The difference between early 291 and 288 compression drivers?

[roberthompson]

Hello! The 291-16A looks virtually identical to the early 288, to my eye. I use 291-16A drivers on my horns, and love them. However, when I was searching them out a couple of years ago, I noticed the early 288s were twins. In looking at a several of these compression drivers together, some were missing those stick on labels, and I couldn't figure what they were. Some early 288 diaphragms were different in appearance, but minus the diaphragm, I couldn't guess. I read the spec sheets of course, so I have a sense of that, but noticed that later 288s had the same frequency range as early 291s!?! I'm just a hobbiest and no expert, but some folks here are, so an enquiring mind wants to know! Thanks in advance for your time taken in responding to my question. Robert

[Old Guy]

The early 288...by which I mean B and prior....have minor differences that require machining to use current diaphragms. The differences are not visible outside.

The later 291/288 are identical mags to each other but different frams. Neither will fit the old driver without machining.

IE 291A is same as 288 C. 291B is same as 288G.

As far as specs the newer ones may just be more honest.

291 were supplanted by the 299. It has better highs. 299 frams will retrofit to earlier 291.

Easy to identify the 291 fram by the brown ring around the dome. If it's metal instead it is a 288 or 299 fram.

[magnarc]

Note, these are general observations, not applicable to EVERY 288 B or C unit!

There are some subtle differences in the finish of the drivers between the 288 B and the C, the B tends to be a crinkle type rougher finish, the C smoother. Early C can be more hammertone than later ones as well.

The B and early C have larger romanesqe typeface on the Altec logo stamped on the cover, and say Altec Lansing Corporation(sometimes with Hollywood), the later Cs just have Altec Lansing.

Most Bs have a rope type surround on the cover, most Cs have a rubber seal.

The other posters are correct about the mechanical difference, it is the depth where the voice coil fits into the driver. The B is much shallower than the C.

[Old Guy]

Sorry...assumed we were only addressing mechanical difference. Finish variations on Altec could keep us busy for a few weeks.

[magnarc]

Hahaha

Indeed, even label variations would take a week.

To me, the definitive difference is the mechanical differences you mentioned, mainly the short magnetic gap on the B and earlier units.