In a vacuum triode (positive terminal to anode, negative terminal to cathode), how do the electrons flow from the control grid to the anode?

Question

In a vacuum triode (positive terminal to anode, negative terminal to cathode,) once we apply the positive charge to the control grid it will start to attract free electrons, which are released from cathode due to thermo-ionic emission.

• How do the electrons flow from the control grid to the anode , since both of these terminals are positively charged?
• What is the driving force that helps in moving the electrons from the control-grid to the anode?
• How come placing the control-grid is effective, when there is voltage required to move free electrons from the cathode to the anode? Zero voltage on control grid = voltage required to move free electrons from the cathode to the control grid + voltage required to move electrons from control grid to anode.

Please help me to understand better.

Answer 1

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 "feels" 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 "transparent" 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).