A directed heater electron tube is like infinte electron tubes is parallel

This should be because the potemtial of the cathode varies from 0V to say 1.3V along its length, if the potemtial difference between its terminals is 1.3V.


So there is a different grid bias depending on which electrons and where.


It must have been read on the Fundamentals of Radio-Valve Techniques - Deketh.