Abstract: A travelling wave tube which amplifies a signal and includes an electron gun, a D.C. voltage generator, an electron accelerator and delay structure loaded with dielectric material. The electron gun is at approximately ground potential and emits an electron beam. The D.C. voltage generator generates a very high D.C. voltage. The electron accelerator receives the high D.C. voltage from the D.C. voltage generator and accelerates the electrons in the electron beam. The delay structure is at approximately the same high D.C. voltage as the electron accelerator and includes an input port and an output port, the signal being received in the input port, travelling through the delay structure, and being output at the output port. The rf input and output windows are insulated from the high DC potential of the delay structure by dielectric wave guides.