(Such Pleiades experiments have been usually taken place with an EF183 electron tube triode connected). (An ECC82 12AU7 may be used too, it works very well with the Pleiades bias on various Pleiades preamplifiers operating with 6 volts at the anode).
When an 8MΩ resistor is connected from anode to control grid and the cathode is cold the potential of the grid becomes the potential of the anode.
So if the anode is supplied by 3V, so is the potential of the grid.
As soon as the cathode heats up the control grid potential drops to a much smaller value and may even become negative.
This is what we want. The potential is taken care of by nature itself.
It shows there is a current through this external resistor. (Since there is voltage drop at this resistor there must be current through it).
This possibly means some of the cathode electrons are sucked by the grid and sent externally to the anode circuit through the high Megohm resistor.
But the presence of this slight negative bias makes the tube operate at a very low anode voltage.
If we do not connect the high Megohm resistor then the self summed potential difference from cathode to grid is many hundred of millivolts. The cathode is positive as it missed electrons to the electron cloud. So the grid is negative with repeat to cathode. And there is no anode current for low anode potentials.
See also part 3 on the next euroelectron post.
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