The invention concerns a micro-wave tube comprising at least an electron gun (10) for transmitting electrons to an input waveguide (11) through which they pass transversely and wherein an input signal (Pe) to be amplified is to be propagated, injected at an input end (113), said input waveguide (11) being followed by an output waveguide (12) likewise passed through by the electrons and from which is extracted the amplified signal (Pd) at an output end (119). The electrons are directly density-modulated in the input waveguide (11) by the input signal (Pe) and accelerated, when they come out of the input waveguide (11), by the output waveguide (12) brought to a potential higher than that of the input waveguide (11). The invention is applicable to broadband power micro-wave tubes.