The concept.
A high megohm resistor, typically 4 MΩ, connected from anode to grid makes electrons free so that the electron tube can operate at an anode potential of less than 6 volts.
Suitable tubes are ECC82 and even better an EF183 triode connected ie all grids except the control grid connected to anode. Tubes that operate best are very high transconductance (gm) and at the same time vari μ tubes therefore having remote cutoff. A perfect example is the EF183.
A typical value of input impedance of the Pleiades preamplifier is 100 KΩ which is ideal as a load for the secondary of a microphone step up transformer for example.
The heaters are supplied too with 3.6V, therefore the cathode is deliberately underheated. The low heater and anode voltage minimize any secondary electron emission effect, unblock the tube by negating the positive electron attracting cathode potential with respect to grid with the aid of Rag and ensure possibly the quietest amplifing operation ever achieved.
+3.6V
|
<
> 10KΩ
____ < ______________ out
4ΜΩ | |
< _|_
> EF183 triode connected
in ----| |-- <----- - - -
___
|
____ _______|_______________ 0V
The Pleiades preamplifier
To summarize, the grid should not be negatively biased 2 times; one internaly by what nature is doing and one externally of the electron tube if low voltage operation is desired.
Acknowledging Hliana Stalika for insisting that the grid should not be negatively biased inspired by a Montalk article relevant to preventing ourselves from negative bias and making us free.
Pleiades concepts are open source, public domain.
References:
Operating features of the Audion - Edwin Howard Armstrong
On preserving the transconductance of electron tubes at very low anode potential - euroelectron blogspot
The use of Multigrid tubes as Electrometers - J.R.Prescott
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