Ampex 400 Tape Recorder/Reproducer

Message Board - AMPEX Model 400 Tape Machine

Not long after joining Ampex, John Leslie teamed up with another Ampex engineer, Al Dinsmore, to build a tape recorder of their own design. It was not a secret that they were doing it, but they chose to do it outside of Ampex activities. Al had a rather complete machine shop in his garage, and they worked evenings and weekends to complete it. It had two motors and used a tape-pusher approach into the head assembly. Compared to the 300, it was much smaller, much lighter and less expensive to build. The electronics were simpler than those for the 300 and worked well.

They demonstrated it to Alex Poniatoff, Walt Selsted, and others. They were impressed and decided that it provided the basis for another new Ampex recorder. Harold, and the rest of his team, designed the transport and Frank designed the electronics. The combination gave Ampex the Model 400. It was introduced to the world in late 1950. It could be either a full-track recorder or a half-track. It was a two-speed machine, switchable to either 7.5 or 15 ips. In early 1951, Frank tried an experiment on the Model 400; he mounted two half-track heads on the transport with one for the upper track and the other for the lower. This was Ampex’s first venture into stereo recording of audio, but it was with staggered heads which was the industry practice at the time. It wasn’t until the summer of 1952, that Frank used in-line heads on the Model 400 for stereo recording and playback and established a new industry standard.

1951 was a year of catch up! Myron brought in even more contracts for special instrumentation recorders. Ampex was having hiccups putting the Model 400 into production. It was a machine that would be excellent if appropriate tooling could have been afforded for high volume. Unfortunately, there was too much variation between batches of the same parts. Also Ampex was expanding so fast that it was necessary to move to larger quarters. This was a year that Alex Poniatoff made another of his excellent decisions: he asked Walt to become chief engineer.

In 1952, Ampex introduced its first production run of inline-head-stereo recorders. Both two-channel and three channel versions of the Models 300 and 400 became available. Bill Cara, an audio salesman with Kierulff Sound in Los Angeles (and later with Ampex) used a three-channel Model 400 to record his famous train demonstration tape. Cara’s demo tape also included music from a Wurlitzer pipe organ and classical material played by the Santa Monica Symphony. The tape was played every hour during the Audio Fair in Los Angeles the following January. That demonstration is often cited as the catalyst that moved the audio profession into stereo.

Foundational text courtesy of Ross H. Snyder and John Leslie from the AES Historical Committee paper, “History of The Early Days of Ampex Corporation”.