JBL speakers with changed serial numbers

[glen]

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?

[Don C]

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.

[Thom]

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?

[GordonW]

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.

[Don C]

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

[glen]

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

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?