Getting a bit esoteric now aren't we?G
You may also want to do a Google of "tunnel diode oscillator circuit".
Actually Nigel the first tunnel diode oscillator I built was as a kid and maybe the early 60s. It actually ran in the 6 meter ham band or about 50 MHz. I do however agree I was on the esoteric side. OK, we will let it go with a unijunction relaxation oscillator.Getting a bit esoteric now aren't we?
It also seems fairly obvious that he's looking at audio oscillators, as far as I can recall (never having even seen one) tunnel diode oscillators were for microwave frequencies.