Identifying a Carbon Mic Element~

[Astronaurt]

Hey all, I suppose this is a wee bit of a tangent off from pedal design, but I'm looking to try putting a Carbon Microphone Element into a guitar, kind of a la Jack White. I'm having trouble finding information on identifying the element itself in order to get specs on Current draw, maximum bias voltage, output impedance, etc. etc. so I'm wondering if anyone has got a direction I could wander in to get that info?

I've got several elements like this one, no brand name is apparent on, but what looks like an identification number on the back of it reads "75555 2-68"

I guess I understand that the Shure 104C is as ubiquitous as any carbon mic element, could this be analogous to that? Thanks Stomperz, Peace,

[EBK]

Well, I can tell that probably came out of an old telephone.  Maybe try searching generically for specs on telephone carbon mics. 

[EBK]

Here's a tiny bit of info
https://nationalstocknumber.info/part-number/75555_006610469

[Quackzed]

https://en.wikipedia.org/wiki/Carbon_microphone
some info here on carbon telephone elements

seems they dont NEED alot of voltage, but low v may not be ideal...

Most all-electronic telephones need at least three volts DC to work, and so will often become useless in such situations, whereas carbon transmitter telephones will continue to work down to a fraction of a volt.

[PRR]

That sure looks like the carbon mike from most late-20th-century US telephones.

Put the ohm meter to it. (Ring and button.) Telephone carbon mikes were 30 to 150 Ohms (memory fades). This is similar to some dynamic elements, true. So rig up to TAP (not hold!) a 1.5V battery on it. A dynamic mike is also a (poor) speaker. It will click. A carbon mike is utterly one-way.

And of course if you really gonna do this, you may as well prepare a 1.5V battery and 150 Ohm resistos, blocking-cap, output jack, for tests.

A very old carbon mike may have settled. Bell Labs did a bunch of work to reduce this, but the dust clumps when long idle. Tap it on the desk in all directions. "Tap" like you might tap a nice shot-glass to get another drink, not like "tapping" (slamming) a weed-wacker to get more string.

[Astronaurt]

Here's a tiny bit of info
https://nationalstocknumber.info/part-number/75555_006610469

Awesome, thanks for the link! Very helpful. I wonder if I can find a different element in this batch that demands less current... is it possible to starve current to the element and still get a signal out of it, or would it just cease to function consistently or predictably when given too little current?

Telephone carbon mikes were 30 to 150 Ohms (memory fades).

And of course if you really gonna do this, you may as well prepare a 1.5V battery and 150 Ohm resistos, blocking-cap, output jack, for tests.

Interestingly enough, I'm getting wildly inconsistent readings on my ohmmeter. First, 1.5k, tapped it a bit, then about 2k, tapped it out a bit more, then about, 200Ω, 150Ω, now it's back to 1.5k. If the National stock number entry matches this one and it's supposed to be 40Ω, seems like it's been spending way too much time in a box.

[EBK]

A random-value tap pot?  Hmm....  That could have interesting uses....

[PRR]

Carbon mikes have a MAXimum current, which will burn them up.

In old-school passive telephony, you want to run a large fraction of the allowed current for large output.

In a modern studio filthy with preamps, you can run much lower current. 1.5V through 150 Ohms should be fine. One D-cell should power it for 2 months non-stop, or 2 years as a studio effect.