[WaveyDipole]

Thank you for the link and the data sheet.

The linked article is interesting. I have previously come across an oscillator circuit that used a filament bulb as a stabilising device.

This TG66A instrument has some instability which appears as a rhythmic bounce at the lowest frequency end but otherwise seems to work well. There is one electrolytic cap that could possibly do with being replaced but I note the article mentions that noisy pots can cause instability. However, I would have expected that to have been a rather random effect and did wonder whether the ageing thermistor might be a contributing factor. Curious that the guy in the article ended up with two failed thermistors.

Anyway, the replies confirm that it is a thermistor and kan_turl has kindly provided a datasheet so that answers both of my queries. Thanks.