Permanent Magnet as a Philco Replacement Speaker? - Printable Version

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+--- Thread: Permanent Magnet as a Philco Replacement Speaker? (/showthread.php?tid=318)

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Permanent Magnet as a Philco Replacement Speaker? - Guest - 04-04-2006

I recently acquired a Philco model 38-5 tombstone that is missing the speaker. I have placed requests in the "Wanted" sections of various radio sites but have come up empty. As a temporary solution I would like to install a permanent magnet speaker until I someday find an original. While I've restored a quite a few radios in my time I have never had to replace an electrodynamic speaker with a permanent magnet type and am unsure how to proceed.

I'm missing:

- the output transformer
- the field coil

(are the output transformer and the field coil the same thing?)

- the voice coil

I've done some searching and it sounds like using a choke of the appropriate rating is preferred to using a resistor and bumping up the capacity of the filters when replacing the field coil so I'd like to go that route.

In the absence of the original speaker how can I determine what the voice coil resistance is?

I'm not even sure how to begin . Any advice would be appreciated.

Thanks,
Geoff

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- Chuck Schwark - 04-04-2006

The Model 38-5 used two speakers, one in the table top, or B cabinet and another in the console, or X cabinet.

The B cabinet uses a Model H-29 speaker.
The X cabinet uses a Model K-39 speaker.


Both speakers have the same field coil which has a D.C.
resistance of 660 ohms. Both also have the same voice
coil impedance of approximately 2 ohms at 400 Hz.

A PM speaker of 4-8 ohms should work just fine however.
A replacement power resistor to sub for the field coil can
be a 680 ohm, 10 watt unit to run cool.
This is assuming a max current of 90 mA.

Alternatively, you could starts looking for a H-29 or K-39 to fill the bill.

The Model 38-3 and 38-4 also used the H-29 speaker.

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- Guest - 04-05-2006

Thanks for the info Chuck. I posted the same question elsewhere and got some good follow up:

http://antiqueradios.com/forums/Forum11/HTML/007746.html

I also noticed in the schematic that the 38-4 console uses the same chassis with an H 29 speaker (my table top model 38-5 uses the K 39). There is an H 29 speaker on eBay right now but at 11" I think it would be too big for the table top model. And from the description it sounds like it has been modified:

http://cgi.ebay.com/6619460062

There is also this H 21 speaker but I'm not sure if this would work for my application:

http://cgi.ebay.com/6619301772

Regards,
Geoff

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- Chuck Schwark - 04-05-2006

Yes, that's the differeence between the two speakers, just the size; smaller one in the table-top cabinet, but with same electrical specs.

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