Variable-mu compressor
A tube compressor that reduces gain by shifting the bias on a valve whose amplification factor, mu, changes with it.
The trick is in the tube. In a remote-cutoff valve, gain falls off gradually as the grid bias goes more negative instead of cutting out abruptly. Let the detector drive that bias and gain drops the louder the signal gets. The result is a ratio that rises with level: a light touch on moderate material, a firmer hand on peaks. In other words, a soft knee built into the physics.
The Fairchild 660 and its stereo sibling the 670, designed by Rein Narma in the 1950s, are the archetypes. They weighed around 30 kilos, contained some twenty tubes and still turn up in mastering rooms. Used units sell for the price of a car, which explains why most people who say they used a Fairchild mean a plugin.