"Gray Shockley" wrote in message
.com...



The 5-tube/valve AC/DC's are probably the
most dangerous radios ever made.

You might wanta take a look:

http://antiqueradio.org/safety.htm


gray


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.

Not all hot chassis radios used tubes. I've got a hot chassis solid state
RCA from around 1967 in my pile 'o junk. If I recall, the radio circuits
are used as a sort of emitter resistance (bypassed with an electrolytic) for
the audio power output transistor. They went to a fair amount of effort to
eliminate a small power transformer in a transistor radio, and they ended up
having to use an audio output transformer instead. I think I've read that
DC was still used in some parts of the east coast up into the sixties, and
radios such as this may have been designed with that market in mind.

None of this compares with the direct coupled output transformerless tube
amps some homebrewers like to play with. They use four of the 25 volt
heater variants of the 6080 voltage regulator tube in a series heater
string, with another tube, something like a 12AX7, for a phase splitter.
Half of the power triodes are hooked up as cathode followers directly
coupled to a speaker, and the other half of the triodes are used as an
active load for the cathode followers, much like the common totem pole
output often seen in transistor output stages. The power is rectified right
from the line cord with two solid state rectifiers, one for the positive
supply, another for the negative supply. Yikes!!

Frank Dresser