Neville Fletcher


Oral History – Neville Fletcher (Continued)



These are photos of two of the experimental X-78 germanium power transistors designed and built by Neville Fletcher in the summer of 1953. Although the X-78 was not placed into large scale production, this pioneering work was the basis for later extensive commercialization of germanium power transistors by Clevite Transistor Products, beginning in the late 1950s.  The X-78 was documented in several key early transistor texts and papers, and was suppied in limited quantities for evaluation purposes.  As shown in the photos, the X-78 was mounted on a copper cooling flange, with the germanium die potted in black epoxy - the base and emitter leads extend from the top.   





Go To Fletcher Oral History, Page 3



Oral History – Neville Fletcher (Continued)

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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