Seventy-five years ago this month, this ad from Federal encouraged radio dealers to make some extra profit by upgrading their customers’ portable radios. Most such sets operated off batteries as well as standard household current. The radio itself had four tubes, along with an additional rectifier tube for use at home, in this case a 117Z6. The low-voltage tubes came to life almost immediately, but the rectifier took some time to warm up. By replacing it with a selenium rectifier, this problem was solved. And even though selenium rectifiers are famous for their catastrophic failures (which produce malodorous and toxic fumes), they probably lasted longer than the tube, and probably outlasted the radio in most cases.

The age of selenium rectifiers lasted through the 1960’s, when they were replaced by silicon diodes.

The ad appeared in the July 1947 issue of Service magazine.