Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Cabinets-Speakers => Topic started by: murrayatuptown on August 14, 2024, 06:39:03 pm
-
Hello:
Exploring field coil/electrodynamic speakers, I acquired on with several US patent numbers but no identification.
The patents I looked up so far are from Karl Jensen, Peter Jensen & Lektophone.
There are two numbers that suggest product ID, and one where >~1960 date cades appear doesn't have date-valid format.
C12B (surely unrelated to modern Weber PM C12B)
7785A
12A204
It has the common drawn metal cup electromagnet cover with a retaining screw, but I couldn't budge the cover after removing the screw (had hand surgery a month ago). I'll try again with better grip.
I didn't take any photos yet because I didn't see any thing I needed to photograph yet.
Look familiar to anyone?
Thank you
Murray
-
Patents are all Jensen except a few for Lektaphone (forgot spelling) who were suing Rola & Miller Bros. over patent infringement in the 20's or 30's about speaker cone manufacturing.
I couldn't figure out another way to add images directly.
There is a lot of similarity among speakers back then like there is today.
7.2 ohm DC for voice coil...call it 8 ohm impedance
6.44k field coil - call that 6500 ohms. aiming for 12 W DC magnetizing field coil power for a 12" speaker, that's 280 VDC, 43 mA
Good thing I took another look at it after eBay delivery...a dozen spider egg...cocoons? Took them outside. I better check the other vintage speakers.
-
Found this speaker's origin.
~1936 Coronado 2CM floor-standing radio with three 12" field coil speakers and 6F6 PP pentode amplifier (only several watts). Speaker #1 had 6400 (not 6500) ohm field coil and was "used as a voltage divider". I didn't relate to that with split-schematic-zone representation of the source sockets and destination plugs being separated. Speakers #2 & #3 had 1000 ohm field coils that were used as filter chokes. I have to revisit the service manual to see if it makes more sense the next time.
This speaker looks more substantial (relatively) than the others and has hardware rather than welded attachments. Maybe this one is more easily refurbished. Thinking outboard cab speaker with field coil power supply to spare the ancient amp unnecessary surgery.
-
add'l attachments in separate posts due to size of images.
I know the origin, but not the mfr.