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Thread: Rebuilding Altec 855a cabinets advice needed

  1. #1
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    Rebuilding Altec 855a cabinets advice needed

    My cabinets are mostly beyond restoration so I need to rebuild but hopefully improve some things. There is no batting on one side and top of the enclosure. Should there be? Should there be more or different damping material? What should I replace the particle board back and bottoms with? Plywood, mdf or use particle board for some reason. The baffles are ok so I'll reuse them. Should I install internal bracing? I like the size of and looks of the cabinets so I don't want to change dimensions. Anything else I should do? Thanks
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    Senior Member RMC's Avatar
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    Hi Getahobby,

    Welcome to LH Forums.

    That's a nice project, yet nobody as dropped in to offer you some help. Below are some things that may help you make a better box acoustically speaking.

    RE: damping material

    Generally, at least three enclosure panels should be lined with some damping material 1" thick or so (back, one side and either of top or bottom). You can line two more if you prefer. The yellow stuff partly seen on the pic may be ok.

    RE: plywood or else

    Personnaly, I hate particle board (except for its initial rigidity and low cost) still never use it for my boxes. Your finances may dictate the type of wood to use. Since you mention reusing the plywood baffles seen you might as well continue on the same track (subject to a note below about vent location vs reusing baffles). Good 3/4" plywood (Fir, if available) will also do the job with proper bracing.

    RE: bracing

    A box this size should have some good bracing.

    Because the drivers are mounted from the inside of cab you need to keep some room to be able to remove and reinstall them easily. Ideally, each panel should be braced lenghtwise (baffle horizontally) plus in view of box size its a good idea to have side to side and front to back braces (i.e. the cross). Front to back brace should not be glued to back panel (only screwed) to be able to remove the rear. Naturally, the existence of the cross bracing depends ultimately on having enough space to remove/reinstall the driver.

    RE: other issues

    The vent should be moved a little further from driver to have some space on front panel near the center to put a front to back brace, as well as the horizontal baffle brace.

    You may not like the following regarding cabinet's front appearance. There seems to be an opaque cloth covering the port which may be creating a more resistive vent. That's not ideal acoustically since air passage in the vent may be somewhat restricted, therefore reducing the vent's benefits on the woofer, such as cone travel and Low-Frequency distortion.

    You should use some weatherstripping tape to make sure there's no air leak from the removable back panel. I would also add some temporary caulking on the outside joint around the rear panel, this can be easily removed if need be for rear access.

    You need to glue and screw all panels (except rear no glue) plus put permanent caulking to seal all internal box joints (except rear panel perimeter).

    Coax drivers have a tendency to be more lossy than some others (air leaks), so doing your best on sealing correctly the other aspects you can control is important, including around speaker terminal connector. I hope the above will be useful to you. Regards,

    Richard

    ALTEC CABINETS 1965.pdf

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    Senior Member grumpy's Avatar
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    Would only add that if you go with plywood, I'd suggest looking into a void-free type less likely to rattle or resonate, such as baltic birch or "appleply"

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    Senior Member RMC's Avatar
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    Hi Grumpy,

    RE: Baltic Birch

    "Your finances may dictate the type of wood to use. Since you mention reusing the plywood baffles..."

    With planning to reuse the baffle panels, which I would not do re vent location, I got the feeling there might be a cost/savings issue involved here. Hence my suggestion of lower cost Fir plywood VS pretty expensive Birch. But he can still use even lower cost particle board if he wishes.

    For the record, below is what JBL says about box material (The Most Commonly Asked Questions About Building Enclosures, P. 5). Regards,

    Richard

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    Quote Originally Posted by RMC View Post
    Hi Grumpy,RE: Baltic Birch "Your finances may dictate the type of wood to use. Since you mention reusing the plywood baffles..."With planning to reuse the baffle panels, which I would not do re vent location, I got the feeling there might be a cost/savings issue involved here. Hence my suggestion of lower cost Fir plywood VS pretty expensive Birch. But he can still use even lower cost particle board if he wishes. For the record, below is what JBL says about box material (The Most Commonly Asked Questions About Building Enclosures, P. 5). Regards,RichardName:  IMG_0585.jpg
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    Money not really an object. Was thinking walnut veneer plywood for a classic look. Keeping baffles unless good reason to replace. Is mdf ok for backs and bottoms or will plywood give better sound? Thanks

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    Senior Member grumpy's Avatar
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    Nice. I stand by my suggestion as stated.
    First time I heard those drivers, they were in a similar cabinet. Left a good long-term memory

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    Senior Member RMC's Avatar
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    Hi Getahobby,

    RE: "Is mdf ok for backs and bottoms..."

    MDF or better yet HDF would do the job for backs and bottoms. The important issue is that it be properly braced to minimize panel vibrations. Whether you use HDF, Baltic Birch plywood, walnut veneer plywood or Fir plywood, boxes this size will still need to be braced. Regards,

    Richard

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    Let's see a few more photos of what you have to deal with there.

    In the end it will get down to your expectations combined with desired finish and budget. Getting the structural integrity thing correct with consideration to the finish is the trick. And frankly, most of these old cabinets are just that, old wood.

    Baltic Birch Ply is the best alternative but MDF is not that far behind. You gotta remember, today's MDf was not around back when many of these cabinets were made in either ply or pressboard. Plus ply was much more affordable and readily available. BB Ply is about 3x the cost of MDF. MDF is very stable, consistent and dense. It is a nice material to apply veneer of your choice and add hardwood accents to.

    In addition, there are some higher level joinery strategies that help with getting the construction sturdy and rigid. We are currently using rabbet/dado with bracing and rabbet corner joining. Once a good strong box is built you can move on to how you want to finish it which includes the usual suspects of painting and wrapping.

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    Senior Member macaroonie's Avatar
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    I'm with Grumpy. Baltic birch is the way to go. It's lovely to work with , is exceptionally stable and strong. By contrast MDF is horrid , dangerous dust and not generally known it shrinks slightly.
    If you post veneer you can buy the BB with face repairs , the expensive stuff is completely free from blems on both faces.

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    Administrator Mr. Widget's Avatar
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    I agree with the Baltic birch recommendations. I use all of the above, but when appropriate I prefer BB. Corners can rip out if you have a dull blade, but generally it is the nicest material to work with and it is remarkably solid and strong.

    Regarding the veneer, take a look at the offerings from these guys. Oakwood Veneer I typically use their paper backed veneers as they are the easiest to work with and if you plan properly your results will be spectacular. I would suggest making your plywood joints mitered cuts. If you do not, over time the wood panels expand and contract with humidity and temperature changes and the end of the butt joints will telegraph through.

    Here are a couple of pics of a project I am working on. My construction is a combination of Baltic birch and MDF. Because the joint at the top would eventually telegraph through I have veneered a 1/16" 2-ply sub veneer that I will be covering with the surface veneer.


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    Senior Member macaroonie's Avatar
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    Oh my ! ... more pics pls

    Oakwood do x2 free samples and $1.50 each after that. Ideal for doing a few small tests to get a feel for the process.
    One other thought , given that these 604's are getting pretty valuable I reckon that they deserve the best of living accommodation not any old condo.

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    Administrator Mr. Widget's Avatar
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    Quote Originally Posted by macaroonie View Post
    Oh my ! ... more pics pls
    In time... and I don't want to hijack this thread.

    Another note about the damping material. Back when your speakers were built it was typical to only cover every other panel. Now days we use more damping. I suggest you use the recycled denim house insulation. It is very good and doesn't make you itch the way fiber glass does. The dacron materials are not very good.

    I would also suggest making the cabinets narrower and deeper than stock if you can. Deeper cabinets usually sound better, all other things being equal. And add judicious cross bracing. Those large panels will flex even with the stiffer BB material. This will make the bottom end less defined and color the lower mids.


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    Quote Originally Posted by macaroonie View Post
    I'm with Grumpy. Baltic birch is the way to go. It's lovely to work with , is exceptionally stable and strong. By contrast MDF is horrid , dangerous dust and not generally known it shrinks slightly.
    If you post veneer you can buy the BB with face repairs , the expensive stuff is completely free from blems on both faces.
    Hey macaroonie, what prices are you guys seeing over there for BB and MDF? Just curious.

    Great thought on the blemished 2nd BB.

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