A valve power amplifier stage for an audio amplifier, which amplifier stage has at least one thermionic valve and a polarization circuit for polarizing the anode, the cathode and the control grid of the thermionic valve so as to determine the maximum output power. The polarization circuit comprises an adjusting module for adjusting the feed voltage of the anode and the polarization voltage of the control grid so as to adjust the maximum output power of the thermionic valve, and a potentiometer coupled with the adjusting module for allowing a user to control the adjusting means so as to select the desired maximum output power from a plurality of predetermined output power values.