Beam tetrode

The beam tetrode produces greater output power than a triode or pentode with the same anode supply voltage.

[6] 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.

In the UK, three EMI engineers (Isaac Shoenberg, Cabot Bull and Sidney Rodda) filed a patent on an alternative design in 1933.

[12][13] The EMI design had the following advantages over the pentode: The new tube was introduced at the Physical and Optical Societies' Exhibition in January 1935 as the Marconi N40.

[4] 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.

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.

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.

[16] This amplifier circuit links the screen grids to taps on the output transformer, and provides reduced intermodulation distortion.

Beam tetrode application circuits often include components to prevent spurious oscillation, suppress transient voltages and smooth out frequency response.