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Re-designing the B-15 cab for today

Discussion in 'Amps and Cabs [BG]' started by Rob Mancini, Mar 9, 2008.


  1. Rob Mancini

    Rob Mancini Guest

    Feb 26, 2008
    I am thinking about making (rather, having one made) a simpler and more modernized version of the B-15 double baffle cab. Basically I'd be taking the cab as it is and putting it in the same box, but eliminate the fliptop and load it with a neo. Sounds a lot easier than it is, though.

    For those who don't know, the B-15 double baffle is a sealed 21"x21"x14 1/4" cab with two baffles. The inside baffle holds the rear-mounted speaker and has eight oval-shaped holes about 5" each spaced out around the edges of the baffle. The outer baffle is spaced out 1/4" from the inner baffle, and the sound from inside the cab is sent through the holes in the inner baffle and out of the hole for the speaker in the outer baffle. This porting system puts out a ton of low end while the small box keeps the high end intact. It works like no other 15 cab I've tried. The big 15"s have a lot of low end but that's all they have. The small 15"s sound constricted. This one is just right.

    The problem is eliminating the fliptop. Being a rear-mounted speaker design, if you eliminate the fliptop, you have no easy way to get to the inside. So you have two choices...creating a removable flush-mount panel, or front-mounting the speaker on the inner baffle. I would rather not have a removable panel if I can avoid it, but I don't want to change the sound of the cab even a little bit. How much of a difference would rear-mounting vs. front-mounting the speaker make to the sound?

    Also, what would be a good neo that could take 300w for this cab? My vintage cab has an Eminence Delta 15A in it and it sounds great, but I wouldn't mind shaving a few pounds off its weight.
     
  2. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    If you have the existing speaker on hand, a valuable starting point would be to measure the response curve and port tuning frequency.
     
  3. I bought a couple of B-15T's around 2000, they're not double baffle cabs but are very similar in sound, design and size, they have four ports, one in each corner and are not too bottomy. They have 4 ohm 150 Watt Eminence 15's in them from the factory, I took the SS heads out (which don't sound too bad actually) and use them with an old SVT head. It sounds very clean and even up the whole bass spectrum. They don't sound quite as good or as loud as an SVT cab but are much lighter and easier to move around and they're cheap when you can find them. But then again I don't think any B-15 sounds as good as an SVT and I've owned several including an early 60's model which I assume was a double baffle. I have a B-15S right now.
     
  4. Rob Mancini

    Rob Mancini Guest

    Feb 26, 2008
    Fdeck, If I understand you right, I should look for a neo driver with a similar freq response and resonant frequency? I'm trying to understand all this speaker stuff, but it's not the easiest for me.
     
  5. Rob Mancini

    Rob Mancini Guest

    Feb 26, 2008
    Bob, I see a picture of it in the Ampeg book, but I can't tell if the speaker is front or rear loaded. Which is it? Looks pretty interesting. It's about the same size as a double baffle, too. I know I kind of limit myself by looking to recreate the double baffle, but it's hard not to love. But the B15T looks like a pretty sensibly designed cab. Wish they weren't so rare.
     
  6. main_sale

    main_sale

    Apr 26, 2004
    Cape Cod
  7. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    My thought is to measure the overall response of the speaker in its box. A near field measurement will also tell you the port tuning, though there are other ways of measuring that. The point is to know what response curve you are targeting with your new design.

    It is unlikely that you will change the port tuning very much, if at all, because of the need to protect against over-excursion. Thus you are left with two variables: Driver choice and box volume. You can run through available drivers and see if there is an approximate match for the curve below a few hundred Hz. Matching the high frequency stuff is more of a crap shoot, and may require EQ.

    At the same time, don't overlook other options such as a vertical 2x10. The less you restrict the design parameters, the better chance you have of finding something that works.

    I suspect that the port topology of the Ampeg speaker is not all that important, and that a conventional port would work the same, with the advantage of eliminating the front / rear mounting dichotomy.
     
  8. Rob Mancini

    Rob Mancini Guest

    Feb 26, 2008
    You really think so? I wish people would make them smaller, then. They're all designed to stack with a 410. Then they stick on a tweeter which makes it even bigger. The thing that makes the Ampeg speaker attractive to me is its huge bottom and smooth top while being in a box that's about as small as you can make a 15". I don't want to stack a 410 on top of it, just a 12". I've tried the Markbass stuff, which is about the only stock cab I've seen that's smaller, and it's significantly brighter. I wouldn't limit myself if I could find something suitable off the rack.
     
  9. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    If it's a really small box, one possibility is that the driver is specialized. I experienced something similar when I tried to mimic the tone of my GK MB150E combo speaker. Every design that I arrived at with the same shape of response curve was also considerably bigger. In my case, it was OK, since the point of my venture was similar tone but higher sensitivity and power handling. But I certainly understand the desire to retain the size of a nice compact box.

    So, the end result of the "research phase" may indeed conclude that there is not an off-the-shelf driver that makes the goal attainable.

    Now, considering my project, had I not wanted more sensitivity, I am fairly certain that I could have achieved response similar to the 12" in the GK combo, using an available 10" driver. So there may be multiple ways to skin a cat. And we all await what the Eminence 3012's are going to look like.

    Still, if you have the time and access to the cab, a measured curve would be the starting point to evaluating options.
     
  10. Rob Mancini

    Rob Mancini Guest

    Feb 26, 2008
    The original speaker was a Jensen C15N, with a JBL D130F offered as an upgrade. As far as I've heard, both of these were stock, but the CTS speaker they switched to in mid-65 was custom. Currently it has an Eminence Delta 15A, which is an off the rack speaker, and it sounds magnificent.

    I have access to both the Eminence speaker, a CTS speaker from 66, and the cab. How do I measure the curve? Do I need special equipment? Is this something any decent amp tech should be able to do in his shop?
     

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