At your risk. Protect your hearing.
It is an inductor connected in parallel with the output voice coil.
If say the voice coil of the headphone or loudspeaker is 800Ω this can be fine for an AC load but the voice oil usualy having a same order DC resistance produces a voltage drop in anode or collector voltage.
Including a coil there is much less voltage drop at DC. But at AC the impedance of the coil progressively increases so there is no shunt loss of music.
So we can have greater power with same battery, or same power with a less voltage battery.
Another way of looking at it is that the coil produces AC EMF (a changing magnetic field produces electromotive force across an inductor) which is added to the supply rail. So output voltage doubles. And of course a coil can also be thought of as an autotransformer with a 1:1 ratio.
Another advantage is that say from 4V plate voltage to 8V plate voltage the anode current increase can be dramatic.
It may be clearly seen how ingenious are hearing aid circuits. Aiming for the the best sound with fewest components possible, less size, less power consumption. Such designers must have been hundreds of years ahead of the time. They kept quality, keeping everything in class A (electrons flowing all the time).
A drawback could be a frequency response not down to the limit of 0Hz but it can be a good thing.
It has not been tried yet on Pleiades circuits.
How would it sound on Pleiades 2N3053, Pleiades V6 as output stage etc?
The schematic is here:
https://www.radiomuseum.org/tubes/tube_df650.html
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