Electrometer tube applications seem to have different requirements from low level AC amplification of signals.
Electrometers need to amplify DC and have very low leakage so that the charge on grid remains constant to be measured.
Signal preamplifiers, for example microphone ones have an always quickly varying potential on the grid. The music signal itself.
But low plate voltage principles seem to apply to them too as the give a smooth and noise free sound. Examples are Pleiades V series battery tube preamplifiers.
On them an anode to grid high megohm resistor is used. It makes the grid an electron accelerator and the signal to be amplified is connected there too through a capacitor. Grid accelerator or rather electron brake release (see other euroelectron posts on self assumed negative grid potential and positive bias neutralization). This makes an ordinary tube to operate normally at a typical anode voltage of 4 volts.
The typical 1μA current on the anode to grid resistor as on the Pleiades V4 seem to be a smooth current and may not contribute to noise as higher voltage practice.
Underheated cathodes are naturally employed too. More on this on previous euroelectron posts and of course Neumann U47 with its underheated VF14 electron tube.
The input impedance of Pleiades V4 is typicaly 100KΩ. Is this due to miller effect from the added Rag, resistor from anode to grid? 100KΩ is not a bad thing. Electrometers must have almost 1 million million ohms input impedance. 100KΩis fine as a load for the secondary of the input transformer damping any high frequency resonance.
Increasing voltage and transconductance may increase ionization and positive? or negative? grid current which may make a worse effect on noise.
It may be interesting to vary the cathode resistor until no potential difference is observed on Rg. Does this mean no grid current?
On Pleiades V4 the cathode is grounded.
It may be nice to build a hardwired pre preamp where cathode resistor, heater series resistor are variable. And Rag variable too. Then careful tuning - listening tests while protecting our ears.
The above mentioned variable cathode for no grid current adjustment may need greater plate voltage.
It is mentioned in the Victoreen paper that plate voltages below 4.5V are great.
The Pleiades preamplifiers with Rag can operate at 4.5V anode voltage with a close to 100μA anode current, great sound, low noise.
Further experiments are needed, for the time being it may be the quietest amplifier on the planet.
Reference: Electrometer tubes for the measurement of small currents - John Austin Victoreen
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