[Milsurplus] Re: [The WS No.19 Group] Command set, CCT46104

[Mike Morrow]

George wrote:

>The ARC5 manual states that "A" type recievers have 8,000 ohm
>output & "B" type recievers have both 8.000 &  600 ohm output.

That info applies ONLY to the SCR-274-N sets, used by the USAAF.  Ben's set is part of the USN ARA system.

The USN used the ARA sets (which the USAAF copied in most respects to make their SCR-274-N).  Later, the USN developed and expanded on the earlier ARA/ATA command set to make the JAN-nomenclatured AN/ARC-5.  

There are many significant differences in AN/ARC-5 transmitter components, compared to similar appearing ATA and SCR-274-N components, plus several significant differences in some of the receiver components as well.

With respect to the AF output transformers in each system:

1)  The ARA AF output transformer produced a 64:1 impedance (8:1 turns) ratio, primary to secondary.
2)  The SCR-274-N AF output transformer produced a 5:1 impedance (2.22:1 turns) ratio, primary to secondary, for A-suffixed receivers.  B-suffixed receivers also had a tapped secondary effecting a 9:1 impedance (3:1 turns) ratio.
3)  The AN/ARC-5 AF output transformer is identical to the ARA transformer.

I've found nothing in my USN ARA/ATA or AN/ARC-5 manuals that states what the nominal AF output impedance was expected to be.  However, my manual for the common USN WWII aircraft interphone system RL-7 states that the interphone mic amplifier's nominal output impedance was 250 ohms, produced by a 12A6 and AF transformer with a 36:1 impedance (6:1 turns) ratio.

I think it is thus likely that a "nominal" AF output impedance for a USN WWII aircraft receiver (ARA or AN/ARC-5) is 250 ohms or somewhat less.  I earlier posted an estimate of 500 to 600 ohms, but that appears to be more of a low impedance midpoint between the ARA and the lo-z tapped SCR-274-N, though practically speaking it's still low-z.  It's all "nominal," for effective impedance varies as the audio frequency varies, and not only did a interphone system have to deal with multiple paralleled headsets, but also multiple paralleled AF outputs from various components as well.

On a related subject, I've often seen it implied that the interphone amplifier systems amplified the output of the various receivers (command, liason, RDF, MB, localizer, etc.) and transmitter sidetone outputs feeding in to it.  That may be true of some post-WWII systems, but every WWII vintage US interphone system I've ever seen, such as the very commonly used USAAF RC-36 and USN RL-5 and -7, amplify ONLY the interphone user's microphone.  All audio from the receivers and transmitters that is heard over the interphone system comes directly from the AF output of each receiver and transmitter feeding into the IC system.

73,
Mike / KK5F