At your own risk.
A Pleiades pre amplifier schematic, similar to the Pleiades K117 mic head (booster) amplifier for iPad or other mic in devices but with an electron tube:
__________________
| | to iPad mic in
2MΩ [] __|__ a
| 7586
------| |------- - - - - g
C ____ c
in | ^^^ h
____________|______________
The 2MΩ pull up Pleiades bias resitor brings the grid to a higher, closer to 0 negative value. The electrons are then free to arrive to the anode.
So it is possible to operate the electron tube with just 1.5V at the anode.
Without Pleiades biasing the grid potential with respect to cathode ie the grid bias is typically -700mV due to billions of electrons that have left (have been emmited) from the heated cathode. So the electron tube anode current is cutoff.
The iPad or any other electret mic powering device has an internal power supply resitor. It supplies power and acts as a load resistor (output voltage developing resistor) for the electron tube or other active device.
Heater supply can be with a battery. All extra attention should be exercised to the inclusion of a suitable fuse in series with the battery for safety.
See also:
yesterday's euroelectron posts
The Pleiades Bias
Pleiades V6 schematic
Operating Features of the Audion - E. H. Armstrong
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