This circuit is from an old translated publication from 1963. It is a hybrid circuit because it uses valves to control the relay and a transistor as the sensor element. The relay is one of the critical elements as it is a 3 000 to 10 000 ohm board relay. The circuit uses a transformer with a secondary 125 V x 15 mA or more and another for the 6.3 V filament of the tube, a double triode. The transistor is germanium, but equivalent can be used as they all have their temperature sensitive junctions. The adjustment potentiometers can be 47k and 4M7 as the next values indicated are no longer found.
