The tape recorder's manual is available with circuit diagrams [1] and more pictures [2]. I'm not seeing an hf bias oscillator in the circuit unless the manufacturers are being clever. It was half track mono with two speeds, 3.5 ips and 1.875 ips.

Someone did a lot of work to restore these recordings!

[1] https://manualmachine.com/concertone/tr100/8544644-schematic...

[2] https://www.radiomuseum.org/r/transicord_tr_100.html

Indeed, no hf oscillator, instead it looks like it uses DC bias via R12. It's also neat to see all the circuitry re-used identically for amplifying the tape in playback, or amplifying the microphone input to record to tape. Was that a standard and obvious plan for the engineers of the time?

I'm also curious if R22, the thermistor, would have been just for thermal protection, or if it was actually used to set the operating point for the output transistors in normal operation? Would it have switched on loud and angry before the transistors warmed up and the bias point cooled down, the opposite of a tube amp?

Could you explain more? I could ask an LLM but I find it friendlier to ask you, ha! If you don't mind of course.

What does a bias oscillator do that would help, and is the ips speed slow ie it's a low "resolution" (so to speak) analog recording?