A QRP transmitter is designed to transmit at really low powers. The accepted upper power limit for QRP transmitters is 5 W, at least for modes like CW using FM or AM modulation.

To wit, we present this compact two-tube “spy radio” transmitter. From the look around his shack — which we love, by the way — [Helge Fykse (LA6NCA)] really has a thing for old technology.