For many years, the March issue of Popular Mechanics had the tradition of carrying the plans for a radio receiver billed the “Little Giant.” A few weeks ago, we featured the 1942 version, and today we offer the version shown 70 years ago in the March 1947 issue.
The circuit diagram for the 1947 version will look very familiar with those who’ve dug into postwar AM radios, since it’s the classic “All American Five” circuit employed for many years by most radio manufacturers. This one has the familiar complement of octal tubes: 12SA7GT, 12SK7GT, 12SQ7GT, 50L6GT, and 35Z5GT. This is also known as an “AC/DC” set, since it could run off either AC or DC 115 volt household current. It’s transformerless, since it rectifies the line cord for the B+ voltage, and wires all of the filaments in series to run directly off the line current.
This circuit does have one interesting postwar twist. Variable condensers were still in short supply, so it uses permeability tuning. Instead of a variable condenser and fixed coil, it uses a fixed condenser and variable coil. The inductance is varied by moving a plastic molded iron core in and out of the coils.