Around 1940, when radio was still in its youth, a company proclaiming itself “the fountainhead of modern tube development” introduced a unique electron tube. It was designed to perform multiple functions in a consumer receiver but was eminently qualified for use in broadcast “speech input” and sound reinforcement equipment. The characteristic that set it apart was that it was a pentagrid.
Designated the 6L7, this octal-based tube boasting five grids would, in souped-up form, qualify it for admission to the Radio Corporation of America’s elite broadcast group under the type number 1612.
RCA VitaVox Schematic Using the 6L7/1612
https://www.radioworld.com/news-and-bus ... een-twelve
This may be the only example that I recall seeing of a non-VCA-based feedforward compressor/expander.