UTC LS-55
Builders kept reaching for this transformer long after the catalogues went quiet, and the archive can finally say something about it.
Overview
The fact sheet for the LS-55 is empty, but the source-document excerpts in the archive are not. They establish that the LS-55 was a UTC "Linear Standard" output transformer. It went in the back of a tube amplifier, between the output tubes and the speaker. That is consistent across a decade of Audio construction articles. It is not a finished product you plug in. It is the iron inside somebody's amplifier.
UTC pushed the Linear Standard line hard in the pages of Audio through the early 1950s. Read the ad copy as advertising, not as a verdict. A 1950 placement promised the "closest approach to the ideal component" in terms of uniform frequency response, low waveform distortion, high efficiency, thorough shielding, and dependability. The same wording recurs in 1948 and again in November 1950. The ads also pushed a "true hum balancing coil structure," a semi-toroidal construction, and multiple coil sections for maximum neutralisation of stray fields. That is marketing. Still, those construction claims are the sort of thing builders cared about in a transformer sitting inches from a phono stage.
The more useful evidence is what designers did with the part. In an April 1951 Audio construction article, the author used a UTC Linear Standard LS-55 in an amplifier designed to eliminate expensive interstage audio transformers, so the money could go into the output transformer instead. The LS-55 was where the budget went. The article is blunt about why: the output transformer is the most important single component in the system, and satisfactory results cannot be expected with an inferior one. A 1958 Audio piece made the identical argument. Its author used an LS-55 with a 5000-ohm primary tapped at 3000 ohms, then turned the primary "inside out" to run the 3000-ohm section as screen taps for Ultra-Linear operation.
An April 1955 Audio console project reports that UTC itself confirmed the LS-55 could be wired for approximate Ultra-Linear operation. The author quotes a letter from the company on which terminals to tie together, and notes that the change requires a different feedback resistor to hold 10 db of feedback. The builder tried it and found the amplifier apparently more stable, with a "richer" sound in the low-frequency range. He did not bother changing the resistor. In the same project, feedback values of 13,000 ohms and a 2200-ohm cathode resistor were found to permit the greatest allowable feedback. Measurements of output-voltage regulation against load variation put the internal output impedance, looking into the 15-ohm secondary, down at 4 ohms.
The LS-55 also carried a line secondary alongside the voice-coil winding, which is why it turns up in substitution tables for both jobs. A 1948 Audio table lists the LS-55 against the output-transformer slot. A later table lists it as the line-output option, with the LS-57 alongside for voice-coil duty. That dual-secondary arrangement made it a general-purpose part.
Power handling needs a hedge, because the sources disagree in emphasis. The 1950s construction pieces treat the LS-55 as a home-audio output transformer, with one amplifier measured at 16 watts at the secondary - more, the author notes, than is required in the home. Writing decades later in Audio Express (July 2003, p.68), J.L. Markwalter recalled using an LS-55 in a single-ended push-pull amplifier built in the 1960s and kept in service for more than twenty years. He stated that these transformers were rated at 20W, and that an 8417 run as a triode would deliver an honest 20W through them. He recommended the 5000-ohm plate-to-plate connection. That is a retrospective rating from a user, not a manufacturer's data sheet, so take 20W as reported.
The archive does not have a complete picture. There are no production years, no original list price, no dimensions or weight, and no published frequency-response figures. There is no independent bench review of the LS-55 as a standalone component either. The period citations here are construction articles and advertising, not test reports.
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