is an electron tube a current controlled current source at grid potential higher than space potential?

There is current flow between cathode and grid if an external path exists between them since the cathode is more positive from the grid due to lack of the emitted electrons.


The space potential of the grid is negative wrt cathode if we apply heater voltage so that electrons are emmited. (Always connect a suitable fuse in series with a voltage source eg heater supply for safety).