Found in old documentation, this circuit has only the most difficult component to obtain the PUT. The sensors are electrodes placed in the reservoir. The circuit directly controls an AC power load.
This is an old test circuit for integrated TTL counters, feeding a Nixie tube, common at the time....
This old oscillator using germanium transistors has a high impedance headphone as inductor to...
This preamp with several inputs using germanium transistors from the time was found in Japanese...