An electron tube includes: a microwave structure having an evacuated envelope including two ends, the microwave structure being at a reference potential, an electron gun including a cathode for providing a beam of electrons, along an axis, at one end of the evacuated envelope, an electron collector for gathering electrons of the beam at the other end of the evacuated envelope, and at least one high-voltage power supply for applying to the cathode a negative high-voltage potential with respect to the reference potential. The tube includes between the cathode and the microwave structure a device for extracting the positive ions including at least one electrode carried to a negative potential with respect to the reference potential so as to extract positive ions from the evacuated envelope, these positive ions being produced by the impacting of the electrons of the electron beam with molecules of residual gas in the evacuated envelope. The invention has application to microwave electron tubes, klystron TWT etc. using a cylindrical electron beam.