The Pleiades Electra II one electron tube class A headphone amplifier has the grid at a positive potential. About 9 volts!
Electron accelerator grid.
The music signal is applied to the screen grid.
This is the space-charge tetrode operation.
It is described at3-9 page 66 of Reich.
There is a reference to the patent US1558437, I.Langmuir, 1913!
For some reason the Pleiades Electra 2 sounds much better with the cathode underheated. at about 9 volts. So everyhing is powered by a 9 (or even less) volt battery including the anode circuit.
Reference:
Principles of Electron Tubes - Herbert J. Reich University of Illinois - McGraw Hill -3-9 page 66 The Space-charge Tetrode
Pleiades Electra 2 schematic
12K5 datasheet
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