THE RCA 2N3055 POWER TRANSISTOR
Supremes and Super Foundation Bass
Following the example set by the American Vox Super Beatle, designed by Sava Jacobsen in late 1965 at the Thomas Organ facility at Sepulveda, California, the largest amps of the English solid state line - the Supreme and Super Foundation Bass - employed four RCA 2N3055 transistors in their power sections. RCA = Radio Corporation of America.
Transistors were not cheap. The retail price of 2N3055 was 22 shillings and sixpence (22s/6d). The retail price of an EL34 valve at the same date: 9 shillings and sixpence (9/6). The ratio - the 2N3055 over twice the price of the EL34 - presumably held for wholesale (bulk) orders placed by JMI.
"Practical Electronics" magazine, February 1967.
The RCA handbook of 1964 ("Characteristics and Applications of RCA Silicon Power Transistors"), p. 291, outlines its potential uses:
"Silicon n-p-n type used in a wide variety of high-power applications in industrial and commercial equipment. This type is particularly useful in power-switching circuits in series and shunt-regulator driver and output stages, and in high-fidelity amplifiers. It is designed to assure freedom from second breakdown and features an exceptionally high dissipation rating."
Click on the images below for the full technical specifications:
In its sections on "transistor applications", it is interesting to note that the RCA Handbook recommends (p. 26) the use, in audio amplifiers, of an interstage driver transformer between the pre-amp and power amp - which is exactly what the Super Beatle and the English solid state amps have. The full text of the handbook is available here.
The power section topology of the second version of the Super Beatle. The V1141 was preceded by the V14 and V114. For more on these, see the excellent pages on the Vox Showroom website, and R. G. Keen's geofex site.
It may have been the reported problems in Super Beatles, notably the failure of power transistors, that delayed the adoption in England of a new range of fully transistorized amps - the 4 and 7-series amps, released in May 1966, stand as a sort of half-way house, the preamps of most of the range being transistor, but the power amps all valve. A factor also contributing may have been the difficulty of obtaining in early 1966 good reliable 2N3055s.
Certainly British hifi manufacturers hung fire. This was recently stated of Quad:
"Exemplifying all that was admirable in British hi-fi, the 33 preamp (£43) and 303 power amp (£55) were Quad’s first commercial solid-state offerings, the company having waited for the new-fangled transistor to settle down before embracing it in 1967."
The two main things that dogged early transistors were high frequency (parasitic) oscillation, which led to thermal runaway, and secondary breakdown, whereby a flaw in one part of the transistor (the base-emitter junction) caused it to draw increasing amounts of current and fail, even when working well within tolerance.
An element slightly lost in the wash, is the "format" of the early RCA 2N3055s - they were "homotaxial" - identifiable internally by thin substrates of silicon and a large chip (die).
On the left, an early homotaxial 2N3055. On the right, a modern epitaxial unit. See this page.
The importance of using original, homotaxial transistors in early solid state amps is explained here. Although modern 2N3055s have better high frequency response, more gain, and higher dissipation, they often produce unwanted effects in amps built around homotaxial units - notably a poorer general sound, and in certain instances, instability.
In the Supreme and Super Foundation Bass, it is important that the output transistors (the four 2N3055s) are matched in order to help minimise, via the biasing, the amount of DC voltage (the "DC offset") present at the speaker output.
Below, pictures of a set of five RCA 2N3055 transistors - marketed to repairmen - two pairs (or one "quad") and a spare. RCA produced sets of valves in the same way. The "Mounting Kit" is the transparent insulating pad for the units.
Date code "7E" = May 1967.
The transparent mount for the transistor.
Vox will have ordered bulk (wholesale) boxes direct from RCA - either direct from the facility at Harrison, New Jersey, or through the RCA in Surrey.
Most early RCA transistors have stamped date codes. The system is a simple number and letter. The number is the year, and the letter the month. The month sequence runs A-H = January-August; and J-M = September-December.
At the top of this page, the code on the 2N3055 is "7M" = manufacture in December 1967.
Naturally it will have taken some time for the batches to be shipped to England, and then to be picked up for use in amp, having spent days, weeks or perhaps months in the Burndept / Vox store.
For date codes on other components in Vox solid state amps, see this page.