진공관
Vacuum tube musical instruments mean electronic musical instruments generating sound with vacuum tube-based electronic oscillators. Hammond Organ Company commercialized it in the late-1930s as Novachord (1939–1942) and Solovox (1940–1948). Especially, new designs introduced on Novachord — subtractive synthesis and frequency divider — were immediately followed by many manufacturers of electronic organs and polyphonic synthesizers during the 1940s-1970s. However, Hammond Organ Company did not adopt these on main products until the late-1960s, except for S series chord organ (1950–1966) and "Solo Pedal Unit" on RT series and D-100 (1949–1969).