Although largely superseded by transistors for small signal amplification, the thermionic valve (or tube) still finds use in high-fidelity audio amplifiers, in guitar amplifiers and in high-power radio and television transmitters, where their power to size scalability has advantages over solid state devices. In the undergraduate experiments detailed here students construct a triode inside a vacuum chamber, and investigate the effects of different designs on signal gain. The characteristics of their triodes are measured to determine the optimal condition for signal amplification. The triode is then used to amplify an audio signal to determine the frequency response, and they can listen to the results via headphones using an input signal of their choice. This experiment hence introduces students to vacuum technology, to electron beams and to analogue electronics.