A full range ESL does require a large voltage swing.

Quad ESL57 have a 300:1 step up ratio tranny and so if 25Vrms is needed
at the low voltage input coil, then without the tranny
the voltage required is 7,500Vrms.

Where does one get a linear 7,500Vrms from?


There's triodes, intended for color TV high voltage regulation (like
25KV), like the 6BK4, with mu around 1000. The plate could feed into an
inductor similar to the inductance of the transformer normally used (to
step the audio up to the high voltage in the usual setup). The plate
would also connect to the electrostatic speaker.