How do electron tubes can act as automatic volume controls

This may even take place inside the VF14 electron tube of a Neumann U47 world class microphone.


A simpler explanation than pepreviously posted on euroelectron is the following.


From page 421 of Communication Engineering - Everitt. "If the grid is free , electrons can flow to it from the cathode but not in the reverse direction (diode rectification effect?) and it will gradually collect electrons and become more and more negative until finally its potential may be sufficient to stop the flow of plate current."


Note that a change of plate or anode current in an electron tube affects the transconuctance (therefore gain) of the electron tube.


Of course the Neumann U47 has the high Megohm resistor connected to grid which gives a release time constant slowly discharging the grid if this effect happens. So possibly the gain is controlled at high levels but then comes back to normal as the acoustic stimulus is decreased.


On many Pleiades amplifiers a high resistor from anode to grid is used so that the electron tube can operate with just a few volts at the anode. This possibly gives a quicker release time.


References:


Communication Engineering - Everitt


Neumann U47 schematic


Pleiades V6 schematic