Is reduced electron emission better for preamplifiers?

On the world class Neumann U47 microphone it is.


The VF14 is specified at 60V heater voltage:


http://www.radiomuseum.org/tubes/tube_vf14.html


George Neumann used it at only 35V, substantially under heating the cathode.


http://gyraf.dk/gy_pd/g7/u47.gif


And this is on the mic used by Leonard Kohen, Frank Sinatra, Beatles, Madonna?


The same principle of low cathode temperature is used on the Pleiades V series prepreamps.


There may be many reasons but one of them could be that fewer boiling electrons, ie less electron cloud induce a less negative charge to the grid?


It is easy to demonstrate this by carefully connecting heater voltage to an electron tube and without any other pin connected anywhere just measuring the cathode to grid voltage with a millivoltmeter. Surprise!


This effect is very important especially if the tube has to operate at only 3.7 volts on the anode as Pleiades V4. There is also a high Megohm resistor from anode to grid to compensate this internal negative bias given by nature.


Are the above a reason why sometimes used electron tubes may be preferable in some applications? Their cathodes emit less electrons when they as supplied by the required heater voltage. So they behave as if it they were an new tube deliberately underheated.


One of the greatest electronic engineers of all time, Edwin Armstrong, his inventions included FM radio, knew that inserting a grid even without connecting it to anything reduces anode current.


References:


Operating features of the Audion - Edwin Armstrong


On preserving transconductance of electron tubes at anode potential as low as 3 volts - euroelectron blogspot