I need more germanium JTs to run down the AA cells to less than a tenth of a volt, so I put this one together using an antique germanium 2N652 made by ETCO. I used a blue-green or aqua LED like the ones used in the traffic lights. It uses a Fair-Rite 2643002402 core. The core has about 18 inches (49 cm) of 30 AWG (.25mm) enameled wire quadrifilar wound with three of the four conductors connected in parallel for the primary winding. The resistor is the customary 1k. The 2N652 is PNP, therefore the negative or brown wire on the left goes to the coil, and the white wire goes to the emitter of the transistor and LED. The LED is “backwards”, the cathode or flat spot goes to the collector and coil winding, and the anode goes to the emitter and positive (white) wire.
As can be seen, the LED is very bright. I used the piece of birch plywood that’s similar to the ones I used the last few days for the flashers. That gives the circuit a solid mounting surface just like a PC board. I didn’t think there was any point in measuring the current at 1.5V because these germanium JTs will for the most part be used on cells that are half dead and under 1 volt and constantly decreasing… Lower, and lower, until the dead cell measures 0.1V or less.
