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Diggin

Picked up a few interesting pieces today.  Above, an Altec 660B microphone circa 1958.  I already had a 660A (same thing, but fixed impedance) but this was too good a deal to pass up.  Altec marketed these as ‘broadcast mics’ but both of my units, while having pretty good top end, have a pretty weak bass response.  The 660B sounds a little bit better to my ears.

The 660B came mounted on this beautiful Shure S36 tabletop mic stand; that’s a push-to-talk DPDT switch mounted on the front.

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Moving on to stranger fare: above, a “Little Mike” as made by the Brooklyn Metal Stamping Company circa 1930.  This one confused me for a minute as it had no markings on it other than a patent date on the rear:

This was enough information to coax Google into revealing the origins of this artifact.  See here and here for the details.

The Little Mike’s rather long stretch of two-conductor cable terminates in these unusual copper discs.  As it turns out, these discs are intended to be attached thru two of the pins on a radio’s detector tube; this will allow the mic signal to come out of the radio speaker.

The question is, naturally: which pins?  The grid and the ground-side filament, I assume?  I can’t figure out how to get sound out of this thing.  I get no DC resistance reading across the two terminals, and no sound when I connect the terminals across a high-gain, high-impedance input.  I am guessing, based on the patent date, that this is a single-button carbon mic, which would mean that I would need a low voltage source and a signal transformer that can handle DC on the primary in order to test it.  Anyone have any suggestions/advice?

***update: read the comments section for implementation information courtesy of M. Shultz, as well as the not-so-thrilling conclusion to the saga of Little Mike.

8 replies on “Diggin”

The common AA5/Champ opt will do in a pinch if you don’t mind the LOW Z output. There are universal outputs from the radio catalogs that will work too.

right, sure… i thought about that, but that will give me a voltage loss of 20 rather than a gain of 20… looking at the old xfrmr catalogs it seems like single button input transfos are about 22:1 step up… 60 or so ohm to 60k or 80k ohm. Now, if a SE transformer (like a champ OPT) can handle DC on the primary, does that mean it can handle DC on the secondary? Cos if could wire it in reverse that ratio would be perfect…

The DC offset on the secondary is the same as that on the primary as far as the core is concerned. The thing is that the secondary has many less turns, so a given current should magnetize the core less. The core of a single ended transformer, like a swinging choke or modulation transformer, has an air gap (Luftspalt) so it doesn’t saturate. Also some universal outputs, or the one from a Collins shortwave radio (available as a repro) has a 600 ohm output.

As with everything else, experimentation and taking notes is key.

You should have this book:

http://www.amazon.com/Practical-Transformer-Design-Handbook-Lowdon/dp/0672216574

It shows up on ebay and alibris reasonably every once in a while or interlibrary loan will give you a copy to nondestructively scan. (Oh what ever happened to the CapShare???)

pmillett.com has some free books that are pretty good too. Reuben Lee is the one Mikey is always yapping about.

It is indeed a single button carbon microphone.

I have the instruction sheet for a very similar unit in front of me. The connections were to be to the plate and cathode (or filament), not the grid and cathode of the detector. That actually makes sense as the plate resistor for the detector allows a few ma. of current to flow through the microphone. The audio signal developed across this resistor is coupled through a transformer or capacitor into the grid of the first audio amplifier stage.

The little button on the back must be pushed in to use the mic, and that disables the detector. So, the mic substitutes for the radio program, you can play announcer, but you can’t “sing along” with the program (probably a good thing). The patent mentioned on the back is no. 1,790,636, which can tell you much more than you ever wanted to know about this gadget.

I would say just try it, but put a milliammeter in series with the mic, the generally accepted current is 10-50 ma. (per Radiotron Designer’s Handbook IV) for a carbon mic.

A typical detector tube in a radio of that vintage draws only about 5 ma with about 100-150 volts on the plate.

I wouldn’t be too optimistic about getting this mic to work. They were basically toys, and very cheaply made. The switch might be dirty, or there may be internal corrosion, a problem that seems to affect many old carbon mics.

Hi Mike. yea i tried it today with a 9V and a champ OPT wired backwards. I got nothing out of it. the case of the unit appears to be welded shut so i could not open it up to investigate the switch situation. I’m moving on. thanks for the tips. c.

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