The mellotron sound in the collective consciousness of us baby boomers is the string sound - Nights in White Satin.
Mellotron had a 30 second tape per key, so you couldn't sustain it for longer than that.
I got to play with one in the 70's when I rehearsed with a band that was never to be - Detroiters may remember a band called The Third Power (contemporaries of SRC and Wilson Mower Pursuit). Founding member guitar/singer was putting together a larger unit and reached out to me as keyboard player - rhodes and mellotron (he had a mellotron). We rehearsed for a few months, but nothing came of it. ---------- The Iceman