Ferrite vs. Alnico facts?!

[Dave Zan]

I've read lots about Ferrite vs. Alnico but not many actual _facts_ except for our fearless leader's excellent report of comments by some of the JBL people themselves.
I know it's bad form to expect real data since it inhibits the expression of opinions:) but I think I more or less understand this now and I'd like one tiny measurement to confirm it. What is the radial distance between the pole piece and the top plate on a typical JBL woofer? From this I can calculate the reluctance of the circuit and hence the operational point of the ferrite or Alnico.
I asked this before but I assume the lack of response was because I didn't have a catchy title for the post:) Surely someone must have a dismantled driver ready for a recone?

TIA

[Alex Lancaster]

:) Could You send a drawing of the measurement You need?

[GordonW]

I think the dimension he is looking for, is more commonly known as the "gap guage" of the motor... the clearance between the pole piece and top plate, where the voice coil goes between the two.

For most of the vintage JBL big drivers (130A, LE14A, etc), the gap guage was around .057". Some of the earlier drivers (pre-1960 D130, etc) and some of the smaller woofers (123A, etc) had a smaller gap guage... for example, the 123A has a .044" gap guage, and the LE10A has a .048" gap guage.

Now, do be sure to note that JBL used different voice coil winding configuraions on some of the alnico drivers vs. the ceramic (SFG) drivers, in order to "compensate" for the increased magnetic flux available with the ceramic magnet assemblies. In most cases, the later ceramic drivers had slightly heavier cone assemblies, with sometimes longer voice coil windings among other slight changes...

Regards,
Gordon.

[Dave Zan]

Thanks for the data.

I've posted my results in a more popular thread.
http://audioheritage.org/vbulletin/showthread.php?p=62635