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. A tetrode is a two-grid vacuum tube. ... Koninklijke Philips Electronics N.V. (Royal Dutch Philips Electronics Ltd. ... Mullard Limited was a British manufacturer of electronic components. ... A grid used in a thermionic valve (also called vacuum tube) to suppress secondary emission. ... In electronics, a vacuum tube (American English) or (thermionic) valve (British English) is a device generally used to amplify a signal. ...

In the UK, EMI 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 were 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.

(Note: diagram to be added) An electrode used in Thermionic valves (tubes) used to modulate the flow of electrons in the cathode to anode or plate circuit. ... A grid introduced into a thermionic valve or tube to greatly reduce the capacitance between two other parts of the electrode structure. ...

The design is today known as the beam tetrode and historically was also referred to as a 'kinkless tetrode'; since it was more or less a tetrode but without the negative resistance kink of a true tetrode; Purists may argue that the beam plates constituted a fifth electrode.

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

• The screen grid current was about 5-10% of the plate (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 a higher intermodulation distortion than the pentode, though this could be eliminated with the application of negative feedback, or (more usually) adopting an ultralinear design in a push-pull circuit. This connection links the screen grids to taps on the output transformer.

• The beam tetrode required higher control grid voltages than the pentode and thus required a high gain driver stage preceding it.

• The beam tetrode had a tendency to oscillate if the circuit was not designed properly.

The MOV valve company (who half owned EMI) considered the design too difficult to manufacture. 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 and later the KT88. This latter tube was never bettered, and both are still manufactured today for the more discerning audiophiles (but not by MOV). Intermodulation distortion: Nonlinear distortion characterized by the appearance, in the output of a device, of frequencies that are linear combinations of the fundamental frequencies and all harmonics present in the input signals. ... Negative feedback is a type of feedback, during which a system responds so as to reverse the direction of change. ... An electronic amplifier is a device for increasing the power of a signal. ... Pair of 6L6GC tubes; (l) General Electric version from 1960s, (r) current manufacture from Svetlana Electron Devices, Russia 6L6 is the designator for a vacuum tube introduced by Radio Corporation of America RCA United States in July 1936. ...

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, either as a beam tetrode or as a true pentode.

Even Philips/Mullard themselves were not immune. Several examples of Mullard marked ECL82s (a signal triode and low power pentode intended for single ended operation) have turned out to actually be triode/beam tetrodes.

The beam tetrode produces the lowest distortion of this class of tube by producing significantly less third harmonic distortion; 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 plate), and in this mode functions more efficiently than a pentode operated in the same manner.