At last great information was found on a military manual.
Techniques for application of electron tubes in military equipment:
http://www.dtic.mil/dtic/tr/fulltext/u2/142061.pdf
It was found while searching on Google for electron tube contact potential.
On page 1-10 of the military manual starts a description of the effect described on previous Pleiades experiments. That the grid of an electron tube becomes negative as soon as the cathode is heated up. It was observed by chance. When applying heater voltage to an EF183 electron tube the potential between cathode and grid become -700mV. Without connecting anything else to the electron tube. Also when short circuiting from grid to cathode with a micro ammeter a relatively large current of the order of μA is measured.
It is possibly this current that the military manual calls initial velocity grid current. In a figure it shows the relationship between control grid current and the grid cathode potential difference resulting from the initial velocity of electrons emmited from the cathode.
More interesting information is provided. Then the usual methods of forcing this bias to change with a relatively small grid resistor, cathode bias etc are given.
On the other hand on typical Pleiades amplifiers the approach is different. A coupling capacitor DC isolates the grid from other circuits. So that for better or worse no DC current flows through the secondary of an input transformer for example. When heater voltage is applied and electrons are emmited from the cathode the electron tube effectively becomes a source of voltage or current between grid and cathode.
The typical -700mV potential at the grid can for example be changed with Pleiades or anode bias using a high Megohm resistor from plate or anode to grid. If for example we supply the anode with 6V we can then use a high Megohm resistor. A typical value is 6MΩ. So current out of grid and external towards the anode circuit is only 1μA.
The grid bias becomes closer to 0 volts and still negative. For example -70mV.
The electron tube can then normally operate with a very low anode voltage. For example 3 volts.
On the other hand if using a resistor from grid to cathode it would have to be very low to pull up the grid from the -700mV bias since the potential of the cathode is 700mV only higher than the grid. The very low resistor would severely reduce the input impedance. The grid would be forced to change potential by a low resistor to cathode. The circuit would not operate well at 3-6V anode potential as the anode current would be too low from a bias that is still fairly negative. The grid to cathode current would possibly be excessive causing noise.
On Pleiades experiments on breadboard when the grid few Megohm resistor was connected from cathode to anode, the anode current jumped to a healthy value and the amplifier came immediately to life with gain and ultra low noise. The anode voltage on preliminary experiments was 6V. Then it was found by chance while disconnecting the heater that at such conditions of low anode voltage a lower cathode temperature gives less noise, more signal, ie more S/N ratio. With Pleiades bias the input impedance becomes of the order of 100kΩ.
Preliminary experiments were done by connecting a dynamic Shure Unidyne B microphone through an Altec 4722 input transformer through a coupling capacitor to grid. Following experiments were done with a Grampian GR1/L ribbon microphone connected to a Pleiades input transformer on a Magnetec 073 nanoperm tape wound core. It was then confirmed that reducing the heater voltage increased the signal to noise ratio of the mic setup as heard on the headphones while singing. The song was Μες αυτη τη βάρκα - Hatzidakis. A Rsalistic disco mixer connected at mic input from the head amplifier was used to drive the Sennheiser HD-580. On other experiments the Sony TC-D5 Pro was used as the mic amplifier following the Pleiades booster head amplifier.
Using Pleiade bias and consequently a low anode voltage com equally a low heater voltage consequently (low secondary emission etc) a very low output ribbon microphone can sound big, natural and low noise.
An example is the Pleiades V6 microphone head amplifier.
Reference:
On preserving the transconductance of electron tubes at anode potential as low as 3 volts - euroelectron
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