Grid Voltage when made negative, it will not allow the electrons from cathode to pass through, as electrons will start repealing from it hence this will decrease the electric conductivity. Whereas when grid voltage is made positive it will start attracting electrons and make it easier for electrons to leave the surface of cathode. Therefore, through grid voltage we are indirectly controlling the plate’s current. Also in triode valve we know that the maximum current is the saturation current, hence by controlling the grid voltage we are in a way controlling the saturation current. Whereas separation won’t make any difference as it is the voltage which is actually plays an important role in controlling the plate’s current. Changing the separation will only effect the amplification.

No matter what is the temperature of cathode is, at the end it depends whether grid allows the electrons to pass through to it or not. Therefore, it is the grid voltage which effects the saturation current.