The "Echoplex" used an infinite tape loop with a fixed record head and a movable playback head that allowed the producer to achieve the desired delay and echo at a particular moment on the tape. All you had to do to produce a longer delay was to move the playback head farther away from the record. Though mainly used to enhance guitar riffs, the echo effect had several applications. By using this technique, one could create a very futuristic sound. In fact, it was often used for a very UFO or science fiction sound that became popular in the seventies.
As the years went by, technology developed rapidly, yet the echo unit remained an effect in popular music. In fact, U2 guitarist, the Edge, perfected his minimalist style using echo rather than more modern effects like flanging and phasing. Even "ancient" devices like the "Echoplex" remain popular because they create a natural, muffled echo that is more real than modern digital echoes --Josh Blessing