At your risk. Take safety precautions. Any voltage or current can be dangerous. A fuse in series with a battery must be used for fire hazard protection.
It is a pull up bias suitable for low level electron tube amplifiers operating with very few volts at the anode.
On around 2000 meeting with Sean Davies in London, Sean Davies mentioned using electrometer electron tubes for condenser microphones.
Some time after back to Athens a CV2269 electrometer tube was investigated for its suitability for a mic booster amplifier.
Initially on breadboard just 2 batteries were used and a tiny microammeter from a broken JVC Nivico 9425W radio. Nothing else, no mic etc.
A grid resistor had been connected to either of the filament leads.
But filament leads are connected to the 1.2V battery.
By connecting grid resistor to the +1.2V terminal anode current dramatically increased.
At some point it was observed to great surprise that leaving out the anode battery and short circuiting the corresponding amplifier terminals made the microammeter needle move, ie electron tube amplifier was operating and there was anode current. Connecting grid return resistor to the positive terminal made an important difference.
So this can be an amplifier with 1.2V battery connected to filament and same battery used as anode supply.
Later on it was realized after Hliana insisted that grid should not be biased negative that in fact by pull up bias (this time with a resistor from grid to either anode or Vb) grid still remains negative but less so making operation possible with almost any electron tube and very low anode potential. Even for 12AU7 (ECC82), 9-12 volts at anode is fine to more than enough.
Smoothness of sound, low electron velocity, low temperature, posiblilty of deliberate cathode under heating at such low anode potential creates a low noise front end amplifier with minimum secondary electron emission effects.
Perhaps now it is time to go back to CV2269 and use the Pleiades V6 schematic to make a hardwired mic booster amplifier of such low power consumption that it can be portable and operate for perhaps days with just an AA battery. Current drain at 1.3V is less than 13mA (including of course cathode filament).
Low anode current (of the order of 15μA) has the added advantage of amplifier reducing bass heaviness due to Fletcher Munson, voice effort curves, and the proximity effect. Thereby assisting in flat frequency response from producer's brain to listener's brain.
Use of military signal transformers as suggested by Lefteris much earlier can further assist in this direction. For example materials such as Mumetal? where permeability decreases with frequency thereby creating a gentle slope high pass filter. Or using in shunt with microphone a Pleiadws (R,L) gentle slope low cut or high pass filter. (By gentle slope less than 6dB per octave is meant.)
World class microphones such as Fostex M80RP, Sennheiser MD441 etc can be connected to such amplifier with the advantage of having a ready produced vocal sound quality without the need of other analog or digital signal processing.
Another advantage is that it sounds loud getting advantage of the psychoacoustic properties of electron tubes with just one electron tube due to the low supply rail. So without having to resort to driving many electron tube stages at mastering or earlier stages of production for a rich vocal sound. The low gain of the Pleiades V6 CV2269 (V7?) mic booster insures that following mic preamplifier will not be overloaded. So even operational amplifiers may be used which otherwise can be unsuitable for live music production. [Hamm]
The Pleiades V7 combines the elegance and excellence of the Neumann U47 schematic [BBC] and the RCA BA-2C mic booster amplifier with the advantage of battery portability and nearly pocket size compactness. [Euroelectron]
A possible disadvantage is not as world class low noise as its bigger brother the Pleiades V6 EF183 or Nuvistor 7586 operating at 3.6V including heaters and anode circuit.
See also:
The Pleiades bias - euroelectron
Operating features of the Audion - E. H. Armstrong
(Flat frequency response from producer's brain to listener's brain) Sound Picture recording and reproducing characteristics - D. P. Lowe, K. F. Morgan - Journal of the society of motion picture engineers
Tubes vs transistors (vs op amps), Is there an audible difference? - Russel O. Hamm - JAES
http://downloads.bbc.co.uk/rd/pubs/reports/1954-23.pdf
(This includes the schematic of Pleiades V6) http://euroelectron.blogspot.com/2018/03/pleiades-v6-offspring-of-rca-ba-2c-and.html
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