A travelling wave tube for two operating conditions: continuous low-level operation and pulsed high-level operation. This travelling wave tube comprises a double electron-gun 1 and 2 and a double HF structure 5 and 6 with a transition 7. Across the double delay line thus constituted, there is coaxially injected a double electron-beam, one cylindrical the other tubular, this under conditions which satisfy the generation of the two aforesaid operating conditions. Tubes of this kind have a particulary significant application in countermeasure or transmission systems of multipurpose radars.