A positive grid wire does not nessesarily mean a positive or negative potential around grid

In the Pleiades V6 schematic the potential at the grid wire may be -50mV with respect to cathode.





But if heater voltage is further decreased or Rag decreased it may become 50mV.


The potemtial between the grid wires is not nessesarily the same [Deketh].


Reference:


Fundamentals of radio-valve technique - Deketh - Philips Technical Library