Anode characteristic curves of the RCA 24-A tetrode vacuum tube. It shows the plate current ("I"b) as a function of plate voltage for different values of control grid voltage ("E"C2). The lowest curve shows the screen grid current ("I"C2). This early tetrode illustrates the negative resistance "kink" in the plate current (labeled "values unstable") caused by secondary emission of electrons from the plate, that caused instability and parasitic oscillations in early tetrodes. Modern tetrodes have a coating on the plate which suppresses secondary emission, eliminating the "kink".