If this external current [Pleiadss V6 schematic] is then it may be good news meaning that it produces no shot noise [Pearson]. Space charge or (charge sea) make shot noise tending to zero as there is not descrete emission of electrons. The sea of electrons is already there.
At the same time Pleiades or anode to grid bias increases anode current, decreases internal plate resistance, increases transconductance. (Therefore thermal anode internal impedance noise. The input impedance is also reduced. Measurements according to the classic paper of Pearson should be made.)
The electron tube can be powered by just a few, typical 3V, at the anode.
The heater voltage therefore can be reduced as the cathode temperature can be lower for a space charge saturation effect.
Secondary emission is reduced, positive ions too and therefore most sources of electron tube noise should be eliminated or greatly reduced.
More measurements are needed according to this classic paper that says so much. [Pearson]
Back to the 1930's where electron tubes were as popular as today there was so many fantastic research and papers written by the pioneers of electronic engineering or control of electron flow engineering or art. For a formal definition of electronics [Gray]
References:
Fluctuation Noise in Vacuum Tubes - G. L. Pearson
https://archive.org/details/bstj13-4-634
Applied Electronics - T. S. Gray - MIT
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