Minerva with the cover closed.
Minerva Tropic Master
AC-DC receiver with all-metal cabinet. This is model W-117. The date on the Rola 6M23 speaker (original equipment for the set) is October 8, 1945. Whether this is the speaker manufacturing date or the receiver's manufacturing date, I do not know. The schematic matching the set is dated June 1945
This 8 tube set covers the AM broadcast band and the shortwave band from 5.5-18 Mhz. It has an RF amp tube and push-pull audio output, a bit unusual in a series filament set. The design wastes one of the triodes in the 6SC7 with the grid and plate grounded. This probably indicates that the designers used whatever tubes were available at the time. The circuit can be found in Sams Photofacts set 6 and in Rider's Volume 15. The Rider's schematic shows two variations of the circuit, one with a 25Z6 rectifier (in this set) and one with a pair of 35Z5 rectifiers. One of the Rider schematics (with the series string incorrectly drawn) shows a pair of 50L6 tubes. This set has a pair of 25L6 tubes as original equipment. Either the 25L6 pair or a 50L6 pair can be used with a simple circuit modification in the series string.
Manufacturing on these sets was obviously planned and started during the latter days of World War II, but it appears many, if not most, were sold after the war ended. A series on restoring the Tropic Master can be found in the April through July and the September issues of Popular Electronics for 1995. According to one letter in those articles, this radio was sold through military post-exchange stores as a morale receiver for the troops. The writer was a Korean conflict veteran. The advantage of an AC-DC set for troops overseas was that power was less of a problem. Either AC or DC could be used. The set will work on as little as 90 volts. For areas with voltage higher than 120, the troops simply hooked a few light bulbs in series to divide the power to the radio until the dial light was about the right brilliance. An ad for the receiver indicates it was "Made for Members of the U.S. Army and Navy. Now Available for Civilians". The price was $75 "Fed. Tax included". At that price, it was half again as much as a Hallicrafters S-38, dooming it to an early demise.
Users of AC-DC sets need to be aware of the hazards, especially with metal-cabinet radios. One side of the powerline is switched to this set's chassis which is insulated from its metal cabinet (A capacitor is wired between the two.) The insulation between the chassis and cabinet must remain intact. An isolation transformer is strongly recommended to reduce shock hazard. At the very least, use a GFCI outlet.
The knobs on this set are not original.
Minerva with the cover closed.