The recordings at Sun Records utilized a subtle trick unique to Sam Philips at the time. The tape had two recording heads separated by just a couple of inches. When the track was laid down, the signal would be recorded twice. Since the signals would be separated on tape, they'd play back just slightly time delayed from each other. The result was an echo-y sound similar to a modern analog delay effect.
It's very obvious in "Whole Lotta Shakin".
