Frank Dresser wrote:



The prewar "curtain burner" cord sets have an interesting approach to making
an AC/DC radio. These radios used a 300 ma series string for the tube
heaters, rather than the later 150 ma string. The 300 ma string added up to
only about 60 volts. The other 60 volts was dropped in a third resistance
wire in the cord. Running the cord under the carpet was a bad idea.
Wrapping the cord up in a neat bundle was a bad idea. As the informal name
suggests, the cord itself was a bad idea.

It's worth mentioning that tube battery sets which have the option of power
line operation are also AC/DC radios. Just about every AC/DC set from about
1950 onwards uses a floating ground bus, which is somewhat safer than a true
hot chassis. I have a prewar GE battery/AC/DC set, which is certainly of
the more dangerous radios ever made. It has a true hot chassis, with one
side of the line cord firmly soldered to the chassis. The chassis is
mounted upside down, and the chassis attaching screws are mounted near the
radio's handle and knobs, without any electrical isolation. It also had a
curtain burner cord to drop voltage for the heater in the rectifier tube.


Some people attempt to correct the AC/DC hot chassis problem by
replacing the plug with a polarized plug, but on AC/DC sets the on/off
switch almost always switches the side of the line going to chassis or
common. So even with a polarized plug, when the set is switched off
there is a path from the hot side of the line through the filaments to
common, so the shock hazard still exists. When using a polarized plug,
the set needs to be rewired so that the on/off switch interrupts the hot
side of the AC line.