The Z tweeters are so darned rare, come up so seldom, I don't mention them. They don't need the Zobel network in the Tobin 104T schematic. The Z tweeters can be problematic depending on when they were built. NEAR kept veering off track and causing manufacturing issues. Rudy had to come back and set things straight. There are specific dates to stay away from.
Just to expand on a few of Biggle's excellent points.
The Z tweeter uses a different cone design than the Y. Both the Y and Z are aluminum-cone drivers, to contrast with the paper-cone X. The cone design for the Z, unlike the Y, is a curvalinear design and it was more difficult to manufacture.
Like other metal-cone drivers, the Y and Z have ringing issues—i.e. circular waves bouncing back and forth on the cone—not curable by changing the surround. Some of you may have seen my warnings about the B-209A relating to ringing. A surround change was the large improvement made for the first aluminum cone B-209A, which required a special external latex damping ring, long since deteriorated, yielding its surround-damped successor the B-209B and descendents Bc and C. Ringing is why the aluminum cones of the Bozak divers—B-200Y and Z, as well as the B-209B/Bc/C—are all sprayed with black latex, most commonly seen in its degraded, flaking form. (The paper-cone drivers, again, have silver dust caps.)
To further address the ringing issues a piece of latex foam was placed underneath the dust-cap of the Y and Z tweeters; this, too, will have degraded and disintegrated. Changes in foam between the Y and Z may contribute to different effects from deteriorated foam. So it may not all be manufacturing in the sense of cone misalignment or other physical errors.
A few years after the introduction of the Z tweeter Bozak's internal quality control apparently suffered serious problems, and many of the drivers from that period had quality issues, but the Z was reportedly particularly bad. (This was related to me by a former Bozak employee, confirming Pat Tobin's comments. No, it wasn't Bob Betts.) The deterioration in quality of the Z tweeter is the problem to which Biggles alludes. Early ones from 1974 are reportedly fine; later units manufactured towards the end of Bozak's existence are often reportedly defective.
I have also been informed of a further issue with the Z tweeter: certain manufacturing dates, likely the early ones, are slightly more efficient than the Y, itself unbalanced (higher SPL) with respect to the other drivers, which means the Z may be out of balance with the other drivers,; the L-pad modification may be useful for better balancing. I do not have any Z tweeters in my Bozak stash so I can't test this.
Nearly all cone tweeters have a rise in impedance with frequency, so the Zobel can be useful for just about any speaker design. I am unaware of anything in the Z design which would tame that rise. But, again, I do not own any Z tweeters so I can't verify this.
I don't want the shiny ones.
