Field coil speaker question

[Murrayatuptown]

Hello:

I have been accumulating some field coil speakers and replacement 'universal' field coils. Like lottery tickets, there will be some losers, but hopefully some winners.

I have one that is marked C12B on the frame along with some other stamped info I don't recognize yet. It absolutely has a field coil, so it's coincidental Weber has a PM speaker called C12B.

I think it predates modern date codes.

There is a patent recognition (credit) sticker on it, and several are from Peter and Karl Jensen. The earlier ones are from a company I don't recognize. That doesn't say who made it, just who they acknowledge/credit to.

Anyone recognize C12B as a field coil era known entity?

Thanks

[johnnyreece]

Are there any other number markings? Your description sure makes it *seem* like a Jensen. If you could find a number starting with 220 somewhere, that would clinch it.

[Murrayatuptown]

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[Murrayatuptown]

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[Murrayatuptown]

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[Murrayatuptown]

J.R.

Someone on an antique radio site told me 'definitely' a Jensen.

Also found that it was from a Coronado 2CM floor radio that could have three 12" speakers, per the service manual, which shows it as also being sold as Wells-Gardner, maybe also Western Auto, and sold at Monkey Ward's.

The service manual was interesting. Two speakers had 1000 ohm field coils and were used in series as filter chokes as commonly expected. However, the third has a 6400 ohm field coil, and the person who helped me find the manual and insists it IS a Jensen says was used as a voltage divider resistance, and essentially in shunt rather than series. So it wasn't a filter choke, just a field coil. It would take quite a bit of voltage since the resistance is so high. Mine measures 6440 ohms for the field coil, and 7.2 for the voice coil. Output transformer was in the chassis.

I had it re-coned & voice-coiled. The phenolic spider looks like the Jensen mid-30's ones. I've not seen them in person, but patents show them. The shop was able to reuse the spider, which they were pleased with.

While there, they tried to entice me with a pair of Capehart/Jensen f.c. speakers. They had C12R marking on the baskets. I forgot the model radio, but current and past sales on eBay for C12R pairs each contributed some more clues. The model of radio (which I have forgotten, but just tells me it was a radio, not a musical instrument amp), another p/n 81-64 and a foil-stamped label on the e-magnet cover that says 7.5 W, 80 V and DC Cycles. Other listings for this speaker reported measured 680 ohm field coil. Some speakers had f.c. resistance on the label, but none of the the Capehart speaker listings I've found had resistance on the label.Crazy prices being asked maybe because someone thinks Hammond (nope, not in mid-30's, but the age is only evident from attempts at educated guesswork.

So I wanted to interpret the 7.5W next to the 80V markings to indicate field coil info. Some old catalogs from Radio's Master that reportedly started in 1935 list both field coil nominal power and voice coil power data. But 80 V and 680 ohms = 9.4 W. 7.5 W and 80 V leads to 853 ohms via Ohm's Law. So maybe I'm wrong.

I did not look closely at the Capehart pair offered to me. I didn't know what to look for at the time. They were in original condition, and the shop owners felt they should remain intact. I wasn't too thrilled with the price, but it was a lot lower than goldminer listings on eBay and Reverb. I might revisit them next week & see if they say what all the others do. I have to keep in mind they are from a huge floor-standing console radio. Maybe OK for modest power MI amp, but not big amps. Many 30's speakers had <10 W voice coil power specs (whatever that meant back then).

Jensen had a wide range of f.c. speakers in the 30's-40's, from lower power 12" to the Concert series that were beefier.

So, JR, back to your questions, I wrote all I could read on the box. I kept the box when I left it at the shop. It looks like they mopped some filth off, and I had a hard time reading everything I did previously. I don't think they had EIA Mfr. codes or date codes using the format we look for, back in the mid 30's.

I am now thinking was NOT marked C12B, because the poorly legible B was not symmetrical in the upper and lower loops (rubber stamp writing analysis?) Now it looks more like an R or K which isn't helping my confidence.

The writing I put on the box from what I read on the basket: C12? (? could be B, K or R?), 7785A, 12A204 and a displaced B further away. The 12A204 looks familiar but in a different era and not a not related to antique radios. The 7785A doesn't suggest a date code very easily, unless the 5 means 1935...going out on a limb far enough to lose my balance.

I wonder if the suffix letter with C12_ indicated the field coil resistance, and this being the highest f.c. resistance I've ever encountered, may make it unique. The Capehart Jensen speakers I think also had a 4-digit and letter marking after the C12R.

I think I should do something with the repaired speaker before entertaining hoarding more. I am going to put the 'Jensen' suspect into a cabinet with my own DC PS (sold in the 30's-40's as a 'field coil exciter'), for use as an EH-150 Echo Speaker. My '150, as purchased, already had the speaker replaced with a Fender/Jensen-Italy 'Special Design' speaker, so this the opposite of the original intent. The amp works well, so I don't want to mess with it.

Some 'modern' Capehart electronics didn't appeal to me at all, but the 30's consoles were supposedly pretty respectable home audio in the 30's. Hard to imagine what that meant with a Victrola playing a 78 rpm record as source material.

I might grab the Capehart/Jensen pair without waiting for the test of the first one I'm asking about. I should be able to get my money back out of them, based on the hysteria pricing these days. Or I can use them.

In left all this info in case someone has insomnia, or might find it useful or interesting.

[Murrayatuptown]

1936 and 1942 Radio's Master Encyclopedia told me about the Jensen C12-R speaker.

Field coil resistances as high as 11500 ohms were catalog items, but any value 'could be accommodated on request'

The A-12 and B-12 were higher power speakers, both in voice coil power rating and field coil power being 18W max, 14 W nominal and 10 W minimum.
The C-12 was typically used in radio receivers, where mine came from. Field coil power was listed as 8W in 1942. The Capehart/Jensen ones I've seen on line were marked 7.5W. The voice coil power rating was between 9 & 11 W

The 18" Jensen's were another beast altogether, in size/weight and field coil power (33W max, 20 W minimum).

The field coil resistance and typical field power allow me to target a safe level in a shunt configuration with an outboard 'field supply' in an outboard cabinet where using a repurposed f.c. speaker is desired without using the field coil as a filter choke in an amp that no longer, or never did, have a field coil speaker.

There was no C12-B; I misread it due to fading or damage.

The stock number, which may or may not appear on the speaker (maybe it's paperwork & packaging had it) distinguishes among the field coil resistances and output transformer primary impedances.

If anyone wants to download old Radio's Master Encyclopedia catalogs, there are 24 separate volumes between 1935/36 and 1970.
https://www.worldradiohistory.com/Radio ... atalog.htm

It's pretty amazing to see what actually WAS available in the 30's & 40's, particularly the simultaneous coexistence of electrodynamic and permanent magnet design offerings vs. the random assumptions and conclusions people offer (understandably) online almost 90 years later.

I also had a Utah 10" electrodynamic (f.c.) speaker repaired. Their data is harder to find and missing a lot of the info Jensen printed.

Murray