Space charge effects (so-called negative-mass instability) as the reason for electron energy spread developing in the region of the electron drift between the cathode and the cavity of a gyrotron are discussed. The quasioptical gyrotron working at third harmonic that is developed at Centre de Recherches en Physique des Plasmas, Ecole Polytechnique Federale de Lausanne, is used to demonstrate the expected efficiency degradation.