>sorry for my ignorance but what's an RA53? Yes, I've searched but found
>no information. :)

STC (the British telecomms company) used to make a family of thermistors, of
which the RA53 was one often used in audio oscillators. The family were tiny
thermistor beads about 1mm dia mounted in a glass envelope about 20mm long
and about 10mm dia. I think the glass envelope was evacuated to minimise the
thermal time constant.

The RA53 was very nice in that its characteristics meant the oscillator had
a 1V rms output level when used in a wein bridge oscillator, without any
set-up problems. Very common device for this use back in the 60's.

I had another thermistor of the same family which gave a different output
voltage, and try as I might I could not change the voltage without a lot of
effort. These devices had a very wide operating range.

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