JBL speakers with changed serial numbers

I have seen a few JBL woofers with multiple serial numbers, usually stamped above the original number on the foilcal. And I recently came across a 130A with a foilcal that appeared to be a little messily glued on and a sticker on the back stating:

NOTE: The serial number of your
speaker has been changed to enable
updating to the standards of our
most recent production models.

Does anyone know why this was done?
Was this a case where the magnetic gap had to be re-machined to allow fitting of a new recone kit?

I have read elsewhere in this forum that sometimes the older speakers cannot take a new cone kit unless the voice coil gap is enlarged. That may be what was done here.

The remachining, is this because they were building them to tighter tolerences back then, or do they reflect an improvement. I figure it was either to save money, or to make a better product. It would be unfair for me to guess on a public forum like this because I don't know.Does anybody out there?

The gap enlargement was done, to improve the power handling and durability of the speaker. This was brought about in the early 1960s, in response to higher-powered amplifiers becoming available, resulting in many failures of earlier model (tight-gap) units.

Harvey Gerst is a regular participant here... and was the actual person who MADE the change to the driver for JBL, back in the day. If you search for his name, you should be able to find more specifics on this issue...

Regards,
Gordon.

A document from the archives.
http://www.audioheritage.org/vbullet...1&d=1056288057

Quote:

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Originally Posted by Don C

A document from the archives.
http://www.audioheritage.org/vbullet...1&d=1056288057

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Thanks for digging that out Don, it lists all three of the drivers pictured here as candidates for gap machining, but it doesn't mention applying new serial numbers. It does say that compression drivers may also require the top-plate to be machined, but the only re-numbered examples I have seen were cone drivers.

The serial number appearing in it's normal spot is the highest number on the examples I've found. I would guess these have had NEW foilcals with serial numbers that were apparently current at the time of service applied to the drivers. And then the OLD (lower number) serial numbers were added on.

The re-machining of the gap would certainly be one possible explanation, but I wonder if it is THE explanation?

I have always found it interesting and a bit humorous that both Altec Lansing and JBL found it necessary to remachine the gaps of drivers built in Jim Lansing's era. Altec used to remachine the top plates of early 288 drivers for a wider gap.

Jim Lansing understood the benefits of a narrow gap. The voice coil air gap provides the vast majority of reluctance in a speaker magnetic circuit. As a gap is widened, the reluctance grows by the square of the distance across the gap. All other things being equal, a wider gap will result in lower flux density in the gap. Looking at it another way, a larger magnet would be required to provide the same flux density in the wider gap.

A tight gap with minimal clearances on both sides of the voice coil/former assembly will result in the most efficient speaker, if tight tolerances can be maintained in the manufacturing and assembly processes. Jim Lansing was able to accomplish this like few if any others of his era. Later on, service technicians must have been pulling their hair out trying to repair older drivers with repair parts having sloppier tolerances than the originals. Their solution was to disassemble the driver, hog out the top plate for a wider gap, reassemble and recharge the magnet. Is it any wonder that collectors seek out the older drivers with original cones and diaphragms?

This LE8T has double serial numbers following the familiar pattern of the higher serial number 23580 in the normal space on the foilcal with the lower number 14929 stamped in the space above the higher number.

And the 1982 memo about remachining the gaps wider mentions the LE8 as possibly having a narrow gap requiring machining to allow a recone it does not mention the LE8T.

Even if re-machining the gap was one reason it was done, that operation would only have to be done once so the existence of triple serial numbers would indicate some other reason to assign an additional serial number to a driver.

Does anybody have an idea why the double serial numbers (and sometimes triple) are used? Some kind of warranted repair work done at the factory?

D130F with double serial numbers, and in this case very close together (?)
Listed in ebay auction:
http://cgi.ebay.com/VINTAGE-JBL-D130...QQcmdZViewItem
serial numbers are 25042 and 25399, less than 400 numbers apart.

I would expect this means that the D130F serial number 25042 left the factory, but only a short time later was returned to the factory for some mystery operation that involved removing the original foilcal on the back. Later replacing it with a new one bearing the number 25399 which would otherwise have gone to one of it's brothers rolling off the assembly line at the time of it's revisit. Then the foilcal was hand stamped with the old 25042 number and left the factory to be returned to it's owner.

I've already asked seller if he knows why it has double serial numbers, maybe one time I can catch one of these being sold by the original purchaser who just might know what's going on.
I'm going to keep adding to this thread until someone comes up with an explanation!

Listed in an ebay auction:
http://cgi.ebay.com/ws/eBayISAPI.dll...ayphotohosting

This pair of 130A speakers have consecutive numbers in the regular place on the foilcal: 130A serial number 24201 and 24202
But the hand stamper smaller number above is the same on both speakers:blink: