View Full Version : Quick question on 2421Bs

07-21-2007, 03:43 AM
Have a line to a pair, purchased from JBL one owner, described as been in moderate home use only.

I believe they have a red dot at + so 16 ohm dias?

Also, seller thinks they are titan but isn't sure. :hmm: If aluminium, they are considered as desirable upgrade to my 2420s.

So, which material are they? Anything else I should know?

Thanks a lot

07-21-2007, 06:16 AM
The 2420 and 2421 both used aluminum in their original diaphrams. The the 2420 had tangental surrounds and the 2421 had the diamond surrounds as in you photo's. The 2421 diaphram would be the most desireable. The problem is the Titanium 2425 is identical to the 2421 except maybe the color. Trying to tell the Aluminum from the Titanimum isn't so easy by eye. The Titanium seems to have a greyish tone to it where the aluminum is a bright silver color.


Mike Caldwell
07-21-2007, 06:29 AM
Judging from your picture those diaphragms look to be aluminum. Titanium diaphragms have a brownish gray look to them.
The red mark is for 16 ohm diaphragms, green is for 8 ohms.

Mike Caldwell

07-21-2007, 08:13 AM
Anything else I should know?
Yea, the mounting bolts for the horn are threaded only into the body of the motor - there are NO high-tensile inserts or nothing. This material is soft - DO NOT OVER TIGHTEN OR CROSS THREAD.

Start each bolt by rotating reverse, until the threads set, then carefully hand tighten.

07-21-2007, 12:41 PM
Funny. May be the lighting or the monitor, but I took one look at the pics and said, "Titanium, obviously...." :p

07-21-2007, 01:52 PM
Definitely ALuminum. I put a set side by side in the sun and it's clear.

BTW get a 1/4-20 bottom tap and bit of WD-40 to clean out the horn mounting holes. A Qtip does a good job of snagging the debris and extra fluid.

And replace the foam - it's rotted as evident by the depression left by a lead wire.


Mike Caldwell
07-21-2007, 09:17 PM
Pictured is a titanium 2425 diaphragm.
Take care of the original 2421 aluminum diaphragms, when and if found
they are expensive to replace.

Be careful on the length of the mounting bolts, you do not want them to bottom out on the alnico magnet that is directly below the holes. Check the depth of the threaded mounting holes by running a bolt in with your fingers until it stops, it's about a half an inch.

Mike Caldwell

07-22-2007, 08:59 AM
Those things don't come along here every day.

Promise to compare / report when arrived and put into operation.

07-30-2007, 05:36 AM
Definitely ALuminum.Yup. :thmbsup:

Got them and did as advised. They tested out flawless with a tone generator. Turned out as even cosmetically quite nice, so here's an own pic of the bundle.