I didn't know you could take a duplex apart like this:)

[Elitopus1]

Thanks bowtie. This is the first time I have seen one taken apart. I always figured you couldn't do that. Is the 515 section of the 604 the same as a regular 515? Like are all the magnet slugs made with the 1" hole in the middle?
There is a 515b listed on ebay that has a hole in the magnet cover. It looks like it was either a 604 previosly or all 515's have that hole that is covered somehow. This one

[Old Guy]

The 515 uses a ring magnet. In the center is a steel pole piece. The hole is thru the pole piece.
In most modern coaxials the hole normally used for back venting is used.

[Elitopus1]

So if the ring magnet is around the outside of the VC the whole center piece is solid steel. This is a pic of my 515b.



That would mean that very early in the production of the woofer it would be built as either a 515 or a 604.

[bowtie427ss]

My bad for calling it a "slug".

All of the Altec alnico motors from the 755 to the 802 to the 406, to the 288 and 515 use a ring or donut shaped hunk of alnico.

You'll really only find solid slugs in the smallest transducers like the 401/ 402a, 2000 and 3000 series tweeters, and microphones. Generally, any speaker motor you see that uses a formed or folded return structure will have a solid(read small) alnico slug that also serves as the pole piece.

These 2000 series tweeters might be an exception, it looks like they have a small donut around a steel pole.



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So if the ring magnet is around the outside of the VC the whole center piece is solid steel.

It's not.(around the voice coil i mean)

The magnet is around the pole piece. Yes, unless it's a duplex where the HF plays thru the center of the woofer's pole, the pole piece should be solid. On your 414 woofers there should be a bolt visible thru the dust cap. I think that pole piece has a step in it such that the bolt clamps the magnet between that step and the back of the return pot that it's bolted to, pretty sure the alnico 416's are built this way too.

If the suspension would allow you to push the cone in far enough, the voice coil former would bottom out on the magnet itself.

The pole piece, return pot, and front plate form the magnetic circuit. The magnet itself needn't be located all that close to the voice coil if the other parts of the circuit are doing their jobs. The magnetic gap that the voice coil operates within is formed by the pole piece's concentric location within the front plate

[Elitopus1]

Ok gotcha. Nice explanation