Abstract: A vacuum tube for amplifying an r.f. signal includes an assembly containing a cathode and grid for current modulating an electron beam derived from the cathode. One of the electrodes of the assembly includes a slow wave structure approximately resonant to the frequency of the signal. A cavity resonant to the frequency of the signal, positioned between the grid and a collector for the beam, is coupled to the beam. In one embodiment, the slow-wave structure is mounted in a support for the grid, while in a second embodiment, the grid is configured as plural, parallel meander lines forming the slow-wave structure. In the latter embodiment, the beam is preferably annular and the meander line geometry, in certain modifications, is adjusted so that there is a relatively small electric-field variation with radius over the portion of the grid through which the annular beam passes. In a further embodiment, the grid is configured as two interlaced spirals, driven by complementary replicas of the r.f.