Raytheon was the early leader in the manufacture of
germanium transistors, with the largest quantities destined for use in
hearing aids. These first Raytheon hearing aid transistors (labeled as
CK718) were crudely made devices using a black epoxy case. Raytheon made
rapid improvements to their transistor manufacturing processes, and
introduced a much more robust, metal cased hearing aid transistor type in
1955, the CK78X series. Each transistor was tested and, based on specific
performance characteristics such as gain, noise and leakage, the resultant
functioning transistors were labeled with the corresponding model number.
Transistors which failed to meet the rigorous hearing aid performance
requirements were not labeled and were used by Raytheon to produce the
CK722/721 hobbyist line of devices. These unpainted CK78X transistors were
encapsulated inside the larger metal case of the CK722/721 type and were
sold by the thousands to electronics hobbyists, eager to use these unique
devices in their next radio or audio amplifier project. Use these
transistors as “mini-CK722s” – the performance is identical!