The helical magnetic structure of B20-type compounds caused by the hierarchy ofexchange interaction: main ferromagnetic exchange interaction, J, together with the antisymmetric Dzyaloshinskii-Moryia (DM) interaction, D, stabilize the helical (homochiral) structure in these systems below Tc, and the weakest cubic anisotropy direct the spin helix along the main axis of the structure. Mn1-xFexGe compounds undergo the transition from the helical state into the ferromagnetic at the critical concentration xc = 0.75. This fact could be related to the unusual behavior of the spin-wave stiffness of these compounds. Recent studies of the spin wave stiffness in FeGe have shown the applicability of SANS in studying the dynamical properties of helical magnets. Moreover, it is shown that Bak-Jensen model has a restricted application and the energy of cubic anisotropy should be taken into account for pure FeGe. The general aim of the proposed experiment is to measure the spin-wave stiffness of the Mn1−xFexGe and its evolution with the temperature and concentration of substituted element on the Fe-rich side of the phase diagram.