In 1926 Radio Broadcast magazine had a series of articles about the so called 'Equamatic' tuning circuit. The basic circuit was described in the September issue, starting on pg. 377 (pg 14 of the pdf):
http://www.vacuumtubeera.net/RadioBroad ... 926-09.pdf
Basically, the shaft of a variocoupler was connected to the shaft of the tuning capacitor. The coupling between the primary (ant. and ground) and the secondary was tighter at the low end of the dial. The coupling would loosen up as the radio was tuned to higher frequencies. This was to keep the tuning sharp across the dial. It wasn't long, however, before the Equamatic was replaced by a true bandpass circuit in the form of the 'Technidyne', in 1927:
http://www.theradioboard.com/rb/viewtop ... technidyne
In the December 1926 issue, the basic 'Equamatic' idea was applied to a regenerative circuit. The idea being to obtain automatic regeneration control. pg. 169 (pg. 30 of the pdf):
http://www.vacuumtubeera.net/RadioBroad ... 926-12.pdf
Oddly enough, I found a patent, granted in 1934, that seems to show the same type of circuit from the December 1926 issue of Radio Broadcast magazine. Looks as if the idea was stolen:
The Equamatic tuner idea seems to have been lost to history, but it's interesting. I think so anyway.