An analysis of the structural parameters of globular clusters (GCs) carried out by using data contained in the Harris Catalogue indicates a clear missing coincidence between the maximum of the W0-distribution and the onset of gravothermal instability. This question, firstly outlined by Katz, is a still open problem because the coincidence is necessary if considering the dynamical evolution of the GCs population. In order to tackle the problem, a selfconsistent thermodynamical model of GCs is developed by considering a different approach with respect to the King one in calculating the distribution function which must be solution of the Fokker-Planck equation. Requiring a Boltzmann form of the distribution function expressed by an Hamiltonian function leads to a solution containing additional term in the total energy. This new term, called effective potential, simulates the effects of the tidal forces induced by the hosting galaxy on the phase space limitation in the velocities of stars. The influence of the effective potential on the virial theorem will be described, together with the related consequences on the insurgence of thermodynamical instability. Interesting results implying a different critical point for the onset of gravothermal catastrophe now in accordance with observational data are presented, as well numerical simulations confirming the presence of the effective potential with the predicted form.