Quoting myself here:
Tim Gillett wrote:So I'm not sure that even around 1950 they couldn't make a repro head with narrower gap, so much as they didn't see the need to do so, to go above 15 kHz as the upper frequency limit for playback...
I did a few few quick calculations and cross checked another's figures. https://electronics.stackexchange.com/q ... -tape-head
It seems for a tape repro head to reproduce 20 kHz at cassette speed (1.875ips), its gap length must be 1.2 microns or less. But at 15ips, 8 times faster tape speed, that gap length only needs to be 9.5 microns or less.
In the early to mid 60's, Akai was making tape heads for open reel domestic recording with 1 micron gap lengths. I still have an Akai X-IV portable (Germanium transistors!) which claimed 13 kHz upper freq response at 1.875 ips (cassette tape speed). In the late 1950's, or even earlier, that they couldn't construct a repro head with a gap length of 0.374 thou (9.5 microns), 8 times less critical than what Akai achieved in gap lengths the early to mid 60's