This is done by capacitively coupling the grid and connecting an accelerating few Megohm resistor from a positive potential to grid.
Reducing the heater voltage may prove further beneficial in actual low level signal amplification applications.
An explanation for the above improvement maybe the reduction of negative (thermal voltage?) potential induced to the control grid when the cathode emitted electron cloud is reduced. (The few Megohm resistor seems to act as a domestic vacuum cleaner, cleaning the area around cathode of excess electron steam cloud which induce a negative charge on grid wrt to cathode). It must be observed that a cathode becomes positive with respect to grid as soon as it heats up and emits electrons. The electrons escaping it immediately make the cathode positive with respect to grid or the grid negative wrt cathode as can be verified with a voltmeter of even a low input impedance.
The few Megohm resistor from a positive potential to grid partially cancels this effect allowing a normal anode current to flow at very low plate voltage.
These principles are applied to the Pleiades low level electron tube battery operated preamplifiers. The Pleiades amplifiers development is open source. Cathode underheating can be observed on the Neumann U47 microphone schematic which shows the VF14 supplied by 35V instead of 60V.
On the Pleiades V series microphone amplifiers an EF183 triode connected vary μ pentode is employed. The cathode is connected to ground. A positive potential is applied by a resistor of a few megohms from anode to control grid. It may be called a grid electron acceleration potential bias resistor or electron brake release. The grid may still be negative but less negative (positive wrt previous value) than without the electron brake release resistor. Other common tubes such as ECC82 operate satisfactorily at slightly higher potentials. The microphone is impedance matched to the grid by an input transformer. The secondary is coupled through a capacitor to the control grid.
The use of anode and heater voltage as low as 3V appears to further reduce electron tube noise on Pleiades V4 for example.
Possible explanation is:
Use of a simple pure power supply of just one low voltage battery. This battery supplies everything and is inside the amplifier.
Reduction of ions formed by gas present on the tube.
Reduction of thermionic grid emission due to heating of grid by cathode filament power.
Reduction of ions emitted by cathode.
Reduction of photo electrons emitted from grid under the action of light from cathode.
Reduction of photo electrons emitted by the control grid under the action of soft X-Rays produced by the normal anode current.
References:
The Use of Multigrid Tubes as Electrometers - Prescott
Operating Features of the Audion - E.H. Armstrong
https://earlyradiohistory.us/1914opr.htm
Schematic of Neumann U47 microphone showing application of reduced heater voltage.
Pleiades V6 schematic - euroelectron blogspot
Other Pleiades open source schematics euroelectron posts
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