[Milsurplus] WWII-era USN headphone question

[Mike Morrow]

I asked:

>(0)  Did the USN go through a high-z to low-z headphone transition
>the way the Signal Corps did?  The ARA and the early SCR-274-N
>receivers likely must have both had high-z AF output.

I've done some research through the manuals for the RU-3/GF-1 (1934),
RU-16/GF-11, ARA/ATA, ARB, SCR-274-N, and AN/ARC-5 since I asked that
question and found that I made a bad assumption.  All these have low-z AF
output, though the ARB is selectable for high-z.

It appears that the USN had been using low-z headphones back to at least the
early RU/GF series, and thus did not go through the transition from high-z
to low-z the way the Army did in the middle of WWII.

My incorrect assumption that the ARA has high-z AF output was based on the
fact that the ARA and the early SCR-274-N receivers were nearly identical,
and the early SCR-274-N sets are high-z.  But one significant area in which
they are different is the AF output transformer.  SCR-274-N A-model
receivers use a different AF transformer that had only high-z output.  Later
B-models had a low-z tap.  The ARA receivers, OTOH, were always low-z.
Presumably that would cause a minor problem if one tried to use an ARA
receiver in a high-z version of an SCR-274-N installation.

I wonder what prompted the Signal Corps to make the transition from high-z
to low-z in the middle of a war?  JAN compatibility?

Mike / KK5F