Thanks to the great work that Derk put into his great Extract Model tool, I helped him to refine and debug the application by tracing the challenging 6e5p beam tetrode. After 4 versions we managed to optimise the model:
The model fits really well including the kink section but given the saturation of the 6e5p tetrode at currents above 50mA there is a slight divergence as the valve cannot reach the same anode current at higher voltages.