At your risk.
Take all safety precautions.
Always use a suitable fuse in series with a battery in order to avoid injury or a fire hazard.
A used 1H4 Tung Sol electron tube was used on Hager octal basse. (See older posts).
1.5V from an alkaline battery is feeding the heaters.
When heaters get hot, the grid becomes negative since cathode has become positive by losing electrons to the electron cloud.
When a miniature 12V battery is connected to anode, anode current become about 50μA which may be plenty for a mic booster pre preamp. See previous posts and schematic of Pleiades V6.
And this is a condition of grid at space potential since nothing is connected to the grid.
Grid can be made less negative by a high megohm resistor to cathode or to anode.
So the anode current can become much greater if needed for perhaps lower noise or more bass.
But then extra bass to listener's brain should be reduced by connecting to a microphone a Pleiades (R,L) gentle slope high pass filter.
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