What the heck is a mellotron? Good question. A definition of this unusual
instrument is provided by an unusual website:
"The Mellotron is an unusual electro-mechanical keyboard instrument which
enjoyed a certain popularity/notoriety from the mid-sixties to the mid-
seventies. It's sound is difficult to describe since it is so varied. It works
by each key of the keyboard activating a tape recording of one note of an
instrument. This could be a flute, a choir, a whole orchestra - anything. In
the early to mid-sixties when they first emerged this was pretty staggering.
The contemporary keyboards being some cheesy electric organs and unconvincing
electric pianos. Unfortunately they cost as much as an average house and were
very heavy and unreliable. This restricted their use to the recording studio
and a few very rich or very daft bands."
So, basically, it's an old-school analog keyboard, using tape loops for each
key, instead of generating the sound thru electronics. The mellotron was used
most prominently on Spaceboy.