Ampex 31321
Not a tape machine at all - it's the alignment tape that every serious deck in the country was judged against.
Overview
The Ampex 31321 is a calibration tape, not a recorder. Newcomers see the five-digit number in a service manual and assume it belongs to a transport. It doesn't. It's a full-track alignment tape recorded in an Ampex laboratory, used to set playback equalization, azimuth, and level before anything else gets touched. The most common variant, the 31321-01, is the 7½ ips NAB full-track standard. It turns up in the test-equipment lists of machines that have nothing to do with Ampex, which says how widely it was trusted.
The tape is a reference, and the specs reflect that. Ampex quotes the standard as ±1 dB at 63 Hz and ±2 dB at 40 Hz. One reviewer's bench measurements against a 31321-01 came in excellent apart from a bass fall that matched the published tolerance almost exactly. As HiFi-Stereo (April 1964, p.32) reported, the Sony TC-500 measured ±3.5 dB from 50 to 15,000 cps when checked with the Ampex 31321-04 alignment tape at 7½ ips. That bass rolloff happens on replay rather than on recording. A technician who recorded a tape on the machine and played it back on a second deck confirmed the recording itself was flat down to 40 Hz. So if your low end sags against the alignment tape, the problem is in your playback electronics, not your record head.
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