Most Pleiades mic booster front end low level amplifier experiments are performed on low anode current and low anode voltage, low bias voltage.
It is precisely these 3 regions that are not recommended for military applications. The non recommended regions are:
Less than 50V at plate
Less than 400μA plate current
Less that -1000mV grid bias
The Neumann U47 VF14 amplifier is almost...
The reason given in the excellent [Whitlock] book is unpredictability and closeness to electron emmision induced contact potential at grid. This is one of the very few books that explains the presence of negative potential (negative self bias) on control grid as soon as electrons are emmited by the cathode.
Nevertheless in practice, on listening tests on the usual Pleiades setup, signal path, the unrecomended operating conditions seem to be very suitable for mic booster low level (front end) pre preamplifiers. Both in terms of big, natural sound and very low noise. Possibly the only unpredictability is great sound. See previous euroelectron posts. See also underheating of the cathode as done on the Neumann U47 VF14 electron tube headamplifier. For example the Pleiades V6 amplifier tests.
A usual signal path is:
Speech level singing voice - moving coil mic - Pleiades V6 headamp operating at low anode voltage, low grid bias, low anode current - Sony TC-D5 pro at mic in, mono mode - Sennheiser HD580
Plate potentials are usually much less than 6V, grid bias more than -100mV, anode current less or much less than 100μA.
It's great fun playing with electrons at such low potentials and currents. Nature seems very generous and quiet at calm, low electric potential low plate current, low temperature conditions. Nevertheless all safety precautions must be followed such as a fuse in series with a battery for safety etc.
Reference:
Application of electron tubes is military equipment - Whitlock - figure 3.3
https://archive.org/details/DTIC_AD0142061
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