The trick is getting as much of the electron tube sound...

as soon as possible.


This is one of the reasons why the front end (first stage) connected to the microphone operates with just 6V or less on the Pleiades V series electron tube prepreamps.


So we have the mic voice coil then the step up input transformer then the electron tube stage operated in class A with few volts at the anode and an anode to grid electron accelerating resistor. In fact it is a resistor that compensates the internal negative bias of the electron tube by the boiling electrons from the cathode. The cathode is underheated too as is done in the Neumann U47 prepreamp.


A second stage can be used to get more instatenious peak limiting


Or a third stage depending on how loud the input acoustic signal is.


References:


On preserving the transconductance of electron tubes at anode potential as low as 6 volts - euroelectron blogspot


Pleiades V6 schematic - euroelectron blogspot or Flickr