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I developed this transistor
during my summer vacation in 1953.
The “X” means “experimental” and the “78” doesn’t mean anything! The
shape of the mounting is rather fortuitous, as I simply made the original
from scrap metal and the people in the drawing prettied it up without
changing anything much. As I recall, the transistor mount was later
redesigned in a better manner and the transistor itself was surrounded by
bent copper-sheet cooling fins. We
then went on to make a better heat sink out of extruded aluminum in the
form of 12 (?) radiating fins, the whole thing being about 1 inch in
diameter. With germanium, of
course, the problem of keeping the junction temperature down was pretty
important.
When I left TP in September
1955 after completing my PhD, the X-78 was not in production, though the
packaging design had been greatly improved and it was a decent looking
product. The idea was to use it for
audio applications (we didn’t do much frequency response testing, but it
was OK at audio frequencies), and also perhaps in small “inverters” to
convert 12V DC to 110V AC. I
developed a small push-pull oscillator circuit for this application. I remember almost all the technical
details of the X-78 and other experimental power transistors, since this
work, including making the devices, was done by me personally – I didn’t
even have a technical assistant!
During 1955 I also developed a design for an “interdigitated” power
transistor that could be extended to cope with almost arbitrarily large
output power. I made only a few
trial devices.
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