2405 Diaphram Question

[nickc]

A conical phase plug is used in 076/2203.

Mine are also 1.71".

Mystery resolved, perhaps.

Now I gotta do a buncha cone-swapping.

[Perhaps they'll work better with the correct ones installed.... ]

Zilch - thanks for you input. I also tend to agree that the 1.71" phase plug is the correct one and I base this on the fact the driver that had been working correctly (the one with the original diaphram) had the 1.71" plug in it. It looks like the other driver I have is not really a "baby-bum" but a culmanation of parts to make it look like one.

As I said before, even the pole piece hole is different between the 2, the working driver having a depth of 0.65" and the other over 0.7". I assume the phase plug hits the bottom of this hole and provides the perfect phase plug to diaphram clearance. If I am error about this please SOMEONE correct me.

Nick

[Zilch]

Zilch - thanks for you input. I also tend to agree that the 1.71" phase plug is the correct one and I base this on the fact the driver that had been working correctly (the one with the original diaphram) had the 1.71" plug in it.

Nope. I'm sayin' my 076's are 1.71". From Rob's pics, it looks like 1.60 is correct for 2404.

I'm gonna have to open an unmolested 2404H here to verify. Looks like Johnaec's doing the same, and we'll have an unequivocal answer soon.

[johnaec]

Well, I opened one 2404H-1, (out of four I have), and it's definitely got the 1.71" plug. I wonder if at some time JBL either lengthened or shortened the plug, resulting in a narrower or wider throat? This one also had the 2402 diaphragm, as suspected for the H-1. Now I'm going to have to check the others just to see what's matched, but that won't be done for at least several days...

John

[nickc]

Well, I opened one 2404H-1, (out of four I have), and it's definitely got the 1.71" plug. I wonder if at some time JBL either lengthened or shortened the plug, resulting in a narrower or wider throat? This one also had the 2402 diaphragm, as suspected for the H-1. Now I'm going to have to check the others just to see what's matched, but that won't be done for at least several days...

John

John & Zilch - I just want to thank you both for going through the trouble of cracking open your drivers for me. I *really* appreciate the effort.

Personally, I think the correct phase plug is the 1.71 but that still leaves the question of the pole piece hole depths being different. I believe the hole should be 0.65 and not 0.7. That extra 0.05 allows the phase plug to go in further thus casing the interference problem I saw with the first driver (the problematic unit form the start and also the one with the aftermarket diaphram & small phase plug).

I would like to ask if you guys could measure the depth of the hole in the pole piece when you have your drivers apart. I measured the dpeth on the outside diameter using the wall of the hole as a guide. I used a digital caliper.

Nick

[Rudy Kleimann]

The phase plug "cone" is supposed to secure the diaphragm by "squeezing" it between the magnet and the phase plug when you tighten down the screw. If it didn't, the diaphragm would be free to move around, probably making a buzzing noise and possibly rubbing the voice coil inside the magnet gap if it moved far enough.


Somewhere on this site I saw a link to a copy of the "JBL Field Service Manual" which contains relevant information to the ring radiators. Perhaps you will find your answers there.

Does anyone here know where the link is?

[Robh3606]

Hey guess what?? The 2404's I have are the small phase plug. The 2404-1 has the larger phase plug. Looks like we have 2 phase plugs for the driver depending on the diaphram or there were changes down the line.

Rob

[nickc]

Hey guess what?? The 2404's I have are the small phase plug. The 2404-1 has the larger phase plug. Looks like we have 2 phase plugs for the driver depending on the diaphram or there were changes down the line.

Rob

Rob:

Well, now this is a problem - how can you tell the difference between a 2404 and a 2404-1? My drivers do not have the JBL s/n backing plate covering the phase plug screw. Is there another way to tell? Both the plastic baby-bum horms appear to be the same - wouldn't the throat be different for the different phase plugs??

Nick

[Zilch]

2404H-1 used 2402 diaphragm, though the tech manuals say use 2405 diaphragms for replacement.

2404H uses 2405 diaphragm with extended HF response.

My spare 2404's are "H-2," which nobody seems to know what are....

If the cones are different heights, the depth differential may keep the gap to the horn constant?

[nickc]

2404H-1 used 2402 diaphragm, though the tech manuals say use 2405 diaphragms for replacement.

2404H uses 2405 diaphragm with extended HF response.

My spare 2404's are "H-2," which nobody seems to know what are....

If the cones are different heights, the depth differential may keep the gap to the horn constant?

The cones (phase plugs) I have are identical in every respect except for the diameter. See my earlier post with the pictures. I don't know what to do about the problem I have - so I have my eye on a set of 2404 horms on eBay - if I get them (hopefully) then my problem will be solved.

Someone mentioned that there is a JBL Field Service Manual available that has all this information - do you know anyone who has a PDF version I could get emailed to me?

Nick

[Rudy Kleimann]

I stumbled onto this thread, but I don't know how to link it here. maybe this'll work as a link, maybe not. It is in:

Lansing Heritage Forums > Lansing Product Forums (JBL, Altec, LMCo) > Lansing Product Technical Help 2402/2405 Diaphr

Click on the folder symbol above-

Read post number 5 and number 11- one refers to diaphragm diameter, and the other refers to diaphragm thickness. Seems to coincide with your different measurements on the phase plug and the magnet hole machined depth.

[Zilch]

The thread cited by Rudy, above:

http://audioheritage.csdco.com/vbull...ead.php?t=1462

[Zilch]

My point about the cone diameters was that, assuming the cone angle is the same for both sizes, the one with the larger base must be taller, as well.

The difference has to be made up somewhere. Either the gap between the cone and the horn throat is narrower with the taller cone, or the cone is recessed, or the horn is moved forward to compensate.

[nickc]

*** UPDATE ***

Just wanted to let all you guys who helped out with great info what happened. I found and bought (great deal too) two 2404H horns on eBay. I was really hoping they were going to be a matched pair. When I got them I cracked them open to see the phase plug diameter and of course the diaphrams. They were identitcal and had the large 1.71" phase plug.

I stuck the pair them into my JBL 4628B cabinets and did some listening tests. Finally! After all that trouble, I finally have a pair of speakers that sound "matched"...

Thanks again to all!

Nick

[Predrag Dukic]

My point about the cone diameters was that, assuming the cone angle is the same for both sizes, the one with the larger base must be taller, as well.

The difference has to be made up somewhere. Either the gap between the cone and the horn throat is narrower with the taller cone, or the cone is recessed, or the horn is moved forward to compensate.

Both diaphragm shapes ( V-angles) are identical. Different ring dimensions create different area of the flat portion of the dia surface.
It also changes the total mass, and total suspension stiffness.Result is different resonant frequency.
Different phase plug diameter changes compression factor, influencing efficiency, available SPL in the lower part of the FR, but also 2nd order (air-overload) distortion.
One of these days I will measure 2404H and 2404H-1 responses on the plane wave tube ( 1/2" 6' )
and also change phase plug diameters in all four possible combinations.
I will post results in the "ring radiator production" thread...

[Predrag Dukic]

John & Zilch - I just want to thank you both for going through the trouble of cracking open your drivers for me. I *really* appreciate the effort.

Personally, I think the correct phase plug is the 1.71 but that still leaves the question of the pole piece hole depths being different. I believe the hole should be 0.65 and not 0.7. That extra 0.05 allows the phase plug to go in further thus casing the interference problem I saw with the first driver (the problematic unit form the start and also the one with the aftermarket diaphram & small phase plug).

I would like to ask if you guys could measure the depth of the hole in the pole piece when you have your drivers apart. I measured the dpeth on the outside diameter using the wall of the hole as a guide. I used a digital caliper.

Nick

PP hole depth is irrelevant, and probably just a production variance. Phase plug is stopped from going further in by the inner diaphragm ring.
None of the pole plugs, if correctly installed should touch any of the diaphragms.
The shroud formed by the bottom phase plug surfaces (circle and the inverted cone) has the same angle in both of phase plugs.
The only change is the resulting throat surface, and therefore different compression factor (Sd/St).