Demaging 2440, Aka, Investigating Possibility Of Converting 2440's To Field Coil
Quote:
Originally Posted by
Mr. Widget
John,
I don't know the answer to your question, but when I queried JBL about working on the 2441, they told me that they demag it before they work on it and implied that it easily came apart once the demag was done. I doubt you can completely demagnetize one without a very powerful electromagnet.
Widget
My two cents,
This is a timely subject since i have been considering converting a perfect pair of 2440's with new JBL 375 / 2440 diaphrams_ still in the box_to field coil format which is probably as close as i will ever come to a WECo 594-A driver. The first issue being: "Is there enough room for a field coil winding in the volume occupied by the 2440 magnet".To this end I plan to investigate the huge crane, electrical magnetic lifting devices in the iron junk yards, aka, "are these devices dc or ac operated".
There is no hope of duplicating the original Weco diaphragm which was made of a rare no longer available aluminum dural alloy capable of heat treating to a fantastic hardness contributing to an improved mass break point. This gem presents a blue gray color from the intense heat treatment.
Not to worry__JBL 375 / 2440 diaphram is just fine. I do not find any significant spurious oscilltions in the high end with "tone burst" examination and certainly not in the quality of the sound.
I do miss the WECo 594-A drivers i had in the 1970's on WECo 26-A multi-cells on top of the RCA 9462 ubangis loaded with RCA MI 1432-A field coil low frequency drivers from the "Shearer W-Bins". All the drivers had the voice coils in the same plane using the MI-9462 which has excellent response down to 30 Hz.
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Cheers, herki
Bullet Proofing Compression Drivers
Circa, early 1950's, RCA engineering decided to stabilize the Photophone MI-9584 series compression drivers by filling the entire driver body cavity with enert epoxy up to the voice coil gap. This reqirement surfaced when the __acoustic seal__ between the voice coil gap and the driver body cavity was questioned by Chief Engineer, Mr. Max Batsel.
This procedure obtained two fabulous results, aka:
(1) The gap cavity was completely isolated, The assemby components were solidly locked in place and the top plate could still be forced off with the cover screws by a procedure similar to Steve Schell's procedure described in his post of 08-29-2005 04:18 pm.
(2)Closing off completely all comunications between the diaphragm piston volume and the driver body cavity, removed a suck out and extended the the driver fs to some 200 Hz.
This modification was installd after the epoxy component was verifid to be completely enert and not capable of absorbing any moisture whatsoever.
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cheers herki
Adding a Note to "Bullet Proof Compression Drivers Text"
[quote=herki the cat;259457]Circa, early 1950's, RCA engineering decided to stabilize the Photophone MI-9584 series compression drivers by filling the entire driver body cavity with enert epoxy up to the voice coil gap.......................................
Purhaps unique to RCA, it is noted that RCA used Massive Permanent Ring-Magnets in every professonal speaker & compression driver from the very begining in the 1930's.
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Cheers, herki the cat