 The beam tetrode design was also undertaken to avoid the patents which the giant Philips firm held on power pentodes in Europe.Because this overall design eliminated the "tetrode kink" in the lower parts of the tetrode's voltage-current characteristic curves (which sometimes caused tetrode amplifiers to become unstable), M-OV marketed this tube family as the "KT" series, standing for "kinkless tetrode".A number of different KT tubes were later marketed by M-OV.