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http://www.bonavolta.ch/hobby/en/audio/t_bas7.htmThese tubes are tetrodes with two more plates negatively charged that will concentrate and direct the electron flow to the plate.
This system allows a higher efficiency and is found on power tubes (KT88, 6550, 6V6GT, ...).
The beam plates have usually the same voltage as the cathodes and this often by internal wiring in the tube.
