A beam tetrode (also referred to as a "Beam Power Tube") is a type of vacuum tube specially designed to produce greater output power than a similar pentode. It has found extensive application in power amplification.
The problem of secondary emission in the tetrode tube (valve) was solved by Philips/Mullard with the introduction of a suppressor grid to produce the pentode construction. 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. 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 compared to the pentode:
The beam tetrode was not without its disadvantages:
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 - the result was the famous 6L6. Not long after, the beam tetrode appeared in a variety of offerings, but for power audio purposes, the best examples were produced by the MOV company - the KT66 in 1937 and the KT88 in 1956. This latter tube was never bettered, and both are still manufactured today for discerning audiophiles (but no longer by MOV which ceased production in 1988).
Interestingly, many tubes that are described as pentodes actually turn out to be beam tetrodes. The ubiquitous Mullard EL34 (6CA7), although manufactured by Mullard as a pentode, was also produced by many manufacturers around the world as a beam tetrode instead. Even Philips/Mullard themselves were not immune; several examples of Mullard-marked ECL82s (a signal triode plus low-power pentode intended for single-ended operation) have turned out to contain beam tetrodes.
The most common beam tetrodes of all time were probably the 25L6, 35L6, and 50L6, and their miniature versions the 50B5 and 50C5, which were to be found in millions of All American Five AM radio receivers.
In military equipment the 807, with a rated anode dissipations of 25 watts and operating at a supply voltage of up to 750, was in widespread use as the final amplifier in transmitters of up to 50 watts output power and in push-pull applications for audio. Large numbers entered the market after World War II and were used widely by radio amateurs in the USA and Europe through the 1950s and 1960s.
The beam tetrode produces the lowest distortion of this class of tube by producing significantly less third-harmonic distortion, and lower intermodulation distortion when used in ultralinear mode. Even-harmonic distortion is automatically cancelled in a push-pull design. The beam tetrode also lends itself to being operated as a triode (by connecting its screen grid to its anode), and in this mode functions more efficiently than a pentode operated in the same manner. An alternative was to connect the screen-grid to the control grid and use the device in a push-pull amplifier, a configuration known as 'Class-B zero-bias'.