C51 Apollo with LE15A: to port or not?

[hifinatic]

Hi All-

First time post here:

The simple question is: should, or can, the JBL C51 Apollo enclosure with the LE15A woofer and the LE85, at about 3.2 cubic feet, be ported? I am starting to think not, but I'm a little confused.

When I got this pair of speakers there was, on the upper right front face of each, a cutout of about 3x5 inches, open to the interior. On the upper left front face of each speaker was screwed a piece of plywood of about 5x7 inches, painted black like the rest of the face. Perfect size to screw over the apertures to seal the speakers, which I did do. It had not ever been placed like this before (no screw holes). But what else would it be for? It had not been covering _another_ opening.

After researching the possibility of porting I concluded that the C51 is too small, and that the opening was for a tweeter in one of the other driver configurations.

But when I look at photos online of the C51, I see numerous different configurations showing the rectangular opening, including one with the opening, but also _with_ a tweeter on the other side.

Maybe I'm not the only one who has been confused by this issue over the last 50 years? I'd just like to know what JBL originally intended. I do understand, correctly I hope, that sealing the speaker will protect the woofer somewhat from being over driven. Please correct me if I'm wrong there.

BTW I had the LE15A's re-surrounded, and I think my repair guy- well known as a pro in SE PA, re-glued the dust cap. They sound great. He advised, when I asked his opinion, that the 50 year old caps in the crossovers should be OK, on the basis that they don't get heat stress like those in amplifiers. I think as long as they sound great I will leave them alone.

Thanks, Clay

[jbl4ever]

Hello and welcome to Lansing Heritage Hifinatic. I have had a couple of pairs of the C51's over the years. As for your question to port or not to port depends on your liking. 1 pair I owned was loaded as the S-7 system which had the 3x5 board removed for porting. It used the LE15A and LE85 components.
Another pair was loaded with the S-1 components which used the LE14A and LE175DH which also had the board removed. In the S-7 system I replaced the block off board and liked it better with the LE15A.
The C-51 with the S-1 components did better with it removed. The C-51 cabinets were sold either assembled or kit form. I think there was 8 or 9 different component loads for these. Remove the 3x5 and give it a listen but do not think you are going to get the last octave that well. If I recall it had a F3 of 45Hz in that cabinet with that design. I rebuilt my C-51 with the S-1 components and changed the crossover to the LX10 and the LE85 with the H91. It then was more balanced. You can also look in the JBL library in the old catalog section to see the different component loads for the C-51. Start check around 1962-67. Hope this helps