Echo, reverb, and tremolo are all quite different sounds.I had a horrible vision that it would be something like that, but I have never heard of one before and certainly never seen one. However back in the day, the early 60'sw we used to make our own using a short length of audio tape made up into a loop , running continuously past an erase head, a record head then 4 or 5 playback heads each one with its own gain control, some of them even had a variable speed control. we called them echo boxes.