In a vacuum triode (positive terminal to anode, negative terminal to cathode), how do the electrons flow from the control grid to the anode?
First, one rarely uses positive voltages on the grid. This will lead to a grid current (the grid catching some of the electrons) that most tubes don't like much. It distorts the input signal and heats the grid. The usual setup of a triode is the grid being somewhat negative in regard to the cathode.
Second, the grid voltages are usually 10..1000 times smaller than the anode voltages. Once near the grid, the electron &quot;feels&quot; the field created by the anode and accelerates towards it.
If the grid is too negative, no electrons will ever get near it.
If it is less negative, some electrons will reach it and then accelerate towards anode. Zero-voltage grid is &quot;transparent&quot; and the triode works like a diode.
Positive-voltage grid accelerates some more electrons that otherwise would return to cathode (and catches some of them like a small anode).