The 436 series of compressors were all-valve, vari-mu designs, meaning that the amount of compression taking place depends on the level of the signal that is fed in.
Early Altecs were inexpensive and basic; the original 436A model had fixed parameters and no user controls whatsoever. 1958 saw the release of the 436B, which featured an input gain control, then, a few years later, the 436C included threshold and release-time controls. These modifications came about as a result of Altec’s engineers discovering that the cheap 436s were being modified by studio engineers to make them more suitable for processing pop music in recording studios.
The Altec 436Bs that EMI bought in 1959 (three years before The Beatles’ first sessions) were heavily modified to include an output attenuator and ‘recovery’ switch, which controlled the compression release-time.
A unique feature of EMI’s six-position recovery switch was the Hold setting, which prevented a compressed signal from returning to its original level when in use.
