Controlling the performance of the thermionic tube (102) having a cathode (103), a plate (104) and a grid (105) is disclosed. The tube is configured to provide amplification of an audio derived signal (106) and is arranged to apply a grid bias voltage to the grid. The absence of an input audio signal is detected whereafter output current between cathode and plate is measured to identify actual output current. The actual output current is compared against a preferred output current and the grid bias voltage is adjusted so as to bring the actual output current value towards the preferred output current value.