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Porting (properly) an Ampeg B25B with Altecs

Discussion in 'Amps and Cabs [BG]' started by Soulfinger, Dec 21, 2016.

  1. Soulfinger


    Sep 20, 2004
    Houston, TX
    So, about a year ago, I got a V4B head, then a few weeks ago, this happened:


    These two were made to be together---it's amazing that they've spent their lives apart yet ended up looking quite the same. Compared to my other cabs, this one (with Altec 421As) doesn't have a ton of low end, which is good in a way because it's not boomy. I am worried a bit about blowing the speakers even with 100 watts---I've looked, and there's a lot of disagreement on how much power and excursion these things can handle. The cab isn't really ported---it's basically "leaked", with a small hole in the baffle. Based on some WinISD work, it's effectively tuned to 30 or 35 Hz, with performance very similar to being sealed.

    Here's my plan-- I'm going to block the current hole in the baffle, and make a new back for the cab. I'm going to use four 2.75" round port tubes to tune the box to about 55 Hz. WinISD says this should go a long way towards controlling the excursion, and give me a bit of a bump in the midbass around 95Hz, which should thicken up the sound a bit. If it works well, I'll likely do it to the original back--this cab isn't some piece of vintage magic. Bonus feature is that if I plug three out of four port tubes, I'm basically back to the original cab tuning.

    Any thoughts, suggestions? Anybody ever done this?
  2. DukeLeJeune

    DukeLeJeune rational romantic mystic cynical idealist Supporting Member Commercial User

    Nov 24, 2008
    Princeton, Texas
    Owner & designer, AudioKinesis; Auth. mfg, Big E (Home Audio only)
    I like your plan.

    55 Hz might be a bit high as far as providing adequate protection against over-excursion for low-B, but if you tune to low-E, it's probably fine. But then your design gives you the option to plug ports and lower the tuning frequency, so you can easily adjust as needed.

    If you're planning to use expandable plumber's test plugs to plug your ports, make sure you can get plugs that will fit that 2.75" internal dimension. I use Precision Ports not only for the advantages their flares offer, but also because their internal diameters correspond with the expandable plugs that are available in the plumbing department (which are typically sized in one-inch increments, like 2", 3", and 4"). In fact, you need to actually check the internal diameter of the ports you get - the diameter claimed may be an estimate, and the actual internal diameter may be too far off to fall within the range that the expandable plumber's plugs can be adjusted for. I've run into that with brands other than Precision Ports.
    Last edited: Dec 21, 2016
    anderbass and AstroSonic like this.
  3. Soulfinger


    Sep 20, 2004
    Houston, TX
    Thanks for your input Duke. While I had identified the option of plugging ports, I hadn't really considered what I was going to plug them with. While I've used Precision Ports before, I'm not sure if I can make them work for this---the way they are designed, there's a minimum length you can make them, and for the tuning/box size I'm looking at, I would have to use four 4" ports. That's way overkill, and I'm not sure there's enough room in the box to even fit all those ports (it's really only about 5.4 ft^3 and quite shallow). Parts Express has some closeout 2.75" ports--I can get those into the length I need without taking up too much box space. That said, I may never actually plug them....I mostly just play 4-string, and if I were to play a 5, I probably wouldn't do it through this cabinet.
  4. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Yours is different but Ampeg also offered the V4B folded horn cabinet -- see attached. Those cabs have a reputation for working well, long throw, out in the audience, but sound as you described on stage.

    Attached Files:

  5. Ric Vice

    Ric Vice Supporting Member

    Jul 2, 2005
    Olivette, Missouri
    I played through a V4B head and it's matching folded horn cabinet at the Muncie County Fairgrounds when I was in college. It can get pretty loud but you couldn't actually hear what you were playing on stage. I prefered the 2x15 front firing B25 cabinets. I've owned a B-15NC with a single 15"
    Altec 421-8H Series II. They can handle power very well. A pair of them should work just fine.

  6. BogeyBass


    Sep 14, 2010
    if I remember correctly the fs or resonate frequency of 421 is around 33/35 Hz.

    not necessary to tune 20Hz above the impedance peak of a driver. better to tune at fs or no more than 10Hz above or below it.

    more realistic tuning would be 40/45Hz.

    and yes a high tune will reduce cone excursions above tune frequency or FB

    but now low e is below the tune frequency and will cause more cone excursion below the tune frequency.

    also transient response will be poor
    aaround fb a downfall of reflex enclosures. so high tune will effect low e and a.

    tune at fs of the speaker or no more than 7/10hz above or below.
  7. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Those triangular cutout ports are still used in current designs. One example is the BFM Simplexx: Simplexx. Stack two 115 cabs and you have something similar to the two chamber Ampeg 215.

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