Bringing 828 cabs up to snuff...

So now that I'm going to be home from work for a few weeks I can't resist catching up on a few projects.

Since my 828 cabs are relentlessly in the way in my garage I thought I'd at least put them on wheels. I got some 2" rubber-wheeled casters from HD and now at least they're mobile.

While I was at it I couldn't resist getting off the backs for the first time since I bought them. I expected a nightmare inside, but they're pretty clean. Here's a few shots:

http://www.hostboard.com/forums/hbmc...2013/05/50.jpg

http://www.hostboard.com/forums/hbmc...2013/05/51.jpg

http://www.hostboard.com/forums/hbmc...2013/05/52.jpg

http://www.hostboard.com/forums/hbmc...2013/05/53.jpg

http://www.hostboard.com/forums/hbmc...2013/05/54.jpg

http://www.hostboard.com/forums/hbmc...2013/05/55.jpg

I really don't want to gum them up with spray insulation. I have plenty of Ultra denim (last pic) that I can use to further insulate, but where should I add bracing to stiffen her up a bit?

And I guess the first order of business is to build a port board for the bottoms. 6" round? 4" deep?

Oh yeah, the business end:

http://www.hostboard.com/forums/hbmc...2013/05/56.jpg

And even the backs are not too bad. I think I'll keep the battle scars rather than paint them.

http://www.hostboard.com/forums/hbmc...2013/05/57.jpg

I just picked up some N801-8A crossovers. I guess they'll fit nicely in that center panel in the back?

Pair Vintage Altec N801 8A 2 Way Crossover's 800Hz | eBay

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I guess they'll fit nicely in that center panel in the back?

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Yes, provided they're cut to spec.

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but where should I add bracing to stiffen her up a bit?

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Everywhere! But, particularly behind the horn flares. The two factory braces are minimal, and i notice those aren't even there? Next place is the wide open panel acreage, especially the side panels behind the vent panels. A side to side brace from approximately the middle of those areas can do wonders to take a lot of the bounce out of them. IMO, the one piece back also wants for a front/back brace just below the woofer.

What bowtie said. Knocking on the cabinet can give you an idea what needs bracing. Oak dowels, maybe 1" or 1 1/4" diameter, work pretty well, as do 1x2s etc.

Thanks, guys. There's a part of me that wants to hear them unbraced first. But I'll have to wait for my crossovers.

Maybe I'll cannibalize the N1285s from the A6s. This way I'll be able to see what they sound like at 500, 800 and 1200. I'll have to see how they're hooked up to the two woofers. Anyone know if they're series or parallel?

I also forgot to mention the "horn mounting board" as Altec called it in the 50's/60's. That's the "L" shaped piece with t-nuts spaced for 811/511 mounting that spans the top of the cabinet across the top panel of the horn, and as such forms a very effective stiffening brace for that panel. You can get the basic dimensions from the 60's enclosure guide, but you'll need to modify the dimensions for 3/4 inch material rather than 5/8.

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Originally Posted by alancohen

So now that I'm going to be home from work for a few weeks I can't resist catching up on a few projects.

Since my 828 cabs are relentlessly in the way in my garage I thought I'd at least put them on wheels. I got some 2" rubber-wheeled casters from HD and now at least they're mobile.

While I was at it I couldn't resist getting off the backs for the first time since I bought them. I expected a nightmare inside, but they're pretty clean.

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Those look VERY clean! You scored a nice pair there, Alan.

Here's where I'd start in shoring them up (sorry for the crude drawing):
http://img109.imageshack.us/img109/2845/alans828.jpg

Brace the bottom panel, too.

Yeah, with castors, doubling up on the bottom's panel thickness wouldn't be overkill.

GM

I think pine braces on the sides would act like bracing on a guitar sound board. They would still vibrate. Oak braces might be better, but that's a lot of oak ($$$) and a lot of weight.

Since I also build balsa model planes, I'm used to trying to add stiffness without adding a lot of weight. I was thinking along these lines:

http://www.hostboard.com/forums/hbmc.../2013/06/1.jpg

I got some 1-1/4 x 1-1/4 x 36" deck bannister blanks at HD for $.84 each. I think I can trim them for a tight fit and get away with installing with just wood glue. I'd rather not put screws in the sides, but I would if necessary.

I know in order to retard unwanted vibration, which causes feedback on hollowbody guitars, they will tie the soundboard to the back under the bridge with a block of wood. It loses most of its acoustic properties this way, but it stops feedback cold.

And yes, I also broke down and bought some gap-filling spray insulation. Yuck.

BTW, I can also add a couple from the front to the back (lighter red). I can attach struts under the LF mount and then screw them to the back. Two screws through the back will secure them tight.

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Originally Posted by GM

Yeah, with castors, doubling up on the bottom's panel thickness wouldn't be overkill.

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The casters are not only for ease of moving. My house is built on a slab, but the living where these will be going is built on a raised floor. Since I have HW floors and area rugs, I'd rather insulate the boxes from the floor as much as possible to reduce unwanted vibrations. I want to hear the speakers, not my floor :)

I suppose I could add two more full-length from top to bottom behind the LF mount.

Also, as far as the port, I've read 6" is in the right range? But I don't have a 6" hole saw and prefer not to drop $45 on one.

I do have a 4-1/4" hole saw, and I think 2 ports this size yield about the same area as a 6". But I don't have/can't find material to make 4-1/4" port tubes. I do have 2" thick-walled cardboard tubes. I think 9 2" holes would yield the same area.

And hears an AK thread on upgrading the N801-8A crossover: http://www.audiokarma.org/forums/showthread.php?t=85192

I'm trying 8" concrete form tube. and just cut the hole with a jigsaw. if you mark it with a ball point pen and cut it right at the line it comes out nice. GM suggested the 8'' by 6 or 7'' long port size on another thread. so i'm going to play with that a bit and see what happens.

Ahh, I thought 6" was the number. 8 it is then.

I remember someone, maybe AB post the paint concoction to Altec Battleship gray. Or maybe I'll just paint the port panel a flat black.

Oh, the concrete tube just reminded me. I have a 6-1/2' x 14" Bakelite tube. it was the core of some DuPont polyester film. That would yield 33 sq in. Which drivers are you using, Phil? I'll be loading 515Bs. If you're using something else maybe the number would change. Maybe I could impose upon GM???

I'll bet I'd be the only one on my block with Bakelite A7 port tubes.

http://www.hostboard.com/forums/hbmc.../2013/06/2.jpg

Those concrete form tubes are only about 1/8" thick. Is that enough to not deform under bass wave duress? I guess you could cut a thin strip out one section to reduce the diameter and glue it into a second outer ring. It would be nice if you could form a slip fit tube so that you could just slide it in and out of the outer tube to fine tune the length and glue later.

Need a recommendation. I still don't have a proper horn for my A7s. I was going to borrow the MR564II from the A6s for testing, but I still need other horns. I've been looking for some 805B, 1005B etc but can't find anything local or reasonable. Building a set might be in the cards. But for the time being I'm thinking 511 or 811.

511s would be a no brainer, but my listening room is wider than it is deep and my listening chair is only about 9' away. The 288s have been replaced by the 802s for this reason.

And for what it's worth, the A7s will eventually be utilized in my TV area and will mostly be used for TV and movies. For serious music listening it will be the 604Ds with the 300B monoblocks in another area. The A6s w/288s and MR564IIs will be in the shop where they will have room to spread their legs. At least that the current plan.

So...811 or 511 for the A7s

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Originally Posted by alancohen

I think pine braces on the sides would act like bracing on a guitar sound board. They would still vibrate.

Since I also build balsa model planes, I'm used to trying to add stiffness without adding a lot of weight. I was thinking along these lines:

I got some 1-1/4 x 1-1/4 x 36" deck bannister blanks at HD for $.84 each. I think I can trim them for a tight fit and get away with installing with just wood glue. I'd rather not put screws in the sides, but I would if necessary.

And yes, I also broke down and bought some gap-filling spray insulation. Yuck.

The casters are not only for ease of moving.

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Not really since there?s nothing to excite it [them] due to the increased rigidity raising the panel?s Fs high enough and the minimal damping required to quell any spurious HF off the back side of the driver and/or internal reflections.

Right, bracing increases rigidity much quicker than adding mass that only doubles with a cubing of thickness, so for bass cabs one wants to push the cab?s Fs above its intended operating pass-band [(2*Fs/effective Qts at minimum] since to lower it to at least a half octave below today?s typically low tuning [Fb] would require a concrete mini-bunker or similar massive construction.

Yes, cross bracing that ties all panels together to ensure the cab can?t ?breathe? is the best overall with the material[s] stiffness [MOE] determining how much is required for the bracing grid with the number of grids being determined by number of panels, size [area].

Right, a screw joint just weakens a properly bonded joint, so if the cab will ever likely be ?bounced? around much then it?s best overall to remove them and fill the holes with epoxy or similar.

WRT using castors, a cab ideally needs to be coupled to something rigid/massive, otherwise it needs a massive ?foundation? of its own and since these cabs are already fairly heavy, doubling up its base is usually sufficient, though in one case we used concrete blocks to both mass load and reduce net Vb in a semi-permanent install, so that significantly lower Vas, higher power handling JBL woofers could perform ~optimally in them.

GM

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Originally Posted by alancohen

Also, as far as the port, I've read 6" is in the right range? But I don't have a 6" hole saw and prefer not to drop $45 on one.

I do have a 4-1/4" hole saw, and I think 2 ports this size yield about the same area as a 6". But I don't have/can't find material to make 4-1/4" port tubes. I do have 2" thick-walled cardboard tubes. I think 9 2" holes would yield the same area.

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Over the years I?ve posted numerous different tunings for A7s depending on the needs of the app being discussed, though a number of folks with them that?s posted here over the years seem to like a ~75?^2 [Av] vent best overall [~9.77? dia. hole/~0.75? thick baffle].

When the vent isn?t a shelf design, it?s always best overall to use a single round vent since it will have the shortest length for a given tuning [Fb], but if you want some tuning flexibility with this Av using a 4? i.d. dia. tube it will require six of them, so will start out >0.75? long, though you?ll have to find its actual length of this ?array? empirically if you want to ?hit it on the nose?.

To find the number of 2? dia. vents required: [~9.77?]^2/[2?]^2 = ~24, so their length will be substantial just to match the single large vent?s Fb due to friction, mutual coupling losses.

Back when I had A7s, I fiddled with a shelf vent at the bottom and instead of damping on the cab floor, I put it at the top above the driver and on the bottom of the flare, but if you have a router and/or large enough fly cutter and drill press, then using a 10? concrete former tube is what I recommend if you want to start at this relatively high tuning.

GM

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Originally Posted by alancohen

I remember someone, maybe AB post the paint concoction to Altec Battleship gray. Or maybe I'll just paint the port panel a flat black.

I have a 6-1/2' x 14" Bakelite tube.

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Altec has used different shades of gray over the decades, so you?ll need to have yours matched if you want it ?OEM?. FWIW, touch-up spray cans of Ansi 49 industrial gray enamel perfectly matched some 825 cabs delivered to Atlanta in June ?64.

Just a 6.5? dia. hole tunes it lower than all but a few folks prefer and even then these tend to be corner loaded. At 14? long, it will be tuned below 20 Hz, so running it sealed would be better.

GM

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Originally Posted by VolvoHeretic

Those concrete form tubes are only about 1/8" thick. Is that enough to not deform under bass wave duress?

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No, a vent is a 1/2 WL resonator, so the stresses are concentrated at its ends: http://hyperphysics.phy-astr.gsu.edu...opecol.html#c2

For a long vent then, bracing it at the end inside the cab is a good plan to keep it from ?ringing? if thin walled and due to weight if a massive one.

GM

Thanks GM.

So if 75"^2 is the goal at .75" deep, and the opening at the bottom is 8" high now, can I just forget about cutting holes and just block off all but 9.4 linear inches of the vent to yield the 75"^2? Sure it would be a rectangular port, but I can get it almost square making it 9.4" wide and 8" high. I'm not looking to "hit it on the nose". I just want to connect somewhere on the face.

I'm not clear on what a 'shelf' design is. And although I have a fly cutter and router and drill press, I would prefer only using the existing vent and not cutting into the cabinet. I assume cutting a 10" hole would require this.

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Originally Posted by alancohen

Need a recommendation. So...811 or 511 for the A7s

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Depends on the desired performance parameters. Both horns have the same HF design, only the LF sections are different.

The larger 511 will control directivity to a lower frequency with the trade-off of higher HF damping due to increased acoustic loading, collapsing directivity [beaming] in the process. With a high XO point, it will also have a flatter frequency/polar response due to less compression horn loading, acting more as a wave-guide.

If max efficiency isn?t a goal, then depending on the desired top octave response, the 511 can be the best choice overall regardless of XO point, though personally I would opt for the MR994A.

GM

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Originally Posted by alancohen

Ahh, I thought 6" was the number. 8 it is then.

I remember someone, maybe AB post the paint concoction to Altec Battleship gray. Or maybe I'll just paint the port panel a flat black.

Oh, the concrete tube just reminded me. I have a 6-1/2' x 14" Bakelite tube. it was the core of some DuPont polyester film. That would yield 33 sq in. Which drivers are you using, Phil? I'll be loading 515Bs. If you're using something else maybe the number would change. Maybe I could impose upon GM???

I'll bet I'd be the only one on my block with Bakelite A7 port tubes.

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i have 416 8Zs (from a set of 846b valencias), similar to the set in panomaniacs a7-500s http://www.hostboard.com/forums/f700...ey/index9.html and after reading this part of the thread it was pano who suggested the 8" x 6-7" port. but thats what i'm working with. when i get what i want. i'll go ahead and cut a new bottom panel and finish it.

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Originally Posted by alancohen

Thanks GM.

So if 75"^2 is the goal at .75" deep.........

I'm not clear on what a 'shelf' design is.

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You?re welcome!

We don?t know if it?s your goal until you try it and other tunings, though looking forward to what you wind up with.

Well, the way we typically experimented with different reflex tunings way back when was by blocking off increasingly larger portions of the stock vent with scrap wood clamped to it and once the desired tuning was found, then a proper vent large enough to handle high power was designed/incorporated, which just requires making a new baffle board to replace the existing one or two depending on whether it?s an 825 or 828 series, so no cutting into the cab required unless you just want the vent[s] on some other side[s].

After T/S became popular, folks tended to add a removable gasketed vent board with whatever size/number of tubes was required and make the tube[s] too long and shorten them as required. Others just left the baffle in place and slid the tubes in with a tight slip fit and not pushed in all the way, so that they could easily be removed, otherwise any loose fit could be temporarily sealed with cloth/whatever shims or with blu-tac or similar, i.e. no need to flush mount them for testing.

A shelf vent is one where a panel is used to box in three sides of a cab [AKA tunnel or slot vent]. As you can imagine, these are a bit harder to experiment with in existing cabs.

GM

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Originally Posted by Phil-G

i have 416 8Zs (from a set of 846b valencias), similar to the set in panomaniacs a7-500s http://www.hostboard.com/forums/f700...ey/index9.html and after reading this part of the thread it was pano who suggested the 8" x 6-7" port. but thats what i'm working with. when i get what i want. i'll go ahead and cut a new bottom panel and finish it.

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IIRC he has hybrid A5?s with custom XOs and 416s. Regardless, this vent puts tuning down around Fs [my preference for wall or corner placement] and if driven by a high output impedance, then making an effort to tune it exactly to Fs is normally the best overall performer.

GM

And thanks again, GM!

I just had a weird thought. Since I'm in the process of building crossovers for my 604s and getting my 828 cabs ready for their "close-up", I was wondering how my 604Ds would sound in an 828 cab. I'm sure someone has done this, drunk as they might have been at the time.

You're welcome!

Not too weird, a few of us thought the 604E sounded pretty good in an 825 plus these folks took the idea to a whole 'nother level of weirdness: 1979 STANLEY SCREAMER

The 'daddy' of this weirdness was the original A6 though and what spurred me into trying a mini-me version: http://www.voiceofthetheatre.com/ima...att1945.10.jpg

In this pic you can see how much bigger the A110 cab was Vs the 800 series:

GM

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Originally Posted by alancohen

And thanks again, GM!

I just had a weird thought. Since I'm in the process of building crossovers for my 604s and getting my 828 cabs ready for their "close-up", I was wondering how my 604Ds would sound in an 828 cab. I'm sure someone has done this, drunk as they might have been at the time.

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Somebody did it because I remembering them saying they did it...They said it wasn't too bad sounding either.

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Originally Posted by GM

You're welcome!

Not too weird, a few of us thought the 604E sounded pretty good in an 825 plus these folks took the idea to a whole 'nother level of weirdness: 1979 STANLEY SCREAMER

The 'daddy' of this weirdness was the original A6 though and what spurred me into trying a mini-me version: http://www.voiceofthetheatre.com/ima...att1945.10.jpg

In this pic you can see how much bigger the A110 cab was Vs the 800 series:

GM

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That's enough speakers to fill a small stadium.

Well, you know what this means...stay "tuned"...

Thanks!!

So I got my N801-8As the other day and finally got around to mounting my 515Bs in the 828s and hooking them up.

I get no HF from one crossover. The other seems fine. My question is, for $150, should I just send them back and build my own or are these worth fiddling with? I can probably repair it, but for $150 I shouldn't have to.

BTW, the 515Bs sound pretty darn good, even with the backs off!

So I think I'm going to return the N801-8As, I'll probably end up bi-amping these anyway. For the time being I cannibalized the N1285-8Bs from my A6s. I cut out a replacement rear panel and got some matching paint from HD. I might just have to sand them down and paint them since I had to buy a gallon to get the color. I used a floor paint low luster base. Looks pretty good.

I need new binding posts since the originals got snapped off somewhere in their travels.

http://www.hostboard.com/forums/hbmc...2013/06/27.jpg

Are you using your 300B OTLs with these speakers? Bi-amping and getting a good match could be problematic. A digital Xover and another amp in your future?



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Originally Posted by zelgall

Are you using your 300B OTLs with these speakers? Bi-amping and getting a good match could be problematic. A digital Xover and another amp in your future?

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Probably not and yes. I have a Behringer DCX2496 still in the box, but I think the 300Bs need to be on their own. I thought about using them to power the horns and my 15W OTL to drive the bass. But the bass is so sweet with the 300Bs, albeit not going to blow the windows out at 1.5W. It has astonishing peak power and feels more like 8W in the bottom than 1.5. Then again, 300Bs are known for their mids. I guess I can always try it. My 15Wer driving the 99dB 515Bs and 1.5W pushing the 109dB 288s crossed over around 500Hz might be a good matchup.

I have a nice 3W Cathode follower transformer coupled amp and the parts to build another. I was thinking about using the two of them to bi-amp the A7s. They're KT88 based and I like that tube. I would leave the 300Bs to play nicely with the 604s in another room. The A7s would be my home entertainment system for movies and some music. The 300Bs and 604s would be for "serious" dedicated music listening.

Then I guess if that works out, the 15W OTL would get moved to the BR or shop. The cathode followers have quite a bit of gain and don't really need a preamp. The 15W OTL likes a preamp.

Anyway, that's what swimming around in my head. All subject to change without notice.

That sounds great Alan. It sounds like you've got a well balanced system. I'll bet it sounds amazing. Nice work.
And a great description of what you did.
I'd like to hear more about the otl method you used.

I went with 3/4" MDF for the port panel and a 6" diameter x 6.5" bakelite port tube.

http://www.hostboard.com/forums/hbmc...2013/06/29.jpg

http://www.hostboard.com/forums/hbmc...2013/06/30.jpg

That looks like some kind of phenolic material to me......... and after a quick Google search I see that Bakelite falls into that category.

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Originally Posted by Audio_by_Goodwill

That looks like some kind of phenolic material to me......... and after a quick Google search I see that Bakelite falls into that category.

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Yep, you are correct. I buy, reroll and resell polyester Mylar film from DuPont that comes on these tubes. I'm not sure why they use it; I'm sure there are cheaper solutions. But considering it's DuPont, I'm sure they have a reason. Also, a roll of this film is about $3K so they can absorb the cost. I have several laying around the house, and while I could never find a use for them, I couldn't seem to throw them out because they're Bakelite.

So all you pack rats out there have been validated once again. Keep on saving crap. You never know when you might need it ;)

I removed all that nasty fiberglass insulation. I'll be replacing it with Ultradenim.

This is what I've come up with for the internal bracing so far. The bottom panel had quite a bit of "bounce & boing" to it. It is dead now. These should also kill a lot of vibration at the bottom panel of the horn.

The side panels still boing a bit. I'll be putting two braces per side between the horn flares and side panels.

http://www.hostboard.com/forums/hbmc...2013/06/31.jpg

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I'm not sure why they use it; I'm sure there are cheaper solutions.

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It's strong and inherently dimensionally stable.

"Rags n lacquer" is what one of my old boss's always called phenolic material.

In a previous life i spent some time running a "slitter" that used a rotary knife to cut bulk rolled paper and film stock from it's 6 to 10 foot wide master rolls into rolls of smaller widths for use in both offset printing and packaging on an industrial scale.

Some paper/fiber cored rolls would have to spend a period of time in the environmentally controlled warehouse before the rolls would fit the mandrel of the slitter properly. The paper/fiber absorbs humidity, and in humid enough conditions it swells. If it swells too much, it may not return to it's original dimensions which will preclude some machinery from handling it.

That makes perfect sense. The film that came on the roll is .5 microns thin. 29,000' of it is about 3/16" thick on the roll. Dimensional stability would be a very good asset when handling it.

Speaking about dimensional stability...here's a morning's work on cab #1.

http://www.hostboard.com/forums/hbmc...2013/06/32.jpg

This has turned out to be quite a project. It should be worth it. There is a nice dull thud when you bang on it now.

I tell ya one thing; fine cabinetry, these things are not.

The front panel above the port panel was a little messed up. I thought of making a new one, then I thought of repairing this one, then I opted to just turn it over and paint the other side. It's been said, when you need to find the most expedient way to do something, ask the laziest person available.

http://www.hostboard.com/forums/hbmc...2013/06/33.jpg

http://www.hostboard.com/forums/hbmc...2013/06/34.jpg

All painted. I didn't bother filling in dings and scratches; 'cuz then I'd have to worry about dinging and scratching them.

http://www.hostboard.com/forums/hbmc...2013/06/35.jpg

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Originally Posted by alancohen

So all you pack rats out there have been validated once again. Keep on saving crap. You never know when you might need it ;)

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HAHAHA. That's fantastic!!!! :-) I like picking up cat litter buckets........ they're just as handy as 5 gallon buckets, but have attached lids and fit in smaller places. They don't make very good port tubes, Though!

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I've wanted to use the recycled denim for awhile. I think you are the first I've seen to use it and post pictures.

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Originally Posted by Audio_by_Goodwill

I've wanted to use the recycled denim for awhile. I think you are the first I've seen to use it and post pictures.

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The color matches perfectly. It's like it was made for it. And there's just something poetic about Altecs and an old pair o' jeans.

the blue sure looks nice.