DISCLAIMER: USE THE INFORMATION PRESENTED HEREIN AT YOUR OWN RISK. HIGH VOLTAGE DIRECT CURRENT, SUCH AS DESCRIBED BELOW, CAN BE LETHAL. IN ADDITION, EVEN SMALL GEL-CELL BATTERIES CAN PRODUCE HIGH CURRENTS WHEN SHORT-CIRCUITED AND QUICKLY MELT WIRE, DESTROY COMPONENTS, AND START FIRES. IF YOU ARE NOT FAMILIAR WITH ELECTRICITY, THEN ASK SOMEONE QUALIFIED TO CHECK YOUR WORK BEFORE ENERGIZING ANY OF THE CIRCUITRY DESCRIBED.
Older vacuum-tube radios are popular in the prepper world (and rightly so) for their resistance to EMP damage. One useful fact about them that is often unrealized is that many of them can be powered by direct current directly from batteries, without the use of an inverter to make 120VAC.
The All-American Five (AA5)
Many communities in the early twentieth century had 120V DC power, instead of AC. Many radios from the 1940s to 1960s were designed to work off of either 120V AC or DC power, and are of what was commonly referred to as the ‘All-American Five’ design, so-called because they used five vacuum tubes. The earlier designs used large octal tubes (50L6, 35Z5, 12SA7, 12SQ7, and 12SK7) and the later designs used seven-pin miniature tubes (12BA6, 12BE6, 12AV6, 50C5, and 35W4). These sets were designed to have their tube filaments powered directly off of the power line with no transformer required. (The first digits of a tube’s number, that precede the first letter, indicates its filament voltage. Note that the tube prefixes, when added together, add up to 121). Further, the circuitry was designed to run off of a relatively low plate voltage of 120V. This allowed a simple inexpensive design that would run off of either AC or DC power.
There are some variations on this. Later AM/FM radios used a solid-state rectifier to produce the plate supply DC, and used the now-eliminated rectifier tube filament voltage drop for an FM converter tube, to allow the set to receive the FM broadcast band. Some just replaced the rectifier tube with a solid-state rectifier and used a resistor to provide the voltage drop for the filament string. The venerable Hammarlund S-120 uses this system.Continue reading“Powering Tube Radios with Batteries, by Brian H.”