Model of capacity of valve with conic rod, which is used for control of beam current of high voltage glow discharge electron guns, is considered. Because of complicity of valve details geometry, mathematical model of dozed item is improved by suitable choosing of empirical coefficients and its approximation. On the base of modeling and experimental data analyzing of dependence of valve exploitation characteristics from geometry parameters of dozed item is provided. Obtained results can be used for simulation of stability of operation of high voltage glow discharge electron guns inconsisting of technological equipment