[Milsurplus] T-17s, ART-13s and BC-375s

[David Stinson]

One of our members recently wrote me with a question.
I pass it along for general information.
------------------------

 > Something you may be able to answer...
 > I run a couple of ART-13's.  Both have good
 > signals but the speech is quiet.  They produce full 100w+
 > and good carrier, however other Hams are saying
 > they expected more power in
 > the speech. I am using a standard carbon mic T-17b...

I have owned over a dozen T-17s and none will modulate an ART-13
(or TCS, or ARC-5 etc.) more than 40-50% - most much less.
They were designed to be insensitive due to the
high noise environment in the aircraft and age
has not improved them.  Training manuals
specifically instruct the user to hold the microphone in contact
with the lips, to speak loudly, clearly and "like President Roosevelt,"
as one says.  The speech amp in the BC-375 is simple but
more robust than the one in the ART-13, which, in normal use,
was meant to receive its audio from an intercom system.
The 375 speech amp has the gain to work with a T-17,
but you should not attempt to "push" it
to make a stock T-17 work.  If you do,
the modulator will deliver short rise-time pulses of current on
voice peaks and this pulls the MO, adding to the FMing problem.
I use larger carbon mike elements and lower gain settings on
the 375, just "wiggling" the plate meter for full modulation,
which I set using an oscilliscope display;
I don't attempt to go by the manual-specified ~300 mils.
The stability of the transmitter is much improved.
Try a standard telephone microphone element on the ART-13
and/or the BC-375 (TCS, ARC-5, etc.).
This is one instance where "historically accurate"
just won't work, and it's one of the few exceptions
I make to my "original design" rule.
73 Dave S.