Ampex 31321-01
Not a tape deck, not a preamp. This is the calibration standard that every serious tape-machine reviewer in the 1960s reached for first.
Overview
The Ampex 31321-01 is a test tape - a factory-recorded reference reel used to align and evaluate reel-to-reel machines. Anyone who has read period audio magazines has seen the designation dozens of times: "playback response was measured using Ampex test tape 31321-01," over and over, across reviews of decks from a dozen different manufacturers. It is the common denominator in the golden-age tape-measurement ritual. You wouldn't install it in a system.
The tape was recorded full-track, and that detail matters. Reviewers of the period noted that playing a full-track standard tape back on half-track, two-track, or four-track heads produces an apparent bass rise that has nothing to do with the machine under test. The cause is playback-head fringing, which is magnetic flux at the edges of the gap adding extra output at long wavelengths. On top of that come the "bumps" in head response that scale with wavelength and therefore with tape speed. It's housekeeping, but it shows how the 31321-01 was used: as a known quantity against which everything else was judged.
The companion reel, 31331-01, covered the slower speed. That pairing shows up again and again in the equipment profiles of the era. It makes the 31321-01 the 7½-ips half of a two-tape alignment kit, not a standalone product.
The tape holds a set of spot frequencies, not a sweep. The standard test sequence ran 50, 100, 250, 700, 1000, 2500, 5000, 7500, 10,000, 12,000, and 15,000 cycles, with 30 cycles and - where the machine's speed allowed - 20,000 cycles as well. The 700-cycle reference gives away that this is NAB-standard material. Reviewers converting their measurements to a 1000-cycle zero had to note the shift explicitly. High Fidelity (August 1964, p.61) reproduced playback-response plots against the 31321-01 with the zero reference marked at −10 VU recording level, and High Fidelity (February 1964, p.62) ran the same tape through the Ampex F-4460 with separate left- and right-channel NAB playback curves.
Anyone using one today should know the 31321-01 is a tool, not a collectible in the usual sense. It has no faceplate, no knobs, no sound of its own. Its value is entirely in whether the recorded tones are still accurate, and a magnetic tape from the early 1960s is a perishable item. Collectors who find one should treat it as a calibration artifact and a piece of documentation history, not a working reference, until it has been verified against a known-good standard. The reels turn up occasionally in estates alongside the decks they were bought to service.
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