I scored four of these cabs yesterday (freebies) and Im curious what the typical HPF was used on these? I dont want to blow up the loudspeakers in them, and want to get the HPF right.
I used to make dozens of these for sound companies, bands and installations. It was a pretty darned good bass horn.
The K151 was discontinued only a year or two after these plans were made availbale. So very few that I ever sold, used or saw were equipped with the K151. Other JBL 18's (2240, I think ?) worked quite well in this horn but (as Ivan has pointed out) others did not.
Another point is that for every "real" Keele bass horn you came across back then (and would come across now) there was another which has been modified (or just plain built wrong) by folks who knew nothing about what they were doing.
Another point is that when these were introduced few of us were aware of the benefit of using a HPF on a bass horn or subwoofer. We certainly knew of the importance of the LPF, but not the HPF. And very few crossovers around back then had a HPF for this purpose. As a reference: all crossovers were analog and it was just around this time that the first 24dB-per-octave Linkwitz-Reilly filters were being introduced. Prior to this there were very few crossover fliters steeper than 18dB-per-octave.
This link will take you to Don Keele's original document and it looks to me (freq response chart on page 7 plus in text) like a steep HPF between 50-60Hz would be a safer bet.
Edit: Hmmmmm.... conflicting info. Elsewhere in this doc Keele claims it is useable down into the 30-35Hz range. Based on my memory of how low these went and the freq response chart in this doc, I'd not go that low.
Woops. I cannot copy & paste it.
Google: JBL Keele 18" folded horn
and go to the "audioheritage" site that comes up near the top.