I saved the text below from one of the previous AK posts on Rect III -- but forget who the original source is! Hope he speaks up, because this is solid info. Bet if you did a Search on Rect III crossovers, the author would become clear.
I have a pair of v3 I bought from the original owner. One of my favorite speakers -- the least fatiguing and a real sonic bargain for the price. There were a pair recently on that auction site which went unsold for $99. Costly to ship.
For the Archives: What I've learned about Rectilinear III's
I'm testing out some share-ware schematic software, so I thought I'd share some Rectilinear schematics I drew up.
When I obtained some of these nice speakers I could find very little about them on the net, so I thought I'd post what I've learned about them. Feel free to add to this if you have additions or corrections, and I'll add more as I get more, including photos, when and if I get inside the cabs again.
So far as I can tell, there are 4 versions of the Rectilinear III Highboy. I'll call them v1, v2, v3, and IIIa. I myself have two pairs, one is a mismatched pair of v1 and v2, and one is a complete pair of v3. Here's a rundown:
v1: from the late 60's (it is penciled "woofer replaced 1969," so it must be from before then). It has a Jensen 12" Woofer, an alnico Philips whizzer 4.5" midrange (actually a full-range operating as a midrange), and four peerless cone tweeters, two of which are super-tweeters. It has one tweeter pot on the back, and the tweeters have square frames. The crossover schematic is the same as the one attached below, except sub an 8 Ohm resistor for the midrange pot.
v2: also from the late 60's. Jensen woofer, ceramic Philips whizzer mid, and Peerless tweeters with circle frames. Two pots for tweets and mid on the back. The crossover schematic is attached and titled "Rectilinear III 1969"
v3: From the early 70's (the woofer EIA code indicates 1971). CTS woofer, Philips whizzer mid (I assume ceramic, but I haven't opened up the glued-in mid enclosure), and peerless tweets like v2. Two pots for tweets and mid on the back. The port is wider in diameter than the earlier versions, and the connectors in the internal wiring are not soldered like the earlier versions (which makes it much easier to service). The crossover is the biggest difference, since it is a series crossover rather than a parrallel crossover. Supposedly the series crossovers have some advantages when you are tuning the crossovers by ear, since they are self-balancing (please someone correct me on my very limited understanding of this). I've attached the crossover schematic below, named "Rectilinear III 1971"
IIIa: I haven't seen one in person, but I've seen photos. These are clearly marked model IIIa on the back (the III versions 1,2, and 3 are not marked with model numbers). The mids and woofers look like lower-quality drivers, much like pioneer OEM drivers. The mid is not a whizzer, and doesn't look like a Philips. From what I understand, the IIIa was cheaper, and generally not very well received.
Summary: They started with Jensen woofers, alnico mids, and one tweeter pot. At some point they switched to ceramic mids, a later version of the tweets with circlular frames, and added a mid-range pot. The crossover remained pretty much the same. Then, around 1970 or so, they redesigned the crossover, and switched to CTS woofers. I *think* this was around the time they introduced the Low-Boy version. At some later point, they introduced the IIIa, which had lower-quality drivers, and was sold at a lower price point.
