[Boatanchors] BC-348 Output Impedance

[Jack Antonio]

A little more complicated than that,

At the start of WWII, the Air Corps used hi-impedance
headsets, in the 4000 ohm range.  Sometime during the
war they switched to low-impedance headsets, in the 300 to 500
ohm range.  Many radios, such as the command sets and BC-312
series originally only had the hi impedance outputs available,
later production sets had output transformers with both available.
Many sets that had both taps, and were set to the low impedance
had a decal that said "Connected for low impedance"

One of the things you will see in the surplus market are
MC-385 adapters, these were 4000 to 300 ohm transmformers
that would allow the use of the lo z headsets with the
hi-z radios.

As regards the BC-348, be aware that the filter choke
in the output transformer can is a part of the biasing
network for the radio, it is in the B- line and the voltage
drop across it provides bias for the output tube, as well as
providing a measure of voltage stabilization for the radio
(called a "load compensator").

If the BC-348 is otherwise unmodified, the best approach
is to keep the internal transformer and use an external
transformer to step down to your speaker impedance.

Jack Antonio WA7DIA
scr287 at sbcglobal.net

Glen Zook wrote:
> According to the 1949 edition of the ARRL Hints &
> Kinks book, in a special section on the BC-348 series,
> the "high" impedance was approximately 4500 ohms to
> match headphones and the "low" impedance was
> approximately 500 ohms for a "line output".
> 
> Glen, K9STH
> 
> --- Olaf Madsen <Olaf at carolina.rr.com> wrote:
> 
> What are the output IMPEDANCES (speaker) of the audio
> transfomer in the BC-348 ???
> 
> 
> 
> 
> 
> 
> Glen, K9STH
> 
> Web sites
> 
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> http://home.comcast.net/~zcomco
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