(There is another euroelectron post called The Pleiades Bias.)
Even when nothing else is connected to an electron tube but heater voltage, the electron tube becomes a source of EMF (voltage) between cathode and grid.
A typical value can be -700mV, ie the grid being negative wrt cathode. A possible cause is the cathode becoming positive by emmited electrons.
The above is very easy to verify possibly for any tube. (A series fuse to heaters should always be used for safety). Any voltmeter should be suitable even one of not very high input impedance.
Curves of voltage vs current can be seen on Techniques for application of electron tubes in military equipment. The more current we try to get out of this source the less the voltage becomes, like any battery with internal resistance. The graphs show a straight line indicating a certain magnitude of internal impedance, the slope itself.
One can use the Thevenin theorem and obtain the magnitude of this internal resistance by deviding the open circuit voltage with the short circuit current [Neil Storey]. The short circuit current. It is obtained by directly connecting a micro ammeter between cathode and grid.
A typical value of internal resistance might be 20KΩ !
So when we try to operate the electron tube with a few volts at the anode it is not possibly as the grid is already at a cutoff value for such low anode voltage.
By applying Pleiades bias the grid potential is lifted up closer to 0V. A typical new equiplbium value of grid voltage can be -50mV with respect to cathode. The electron tube can now operate normally at only 3V anode voltage. Ultra low noise circuits as possible.
Below is the schematic of Pleiades V6 using a Pleiades bias resistor of typically 6MΩ from anode to grid. The 7586 Nuvistor electron tube may also be used. Another electron tube with great sound so far at low anode voltage using this principle is the 12AU7 or ECC82, see Pleiades V0, V3 amplifier.
Further reading on the classic Armstrong (inventor of FM) paper.
References:
Techniques for Application of Electron Tubes in Military Equipment - Rex Whitlock - page 1-11
http://www.dtic.mil/dtic/tr/fulltext/u2/142061.pdf
Electronics, a systems approach - Neil Storey
Operating Features of the Audion - E. H. Armstrong
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