The Mellotron, built and sold by Mellotronics, Ltd., was based on an earlier instrument, the Chamberlin MusicMaster, which used recorded notes from members of Lawrence Welk’s band—hardly the hippest sounds on the scene when the Mellotron MK1 debuted in 1963. By the time of the MK2, however, the device developed into a powerful multitimbral machine, with a dual keyboard, “containing more than 70 3/8-inch tape players, a reverb unit, amplifiers and speakers.”

Also:
http://www.openculture.com/2013/06/rick_wakeman_tells_the_story_of_the_mellotron_the_oddball_proto-synthesizer_pioneered_by_the_beatles_.html