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Although
existing TA165 transistors were constructed over five decades ago, and
represent the earliest type of transistor technology (point contact), it is
possible to coax demonstrable transistor action from some of these
“workhorses”. The circuit above is a classic, and will oscillate only
with a point contact device - this is because point contact transistors
exhibit a unique property known as “negative resistance”. It is this
feature that allowed the first transistors to be better suited as
oscillators, not linear amplifiers. With the capacitor “C” selected at .1
to .25 MF, the circuit will produce an audio tone. Not all TA165s are still
functional, as the epoxy case material has likely allowed moisture
penetration and resultant degradation of the internal germanium junctions.
You’ll find a recent 10 second sample of audio produced by an active TA165,
connected as above, in this mp3 file:
TA165
Audio Sample
The
fact that a remaining 55 year-old TA165 could produce this audio sample is
a real tribute to the pioneering work done at RCA in the early 1950s by Bob
and his team in the Semiconductor Advanced Development Group – quite an
accomplishment for RCA’s first transistor engineer.
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