission effects. These tubes are usually used for power amplification, especially at audio-frequency.

The problem of secondary emission from the anode (US: plate) in the tetrode tube 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, two EMI engineers, Cabot Bull and Sidney Rodda, produced and patented an alternate design in 1932. Their design had the following features (compared to the normal pentode).

The design is today known as the beam tetrode but historically was also known as a kinkless tetrode, since it is a four-electrode device without the negative resistance kink in the anode current vs anode voltage characteristic curves of a true tetrode. Some authorities, notably outside the United Kingdom, argue that the beam plates constitute a fifth electrode.

The EMI design had the following advantages over the pentode:

Disadvantages of the beam tetrode were:

The 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 resulting 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.

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