Abstract: A microwave tube provided with at least one axial part, supported by at least three spacers and fitted cold into a coaxial envelope. The internal surface of the envelope or the external surface of the part comprises a relief designed to block a spacer so that the assembly formed by the part and the spacers is clamped in the envelope. The blocking is obtained by a relative shift between at least one spacer and the surface comprising the relief. The axial part may be the gun or the collector of a longitudinal-interaction tube or the delay line of a travelling-wave tube.