Acro TO-310
An output transformer built to turn a 6V6 amplifier into something the Ultra-Linear crowd would listen to.
Overview
The TO-310 is a potted output transformer, and Acro built it for one job: converting amps running 6V6 tubes over to Ultra-Linear operation. If you own a 6V6 amp and want to hear what the Ultra-Linear connection does, this is the transformer Acro wanted you to buy.
The published specs are the same headline numbers as the bigger TO-300. Response is ±1 db from 10 cps to 100,000 cps, it's rated to handle 20 Watts, and the response is resonance-free. The company also called the square wave response "unequalled," with low phase shift. Collectors note that Acro leaned hard on that ±1 db / 100 kc figure across the "300" series, and the TO-310 carried it despite being the low-power member of the family. Whether a 6V6-based amp could ever use bandwidth out to 100 kc is a fair question. The transformer was built to a standard, not to the limits of the tubes it was meant to serve.
The physical package is consistent across the line: fully potted in a grey hammertone case, and compact. The advertising gives the dimensions as 3⅜" x 3⅜" x 4⅜" high. The potting seals the windings against moisture, and the laminations aren't going to buzz loose the way an open-frame bargain transformer will.
Acro's advertising was explicit about where the TO-310 sat in the lineup. The Audio (June 1952, p.53) catalog copy lists the TO-310 "for Ultra-Linear operation of 6V6 tubes and EL84 tubes" at $18.75. Alongside it are the TO-300 for KT66 and 5881 circuits at $24.75, the TO-320 for 6Y6 tubes at $18.75, and the TO-350 for 100-watt 6146 operation at $49.50. A December 1952 Audio notice confirms the same $18.75 figure and describes the TO-310 simply as the model "for the Ultra-Linear 6V6." The TO-300, by contrast, was the Williamson and Ultra-Linear-Modified Williamson part. So the TO-310 was the budget entry for the small-tube crowd, and at $18.75 it undercut the TO-300 by six dollars.
There's a wrinkle in the historical record. A March 1958 Audio article discussing output-transformer loading noted that the Acro TO-310 "should also prove entirely satisfactory" in a particular application, "provided a corrective factor of 10,000/8000 is used in computing the output impedance of the secondary." In plain terms, the transformer's nominal primary was 8,000 ohms, and matching it to a circuit expecting a 10,000-ohm load meant recalculating which output tap to use. That's not a defect. It's just the arithmetic of matching a transformer to a circuit, but it can trip up a modern builder who assumes the taps mean what they would on a generic part.
Acro's marketing was relentless about the Ultra-Linear story, and the TO-310 rode along. A 1952 Audio ad (April, p.?) explained that the new Ultra-Linear circuits, credited to Audio Engineering, November 1951, let existing amplifiers, including the Williamson, be converted "readily and simply" by swapping in an Acro "300" series transformer. Distortion decreased and, for triode circuits, power approximately doubled. The TO-310 was the 6V6/6AQ5 member of that series, which is why it turns up in so many period conversion projects. Acro also pushed a free 16-page booklet with seven pages of circuits, and offered a conversion kit for a dollar.
Collectibility on the TO-310 is a narrow thing. People don't hunt these the way they hunt a TO-330 or a TO-350. It's the working-class member of the family, and it turns up attached to period 6V6 amplifiers more often than as a loose part. The value is in originality: a potted Acro with intact hammertone and legible markings is worth more than a stripped one. The potting means you can't easily inspect the windings, so buying sight-unseen carries the usual risk.
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