Diggin

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• 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.

  • very helpful. thank you. c.

• 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.

  • Thank you Mike. If I implement this circuit (http://stonito.com/images/picture/?id=153) to try out the mic, do you have any idea how many VDC it would require? Seems like there is probably a 100v difference b/w the cathode and the filament of a detector tube, but could this circuit work with a 9V battery?

    • 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.