"Henry Kolesnik" wrote in
message ...
Getting old is not recalling the alternatives offered, so
I'm a dummy. I found the filament xfmr first and tried it
but it reduced volume so I assume my radio which is
non-military doesn't have 600 ohms like the military
manual I have. So I guess mine is commercial, either a
SP-200-SX or more likely a SP-210-SX and I think it has a
3.2 ohm output transformer.
I can't find any info and I wonder if anyone knows for
sure on the commercial models output transformer's
impedance.
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Thanks & 73
Hank WD5JFR
All of the Super-Pro models from the SP-100 on had 500
ohm outputs for the loudspeaker and a loosly coupled 8K
winding for headphones. The 500 ohm output will drive a
loudspeaker at much reduced volume. A 12.6V filiment
transformer has about the right impedance ratio (100:1) to
drive a 4 to 8 ohm speaker from this tap. These tramsformers
have enough core material to work down to low audio
frequencies and surprizingly good high frequency response.
70V public address transformers also work very well as Scott
Dorsey pointed out. The original Hammarlund speakers for the
Sp-200, 400, 600, had matching transformers built-in.
The SP-100 has a front panel switch for the loudspeaker
but the later versions do not. You need to mount a switch
(maybe in the loudspeaker cabinet) to cut it off. Use a DPDT
switch with a 500 or 600 ohm resistor switched in when the
LS is turned off to provide a load to the audio transformer.
It should handle a couple of watts.
The headphone winding is not critical of load impedance
and will work with phones from 50ohms to high impedance.
The military and commercial versions of the SP-200 and
210 were identical except for the use of hermetically sealed
transformers and chokes and oil filled filter capacitors in
the military versions, and some variations of panel finish.
There is an owner's manual for the SP-210 somewhere on
the web, maybe on BAMA, I have a copy but don't remember
where I downloaded it.
Most of the communications receivers built up to
perhaps the 1950s had 500 ohm or 600 ohm outputs. A few even
had the output transformer in the loudspeaker cabinet.
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Richard Knoppow
Los Angeles
WB6KBL