Beam tetrode

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 control and screen grids were wound so that the pitches were the same and the wires were in alignment (the pentode used different pitches).
• A pair of beam-forming plates was added at the two ends of the oval grid structure to focus the electron stream into a pair of beams 180 degrees apart (the pentode added a third grid). These plates are normally connected to the cathode.

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 anode constitutes a fifth electrode.

The EMI design had the following advantages compared to the pentode:

• The screen grid current was about 5-10% of the anode (anode) current compared with about 20% for the pentode, thus the beam tetrode was more efficient.
• The design introduced significantly less third-harmonic distortion into the signal than did the pentode.
• The design produced a greater output power compared with a similar pentode.

The beam tetrode was not without its disadvantages:

• It had higher intermodulation distortion than the pentode, though this could be eliminated with the application of negative feedback, or (more usually) by adopting an ultra-linear design in a push-pull circuit. This connection links the screen grids to taps on the output transformer.
• The beam tetrode required a higher control grid voltage than the pentode and thus required a higher-gain driver stage preceding it.
• The beam tetrode had a tendency to oscillate if the circuit was not designed properly.

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.

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. Second-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.