Using the functional reflection method the nonlinear dynamics of the TWTgenerator with delayed feedback is considered. It is shown, that if the feedback factor is small, the region of linear stage beam-wave interaction assumes a character of narrow-band filter. So, the quasi-monochromatic signal is formed to the begining of the nonlinear stage of interaction. The resonance character of the beam instability leads to the strong dependence of the field dynamics on the instantaneous signal freguency.
