Acro TO-290 (1950–1952)
An output transformer that promised the Williamson amplifier's full rated power from 20 cycles to 20 kilocycles, and asked $13.25 for it.
Overview
Thirteen dollars and twenty-five cents doesn't sound like much for a transformer today, but in 1950 it bought a serious piece of iron — the top of Acro's 20-watt line, and the one model in the catalog specifically built around the Williamson circuit's triode-connected 807s. Acro Products Company of 215 Arch Street, Philadelphia, ran the same advertisement month after month in Audio — which in September 1952 reported the new Acro transformers as a "sensational" entry sweeping the high fidelity field in sales — and the pitch was consistent: response flat within ±1 dB from 10 cycles to 40 kilocycles, full rated power delivered across the 20 cps to 20 kc band, and low enough phase shift to permit more than 30 dB of degenerative feedback to be wrapped around the transformer without instability. Those were the numbers that mattered to a builder in 1950, and Acro led with them.
The TO-290 sat above the 12.5-watt models in the lineup: the TO-240 at 4,000 ohms, the TO-250 at 5,000, the TO-260 at 6,000, and the TO-270 at 10,000, all at $8.75 net. The 20-watt group was just two units, the TO-280 at 9,000 ohms for $10.75, and the TO-290 at 12,000 ohms for $13.25. That 12,000-ohm primary impedance defines the part. Acro's own advertising called the TO-290 "specially designed for optimum match with triode connected 807's as in the famous Williamson circuit." It was a single-purpose transformer aimed at one of the most demanding builds of the day. All units had push-pull primaries and secondaries tapped for 4, 8, and 16-ohm voice coils.
Acro's literature used the Williamson amplifier with the TO-290 as the demonstration case. In that service the company reported half-power points at 8 cycles and 50 kilocycles. Intermodulation distortion was under 1 percent at 10 watts output, measured with a 40-cycle carrier, 4-to-1 ratio, and frequencies of 2, 7, or 12 kc, on a 400-volt plate supply. Acro also said the power rating was "very conservative," and that the units delivered full rated power across the whole 20-to-20,000-cycle band. The 20 kc square-wave transient response was offered as proof of the winding quality: tightly coupled, reactively balanced windings, in the company's phrasing.
Buyers had a choice of enclosure. The standard end-bell version shipped as-is. For $2.00 extra, Acro would furnish the transformer fully potted in a one-piece steel case finished in silver-grey hammertone. By late 1952 the company's advertising described all units as fully potted in the grey hammertone case, measuring 3¾ by 3¼ by 4¼ inches high. Shipping weight was listed at 7 lbs. for the 20-watt models. Prices were F.O.B. Philadelphia, and a 25 percent deposit was required on C.O.D. orders.
By Acro's own account, the TO-290 was not a universal part. The company's 1952 advertising had moved on to the TO-300 for Ultra-Linear and Ultra-Linear-Modified Williamson circuits, and the TO-310 for Ultra-Linear 6V6 designs. The TO-290 remained the recommendation for "the original Williamson Amplifier." So it is a period-correct transformer for one classic topology, not a general-purpose drop-in. The same 1952 ad put the TO-290 at $15.75, up from the $13.25 of the 1950 catalog. Acro's literature also noted that other primary impedances, 600-ohm line secondaries, and different power ratings could be furnished to special order.
The TO-290 is one of the names that comes up when the transformer substitution table for a Williamson amplifier gets discussed. A period anthology piece on the Williamson circuit listed the Acro TO-290 alongside the Freed F-1959, the Partridge "Williamson," and the UTC LS-6L3 as transformers thought to be of sufficient quality to permit substitution. The caveats were that no performance guarantee could be offered once substitutions were made, and that the output transformer had to be an exceptionally good one.
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