23744 vs 35480 dia & questions about 904-8A Duplex

[quaramon]

Hi, I recently bought a nice pair (home- but carpenter made) 620 enclosures with 904-8A drivers.
I was under the impression that 904-8A had Symbiotic diaphragms but when I open them up they had 35480 dia.
Unfortunately the two 35480 dia isn't exactly identical, one seems to have a bit wider voicecoil than the other. Se pictures.
They are both stamped 35480 but otherwise the stamping is a bit different (last picture). One says DEC 81 (short vc), the other (wide vc) "86" but unsure if the latter is a date or not.
The wider voicecoil measure 6.7 Ohm, the other 6.3 Ohm. I can't hear any difference between them but will measure them later.

Some questions;
1. Was Symbiotic original in 904-8A?
2. Is it correct that 904-8A is identical with 604-8K except the hf dia and a slightly heavier bass cone that can handle more power?
3. Is 35480 same as the "famous" lightweight but fragile 23744 except for the leadwires and terminals? This is what Todd is saying here (and yes I believe him!): https://www.hostboard.com/forums/f700/51740-902%20diaphragm%20ooos/index2.html#post740963
4. Is it official that there exist different versions of 35480 (I seem to have that, but who knows....)?

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And another question, the backs (don't know what they are called) behind the hf diaphragms, one has a rubber gasket (pretty thick, see picture, the other none.
I have no memory of gaskets here from my previous Altec drivers, what is standard?
And yes, I will test with and without the backs but shall set up the measure equipment first.

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[RonSSS]

I’ve had a pair of 904-8A for many years. These were sold as sound reinforcement speakers. They were available in the Stanley Screamers. Also sold separately for ceiling use etc.

I can verify a few things as I rebuilt mine into 604’s.

Yes the HF diaphragms were Symbiotic. Each had a thin paper gasket under the fram and used loading caps.
I replaced mine with 34647 kits from GPA years ago. Those included felt pads inside the cover. I removed the loading caps. While apart I cut out the bug screens.

Just last year I replaced the woofer cones myself with aftermarket parts from MWA. The same parts would also be used in a 416. So in essence, the woofer section is actually a 416 now. They measure very much the same as a 416 to boot. Which would be expected.
The original cone and voice coil are actually Musical Instrument parts. HD voicecoil wound on a vented aluminum form.
I know they are MI parts because I also re-coned a pair of 418-8H II guitar woofers into 416’s. Same parts.

MI speakers just don’t sound good for audio use.

I detailed all this work over on HiFi Haven in the High Efficiency section if you care to go look.

I can’t speak to the diaphragm parts you have. I’m not familiar with that part number. If the same as the 27344, it’s a winner. The alum is thinner. It can’t be measured w/o harming the fram sadly. 27344 are .0021” thick compared to .0027 for the 34647. I measured some dead ones.

Unfortunately your photos here don’t seem to work.

Ron

[bowtie427ss]

Each had a thin paper gasket under the fram and used loading caps.

Generally, this is a shim used to maintain clearance such that the diaphragm does not contact the phase plug during minimum freq @ max power. I've only ever seen the shims in symbiotik equipped drivers

[RonSSS]

That was my guess as well Tom.

[quaramon]

Thanks for good information, I don't know why my photos doesn't work, they are uploaded with the "Insert image" button in the tool bar.
I'll investigate that.
Yes, if the 35480 are just a later version of 27344 I'm very happy since I'm been looking for those for ages.
Let me come back when I hopefully fixed the pictures and also done some acoustic measurements.
I also have a pair of unused 34647 from GPA that I received just a couple of months ago, can be interesting to compare.

[RonSSS]

Get top and bottom photos of the GPA parts. We are curious how they are built.