Large format alnico driver.

[John]

Well look into the throat and about 1/2 inch into the throat if you see a crack or what looks like a ridge it means the magnet has shifted and has damaged the plastic throat. The entire driver is held togeather by magnetic energy. When I get some replacement throats I will post some more pics.

[tomt]

Hi!

How do You tell if a throat is broken?

Regards: Jakob
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gurgling noise on vocals

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perhaps some bolts should have been used when these
were designed

[alltec]

Hi John,

Did you get the replacement throat?

[John]

Not yet! I asked someone on the forum to look into a pair for me but never heard anything? Maybe I should get off the couch and phone JBL?

[Steve Schell]

I learned a good tip the other day. If you need to disassemble a magnetized Altec or JBL alnico compression driver, you can do so without the bother and the danger of prying up the top plate. Instead, bolts can be threaded into the holes for the mounting studs on the front of the driver. Usually something like a ball bearing needs to be dropped into each hole first, as the threads are tapered and will tighten on the bolt before it reaches the front side of the magnet. Anyway, tightening the bolts down in sequence applies pressure to the front of the alnico magnet, in turn forcing the top plate away from the rim. By the time it is lifted by 1/8" or so, the magnetic force is reduced to the point where the driver can be disassembled easily.

The driver will of course have to be remagnetized after it is reassembled.

[herki the cat]

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

[robertbartsch]

This post is interesting.

I had a pair of 075 alnico tweeters that I sent to get re-maged without including the horn noses in the shipment. When they came back, I tried unsuccessfully to assemble the drivers. I was unable to mount the noses because the bolts would not go through the motors.

Apparently, when the units were de-maged the pole plate (??) got shifted and as a result the mounting bolts could not be installed after re-mag.

Anyway, I sent the units back with the noses and the vendor succesfully de-maged, assembled the nose and then re-maged the units and installed the new frams.

I would have not guessed that JBL drivers are aligned and held together by magnetic force.

Live and learn!

[tomee]

I've attached a pic of a cracked throat from a recent ebay auction. I had no idea there was a plastic part in these. From a design standpoint I wonder if it's more than a cost cutting measure, but a way to sacrifice an easy to replace component to prevent failure someplace else? For example, if the throat was steel or aluminum perhaps dropping it would result in a cracked magnet instead of cracked throat? Given that (when sent for service to JBL) a de-mag has it all in pieces, and rebuild with shims and jigs is required to put it back together and then re-magged, then essentially a plastic throat is no big deal? Of course from the standpoint of making it last 100 years a stainless steel throat would be nice .

Herki - your idea to make a field coil from the 2440s is interesting, but why not start with some rough ones that have cracked throats? I think you'd need to machine a new throat from steel anyway for the coil to go around. I believe there are a few people out there who have done this - a few in the US and a guy in Germany as well. Might check around to see if they sell a conversion kit. As well, I see that Classic Audio Reproductions is now selling field coil drivers so there's always that route, money permitting!

[herki the cat]

Converting Alnico 2440 To Field Coil Driver

[quote=tomee;259246]I've attached a pic of a cracked throat from a recent ebay auction. I had no idea there was a plastic part in these. from the standpoint of making it last 100 years a stainless steel throat support thin cylinder would be nice .

Herki - your idea to make a field coil from the 2440s is interesting, but why not start with some rough ones that have cracked throats? I think you'd need to machine a new throat from steel anyway for the coil to go around. Might check around to see if they sell a conversion kit.
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Hi tomee, many thanks for the picture & suggestions. The thin fragile plastic cylinder appears to extend all the way from the motor housing bottom up through the magnet to the phase plug bottom.

A thin stainles steel replacement cylinder with tapered throat inside the field coil Armco Iron core would certainly provide excellent support for the field coil configuration espcialy during field coil zero dc current excitation conditions where the alnico magnet force normally holds the assembly together.

Also, I agree there is no reason to disturb two priceless working 2440's when distressed 2440's are available.
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cheers, herki

I go crazy when i hear the music since 1930

[herki the cat]

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

[herki the cat]

[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

[Richard Long]

hi guys,

can i use this 2441 still for 100%? the throat has two separat haircracks:

http://i227.photobucket.com/albums/d...g1/Bild399.jpg

http://i227.photobucket.com/albums/d...g1/Bild401.jpg

[Mr. Widget]

hi guys,

can i use this 2441 still for 100%? the throat has two separat haircracks:

You may or may not be able to hear it buzz... when I had the same problem a few years back I shipped it to Northridge and they fixed it... they no longer offer that service. I am not even sure if replacement throats are still available if you can locate someone to demag, replace the throat and remag... I think you are stuck.

Sweep a signal and see how it works. It is possible that the throat is cracked because the top plate shifted... if that is the case you may have VC rubbing problems.


Widget

[Richard Long]

You may or may not be able to hear it buzz...

Sweep a signal and see how it works. Widget

I hope so

Yea I test it and then I see ...