[Milsurplus] T-30 throat microphone

[Mike Hanz]

Good morning all,

I received the e-mail below on efforts to use a T-30 throat mic in a 
1941 open cockpit Boeing Stearman, and was somewhat surprised at the 
report, as my empirical experiments with the T-30 some years ago 
produced about the same results that Bell Labs had concluded during the 
war.  Maybe my Adam's apple is deformed...:-(

I'm not alleging some sort of misunderstanding, but what do y'all 
think?  Has anyone else had the same outstanding performance with a 
throat mic?

  - Mike  KC4TOS


-------- Forwarded Message --------
Date: 	Sat, 10 Jan 2015 13:10:29 +0000
To: 	aaf-radio-1 at aafradio.org


After having received all the parts to make an adapter I tried it out on 
an aircraft at work and them gave it to a pilot to try in the air.
I had a few worries as to the intelligibility but was very surprised!

On the ground the throat microphone worked very well with the interphone 
and when emitting to contact the control tower. My voice wasn't 
distorted and my voice clear.

When my pilot friend tested it, he took off, flew for about 45 minutes 
and landed. When he stopped the aircraft, he asked the control tower if 
at any time they did not understand him or if they had trouble hearing 
him. The control tower replied that it could hear him just as well as 
with a normal microphone! Phew!! Maybe it was the old radios/interphones 
that were the problem rather than the microphones?

*********************************************************************

From: aaf-radio-1 at aafradio.org
Subject: Re: T-30 microphone

    I recently bought a T-30 USAAF throat microphone and would like to
    get it into working order.

    I came across your website and found lots of information about the
    avionics used in WWII aircraft and after having looked all over the
    web and in multiple libraries, I cannot find anything concerning
    these throat microphones. Would be able to help me? I am looking for
    any schematics or wiring diagrams that would be able to help get it
    working or any advice that you may have would be a great help.

Carbon microphones, like those in the T-30, are composed of loose 
granular particles of carbon.  In order to get any audio voltage out of 
them, you need to run current through them. They typically work with 
30mA to 80mA running though them (nominal 60mA), with more output at the 
higher currents.  The entire military line was standardized back in the 
1930s from a design current and characteristics standpoint, so they all 
plug and play in the radios of the era.  Typical amplifier load 
impedances driven by carbon mikes are in the range of 100-500 ohms.  
Attached is a typical circuit used in a command set to employ a carbon 
mike.  You will note that the load resistance at the tube grid reflects 
back to the microphone on the order of 300 ohms.  It isn't critical at 
all.  The same circuit is used in a small two place aircraft interphone 
powered by a couple of D cell flashlight batteries rather than 28 volts, 
so there the 360 ohm series current resistor is reduced accordingly to 
achieve the desired ~ 60mA current when the microphone switch is depressed.

Now here's the bad news:  these carbon granules gradually cement over 
time, depending on storage conditions.  The higher the humidity, the 
more quickly they deteriorate.  You may find yourself with a brick.  It 
is possible to restore them by removing the cemented carbon and 
replacing it with fresh granules - there are on-line sources for it.  
It's a great deal of trouble, and is usually done only for very 
expensive historic microphones.  The T-30 is a commodity - there are 
lots of them around, so you should be able to find another if yours is 
in that shape.  I have seen NIB units advertised on-line.  Of course you 
have no guarantees that even an unopened microphone won't be in the same 
condition.

Finally, while fun to play around with, /nota bene/ the conclusion of 
Bell Labs about the throat mikes they investigated:  "...a good idea but 
for the fact that the sound available from the larynx is basically 
unintelligible".  As long as your expectations are calibrated, then go 
for it!

All the best,
Mike