The discussion of what bias frequencies were used over time keeps recurring. Special thanks to Jay McKnight of Magnetic Reference Lab, Tom Fine, and Brian Roth  for input to this list. I posted this to the ARSC list, but wanted to include it here as well. This knowlege is useful for those who wish to archive the bias along with the audio for future application of time-base-error correction tools such as the Plangent Processes.

In the early days, apparently wire recorders used bias as low as 30-40 kc, but Jay McKnight recalled in the pre-Ampex days, 60 kHz was common.

The Ampex Standard was 100 kc up to the MR-70.

With the MR-70, Ampex switched to 150 kHz bias frequency (and adopted the Hz) [Larry Miller, ex Ampex]

Other later machines used different bias and erase frequencies. We can see with a few exceptions, the top bias frequencies were commonly limited to 250 kHz for audio, with the Sony APR series and the Ampex ATR series in the 400 kHz region. For cassettes, a practical maximum appears to be about 150 kHz.

Ampex AG-440 (A) stayed with 150 kHz [manual]
Ampex ATR-100 144 kHz erase, 432 kHz bias (1:3) [manual]

MCI JH-24 Multitrack 210 kHz bias, 105 kHz erase [manual via Brian Roth]

Otari MTR-10/12 II Bias 250 kHz (erase not spec’d) [manual]
Otari MTR-90 (original) 246 kHz bias, 123 kHz erase [manual via Brian Roth]

Sony APR-5000, APR-24 100 kHz erase, 400 kHz bias (1:4) [manual]

Studer A80VU 80 kHz erase, 240 kHz bias (1:3) [manual]
Studer A80 RC 150 kHz [manual]
Studer A810, A807, A820 2CH 153.6 kHz [manual]
Studer B67 150 kHz [manual]
ReVox A77 120 kHz [manual]
ReVox B77 150 kHz [manual]
ReVox PR99 150 kHz [manual]

Tascam 32/44-OB — 150kHz [manual via Tom Fine]

Technics 1500/1506/1520 — 120kHz [manual via Tom Fine]

Here is a quick sampling of published bias frequencies for two top-of-the line cassette recorders, a better-than-average portable, and an early compact portable.

Nakamichi Dragon (Along with the Nakamichi CR-7A, perhaps  the finest machines ever made for overall audio quality) 105kHz (Service manual dated 1985 (scan) 1990 (copy))
Nakamichi MR-1 — 105kHz [manual via Tom Fine]

Studer A710 (a high-end cassette recorder, without the auto-azimuth that makes the Dragon superior) 150kHz (no date, scan on Studer ftp site)

Sony TC-D5M (a workhorse, good quality stereo portable) 85 kHz (Svc Manual dated 1980)
Sony TC-55 (an early compact — jacket pocket — mono portable) 41kHz (as low as I’ve ever seen) (Svc Manual dated 1972)