[Boatanchors] 70v line transformer question

[WA5CAB at cs.com]

Thanks to Jim, Chuck and/or Don for posting all of the details (which I 
didn't copy back again).  All I ever bothered to remember was that a 70.7 volt 10 
watt transformer was satisfactory for 500/600 ohm military receivers.

Greg, I'll add that although most WW-II and later vintage Navy receivers 
(including ARA and AN/ARC-5 aircraft sets) have audio output impedance ratings 
around 500 ohms, early-War Signal Corps ones are mostly 4000 ohms, to match the 
7000 ohm input impedance of one or two LS-3's or the 4000 ohm impedance of the 
HS-23, etc..  Including the early BC-312/342, BC-348 and SCR-274-N.  Later a 
tap was added at 250 ohms, not 500/600.

Also most of the smaller permanent magnet loudspeakers had 3.2 or 4 ohm voice 
coils.  If in doubt about that, try both output taps on the 70.7 V 
transformer.

In a message dated 11/24/2006 2:20:11 PM Central Standard Time, 
bluebirdtele at earthlink.net writes: 
> Hi:
> Can a  tapped 70v audio line transformer be used to transform a 500 ohm  
> reciever audio output to a voice coil of 4 - 8 ohms?
> I tried it here and didn't work.  Someone told me to try this and I wonder 
> what jumpers need to go in to make it work.
> Thanks
> Greg
> WA7LYO
> 

Robert Downs - Houston
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