UTC LS-22
A push-pull interstage transformer from the Linear Standard line. It sat between the driver and the output tubes in a serious mid-century amplifier.
Overview
The LS-22 is a push-pull interstage transformer. Nobody takes its picture, but it has a lot to say about whether an amplifier sounds clean or smeared. UTC built it as part of the Linear Standard series. The company's own catalog copy in Audio (January 1950, p.52) pitched the whole line as "the closest approach to the ideal component" - uniform frequency response, low waveform distortion, high efficiency, thorough shielding. The LS-22 was one of the pieces carrying that banner. The same full-page ad ran word for word across several years of the magazine.
The LS-22 takes a push-pull signal at the plates of one stage and hands it, still push-pull, to the grids of the next. The primary is split, 30,000 ohms plate-to-plate. The secondary is also split, 80,000 ohms overall with a 1.6:1 step-up ratio. That step-up is the point of an interstage. It lets a modest driver stage develop the grid-to-grid swing that big output triodes demand. The response is quoted at 20-20,000 cycles within ±1 dB, maximum level +26 dB, hum-pickup reduction of −50 dB, and a maximum unbalanced DC in the primary of .25 MA. That last figure means this is a transformer for circuits that keep DC out of the winding, not a parafeed part.
The part earned its reputation in real amplifiers. A 1948 Audio contributor writing up a 6AS7G amplifier (October 1948, p.52) reported using an LS-22 as the interstage. He described the result as so silent that with the tuner cold the ear had to touch the speaker cone before any noise could be perceived. That is a striking claim for a transformer-coupled push-pull design, where hum and leakage are the usual complaints. In the same magazine's earlier write-up of a 300A/300B amplifier, the author noted that the LS-22 "could just as well be used" as the interstage in place of his own mumetal-cored design with a 1:2 step-up.
It also turns up in broadcast-adjacent work. A listing of amplifier and power components for a 100-watt broadcast modulator pairs the LS-22 with 57 triode plates feeding 59 grids, the classic pattern of a small driver tube pushing a larger one through a step-up interstage. In a 1951 classified ad in Audio (July 1951, p.52), a seller offered an LS-22 alongside an LS-57, "like new," for $25. On original pricing, the July 1951 Audio table lists the LS-22 at a $31.00 list price, while a distributor catalog of the period quoted it at $13.20 net. The gap between list and jobber pricing was how this trade worked, and buyers in the know paid the net figure.
Collectors today seek the LS-22 out for the job it was built for: a period-correct push-pull interstage in a triode amplifier, or a known quantity when a homebrew design needs a step-up stage. UTC advertised semi-toroidal, multiple-coil construction across the Linear Standard line. The shielding and the hum-balancing coil structure were not marketing decoration, and the 1948 report of a dead-silent amplifier running one keeps the model on shopping lists. The LS-22 is also a specific tool. 30,000 ohms plate-to-plate and a 1.6:1 step-up suit a particular range of driver and output tubes, and dropping it into a circuit it was not wound for will disappoint. The .25 MA DC limit in the primary bears repeating. Don't hang DC current on this part.
Condition is everything with a transformer of this age. The windings are the whole value, and a unit that has been stored damp, dropped, or run hot may look fine and measure wrong. Buyers should check DC resistance across each winding half and confirm the split primary and secondary sections read as they should before trusting a pair to a build. The LS-22 is not rare the way a famous amplifier is rare, but clean, matched examples with intact leads and legible markings are what the market wants. The ones that surface are usually pulled from equipment or from old stock rather than offered as tested, guaranteed parts.
For anyone building or restoring a mid-century push-pull amplifier who needs a period interstage, the LS-22 is a defensible choice, provided the impedance and step-up match the design. It is a component, not a collectible in its own right.
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