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A traveling-wave tube whose circuit supports a backward wave having oppositely directed phase and group velocities has been found to exhibit narrow-band regenerative-type gain. For low beam currents the tube acts as a high-gain, high Q, voltage-tunable filter. As the beam current is increased, the tube breaks into oscillations whose frequency may be shifted to any point in the structure pass band merely by changing beam voltage. An analysis is carried out which predicts some of the small-signal operating characteristics of the backward-wave tube. Numerical solutions are presented for the start-oscillation conditions including circuit loss and space charge.