If 140μA is plenty of current for an electron tube microphone booster front end amplifier...(see 7596 Nuvistor electron tube at 140μA in a previous recent post)
Then cathode materials could be used that do not have high current emission but operate at low temperatures. Heater consumption must be then very low.
Are electrometer tubes made in this way?
The temperature of the cathode must be a limit to noise as the noise temperature of the anode internal resistance is the temperature of the cathode.
Many of these experiments and thoughts are derived from the Pleiades V6 schematic:
See also previous posts, and...
A study of noise in vacuum tubes and attached circuits - F. B. Llewellyn - Proc. IRE - February 1930
The first few chapters of Fundamental of Radio-Valve technique - Deketh - Philips Technical library
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