Alternative Detector for a Shunt-fed Crystal Radio

After a significant improvement in performance by just adding an inductor to my first crystal radio, the focus was on a replacement for the OA5 which would give a higher output.

OA5 diode
The base-emitter and base-collector junctions of germanium audio transistors like AC130, OC74, AC127, 2N61, AC188 and AD162 were tried out. However, they were all only as good as the OA5.

Germanium transistors
As luck would have it, when the 2N61 was being tried out, an accidental short between its base and emitter leads resulted in a tremendous increase in signal output. Hence the OA5 in the crystal radio was replaced by the base-emitter-shorted 2N61.

Using a wire antenna 60' long, the lone local 612 kHz, 200 kW AM broadcast station could now be heard even at a distance from the headphones! Headphone current measured using a 1mA FSD 60 Ω meter, was 725 μA.

Crystal Radio - Schematic
The other transistors too gave identical results with their base and emitter leads shorted. Results with the base and collector leads shorted trailed close behind.

A dedicated unit of equal performance was built using a 3" x 3" x 1½" electrical bakelite box as an enclosure.

Series-tuned Crystal Radio Enclosure
Screw terminals were attached on the rear for connecting the antenna, earth and headphones. 

Inside the Series-tuned Crystal Radio
Lugs on the screw terminals facilitated soldering of the transistor, moulded inductor and earth link. 

When subsequent trials on series-fed crystal radios showed a degradation in their performance,
it was concluded that germanium transistors, with base/emitter interconnection, outperform germanium diodes only in shunt-fed crystal radios.