This is on why a high Megohm resistor from a positive voltage supply (usually the anode itself) to the control grid of an electron tube is needed for very low voltage operation.
Below is a series of possible events.
Assuming the electron tube is cold.
The cathode is not charged.
At some point in time the heater circuit is closed and the cathode heats up.
Some electrons inside the cathode gain enough peak velocity to escape from the cathode. Like boiling water, water molecules escaping to steam, the analog of electron cloud.
The cathode has now more protons than electrons and therefore is positively charged.
We need to charge the grid with a similar but less positive potential otherwise the electron tube is blocked (electrons can't arrive to an anode of low potential).
Hence the need of a high (typically 4MΩ-8MΩ) resistor from anode to control grid.
The cathode is normally grounded and the input signal is supplied to the grid through a coupling capacitor so as not to upset the potential assumed by the grid.
An example circuit is the Pleiades V6 microphone head amplifier.
Reducing the heater voltage, ie cathode temperature makes an ever lower noise amplifier.
The idea of biasing the grid positive too (the grid potential or bias may still be a bit negative compared to cathode) comes from Hliana who insisted on this for freeing the electrons.
More on part 2 on the next euroelectron post. Reference:
Pleiades V6 schematic
On preserving transconductance of electron tubes at 3V anode potential - euroelectron
Operating Features of the Audion - E. H. Armstrong
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