Triode-based circuits are still the less expensive and more reliable high power sources for industrial applications between 13.56 and 40.68 Mhz. Among them, self-excited oscillators feature the lower cost but with a frequency stability preventing them to address all ISM frequencies and applications. This article demonstrates the possibility to model numerically self-excited oscillators with power triodes using a class-C practical circuit. Advantages over existing analytical calculations and graphical derivations are shown. A method is proposed to evaluate the frequency stability in presence of internal imperfections and external perturbations. These tools offer the possibility to design and analyze more efficiently this family of oscillators especially for demanding applications regarding frequency stability.