SPARC Radio Museum - Rogers AC Tube

The Rogers AC Tube -
A World First for Canada!
Gerry O'Hara VE7GUH
Dec 2009

Advertisement for a
B-battery eliminator using
the Rogers 'AC Tube'.

In the early 1920's, all radio receivers fitted with tubes were powered by batteries - not one, but at least two and sometimes several: a large low-voltage accumulator (the 'A' battery) to power the thirsty tube filaments (a 201 tube used 1 amp at 5 volts) and a high-voltage 'B' battery (90 volts was typical) for the plate (anode) circuit(s). The accumulators, similar to car batteries, leaked acid and were not popular items to bring into the living room or 'parlour'. When the 'B' battery voltage dropped, the radio tended to break into oscillation and become distorted. And, of course, they would expire at that critical moment or when the local radio store was closed on a Sunday. The batteries were connected to the various circuits in the set via a maze of wires that were another source of problems - poor connections, shorts and the like. The design of the tubes of the day was such that they had a directly-heated cathode (similar to the filament in a light bulb). But this meant that if you powered the filament with AC power, the signals passing through the radio became modulated with a loud hum - AC-powered radios were therefore not practical.

Ted Rogers, a Canadian radio pioneer and an ambitious and bright young man who owned his own radio company by the early-1920's, set about eliminating this problem. Rogers realized that to get wider acceptance in the home, radio sets must break their ties with awkward and costly batteries. He left Toronto in April 1924 on an extensive trip to search for his 'AC tube'. His travels took him to Pittsburgh to visit Westinghouse, there discussing the tube problems with Fredrick McCullogh, who was experimenting with AC tubes (unsuccessfully). The wily young Ted purchased the Canadian rights to McCullogh's experimental tubes for $10,000.

On returning to Toronto, Ted experimented relentlessly, trying one idea after another to produce a reliable and effective indirectly-heated cathode (one in which the heating element, or filament, is electrically isolated from the cathode that emits the electrons as part of the tube's function). On August 1, 1924 he finally achieved success with his experimental tube designated the '15S'. The insulation on the cathode of this tube provided better electromagnetic and electrical performance than achieved previously and the tube's structure provided better separation of the AC-powered heater from the other tube elements.

Although the 15S worked, and with little hum present, there was much work to be done to bring the tube into production and have it widely accepted in radio designs. Using it meant re-tooling for many companies (a costly business) and they took some convincing this was the way to go. Also, in order to make an all-AC-powered radio, Ted realized that a robust high-voltage rectifier tube had to be designed for the plate-circuit supply. Ted worked on the rectifier design during the winter of 1924-25 and was awarded Patent 250174 in April 1925 for his rectifier tube. These two inventions finally led to the production of the first AC-powered radio on April 8, 1925, later designated the Rogers Model 120. This set contained five AC tubes and the Rogers 'B-Eliminator' power unit. By the late-1920's, people were throwing our their battery sets by the thousand and replacing them with AC-powered sets.

Ted Rogers built-up a true radio legacy before his premature death at the age of 38 in 1939. He even started up his own radio station, CFRB ('Canada's First Rogers Batteryless'), in Toronto.

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