Transistor Family Tree: [From left] Raytheon subminiature vacuum tubes such as the CK549DX dominated the market for hearing-aid tubes throughout the 1940s. In 1948, the CK703 point-contact transistor went into production, but proved too unstable to unseat tubes. The more robust CK718 alloy junction transistor debuted in late 1952 and rapidly superseded tubes in hearing aids.

CK718s that didn't have the noise and gain characteristics for hearing aids were relabeled CK721 or CK722, both introduced in 1953. The black CK722 was replaced by the blue metal style in 1955. The silver metal case followed in the late 1950s and sold through 1964.

Finally, smaller, better-performing transistors like the half-size CK784 were introduced throughout the late 1950s.