A mechanical model is described which represents the action of a 3-electrode valve when coupled to an oscillatory circuit. A string moving around pulleys, one of which is driven by a small motor, represents by its motion the current through the valve. The oscillatory circuit is represented by a spring and a weighted pulley. By a mechanical coupling between the spring and a brake on the driving motor the effect of the intermittent current through the valve is obtained, and the system exhibits self-maintained oscillations. A note is added on a “negative resistance” effect obtained with a centrifugal friction governor.