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Shure/RCA mic, 99A128 element

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Dec 26, 2009 6:09 AM GMT

I posted some photos of the mic and element in question in my profile as a new album called "RCA mic." This mic looks like it was made by Shure for RCA. It has an element marked "99A128" and "448." Because of the latter, I believe this mic was made in 1948. I've bought it, but have not received it yet, so I can't tell you what it says on the mic's tag. I've done a search, and found conflicting reports on the element. I believe that it is medium impedance. There is some disagreement in the few sites that mention it, but I believe it to be a crystal element, so I'm guessing that this mic is the RCA version of the Shure 707A or 18-51.

Do any of you have a better idea about it?

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Comments (13)

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Dec 27, 2009 1:35 AM GMT
OrthodoxBlues Replied:

I found a now finished eBay auction for a Shure 508 microphone with the same element. The fact that the beat-up mic in the auction still worked was a good sign. More importantly, the auction had a link in it (see below). According to this very old 1941 guide to these mics, the element was "dynamic," but also work on a magnetic principle. So, is this mic what we now call a dynamic? Any chance that this element was a precursor to the beloved controlled reluctance (CR) element?

Dec 27, 2009 4:47 AM GMT
BlowsMeAwy Greg Replied:

Technically, CR's and CM's are dynamic mics. To me, a dynamic mic is any mic that uses electro-magnetic induction as its principal of converting sound waves to electrical energy. That involves moving a magnet relative to a coil, or at least disturbing a magnetic field and sensing it.

Crystals and ceramics are different and use a "piezo-electric" principal. That is where you mechanically stress a crystal and get electrical energy out of it.

I doubt your element is a crystal, because as far as I know there's no such thing as a low imepdance crystals. Dynamic mics can be set to any desired impedance by the ratio of the number of turns of wire on the coil. Clearly the mic in question was available in high and low impedance models.

Once you get the mic I'd like to see a pic of the element....


BlowsMeAway Productions
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Dec 27, 2009 5:00 AM GMT
OrthodoxBlues Replied:

Greg --

i was hoping that you might weigh in on this issue. To tell you the truth, I've been wondering how one would classify the CR/CM mics. Having come to no conclusion before reading your post, I was calling them "magnetic." It is interesting to know that they are a type of dynamic mic, as opposed to ceramic and crystal.

Dec 28, 2009 1:41 AM GMT
OrthodoxBlues Replied:

Joe --

Well, you're probably right. It probably is a carbon element. I think that I'll give it a try in any case, but it may wind up with a replacement element. More photos once I receive it.

Dec 30, 2009 7:18 AM GMT
OrthodoxBlues Replied:

I received the mic today. A bit more completely beat up than expected, but that doesn't bother me. One screw that holds the grille to the body is missing; I'm hoping the local hardware store has something that fits. I will take more pictures, but there is no tag on the mic, and no sign that there ever was.

I wrote to Dave Kott, who hosts the very informative site, and he agreed with Joe that it probably is a carbon element.

I recalled later reading one of the few internet posts about this type of element, and one fellow had just an element with this code, and measured 500 ohms. Is that possible with carbon?

I'll try to connect some wires temporarily and see if I can get some sound, and also get some more pictures. I can't get it to the amp (and mic) tech for at least a week and a half.

Jan 16, 2010 3:32 AM GMT
OrthodoxBlues Replied:

This mic is at the tech's shop now, so I'll know whether it works soon enough. Whether it sounds good with harp may take a bit longer!

Jan 22, 2010 7:18 AM GMT
OrthodoxBlues Replied:

Sadly, the element is dead. I suppose I should see if I can open it up to see what it looks like. I'm considering sending it to one of the guys that resurrects old Shure CM and CR elements, to see if it has any features in common, or maybe they can put the guts of a CM or CR in the case of this one!

Jan 23, 2010 5:25 AM GMT
BlowsMeAwy Greg Replied:

It sounds to me like you might have developed an emotional attachment to this element. Does it have some sort of sentimental value to you? If not, my advice is to cut your losses. Getting bad elements is an unfortunate but very real part of this game.

When CM's and CR's are "fixed" they are disassembled - the coils are rewound, the parts are cleaned and they are reassembled. This can be done with CM's and CR's because of the way their made, and the fact that you can still buy coil winding wire. It cannot be done with crystals because of the way they're made, and the fact that you can't buy the kind of crystals they were made with any more anyway.

Sorry for your loss......

BlowsMeAway Productions
Bluestate on CDBaby
Bluestate on iTunes

Jan 23, 2010 6:01 PM GMT
OrthodoxBlues Replied:

Thanks, guys! I did develop an attachment to this element, and the mystery of what type it is, but the chances that it is worth resurrecting are small, as you both (Greg and Harpaholic) point out. The right thing to do is to put in another element, or let it be a display item!

I seem to be better at practicing non-attachment in other areas of my life ... but not these super-cool old mics! I'm totally okay on the financial side. I paid little enough that it was an okay price for a cool old shell.

I love old, worn musical stuff ... including guitars and amplifiers, too. It seems like they already have music in them.

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