Stephen wrote:

My estimate would be 30 minutes at 3 3/4 inches per second, mono, 10 kHz
frequency response, signal to noise ratio 40dB.

My little tape recorder had a non-standard speed of 4ips and also had
the tape wound in a funny way so it played backwards on a normal
machine. For tapes I used to strip down 1/2" computer tape. My uncle
worked at IBM and he used to get it for me.

Bill