My new-to-me 820s

[wws944]

Here is a photo of a portion of the bass reflex vent. It is between the main grill area and the floor. About 40" wide:

[GM]

IIRC, all of the plans are missing some information and/or has some way wrong dims, so do require a bit of common sense and/or speaker design knowledge to make them 'good enough' once the vent is properly tuned and the damping done to 'taste'.

The 820's vent is hinted at though in the far left 3/4 view where a corner of its framing is shown and its 2-1/4" vertical cutout height is 'hidden' in the section B-B view; so with the implied 5/8" thick side base rails framing the cutout, its width is = to the horn width of ~28-1/8", making the vent area ~63.28"^2 and the 3/4" thick front base rail makes it 3/4" deep. The exposed center brace creates a minor vent divider that combined with the cab's high aspect ratio, multiple vent holes, fiberglass damping and grill cloth shifts tuning lower than typical reflex Helmholtz theory predicts.

Hope you?ll publish your cab?s vent dims for posterity though and eventually measure actual tuning of each in a corner.

The one I cobbled together back when I knew virtually nothing about speaker/horn design proved to be tuned way too low for the cheap ex-stereo console 'woofers' I used, so wound up removing the front base rail and cut out all those resistive holes I'd laboriously made with a hand drill. No wonder my hands ache so much now.

Unfortunately, it was still a bit ?one note? down low, but then I didn?t know that drivers used in reflex cabs had way different design specs than those for consoles, so probably should have just braced the sides and tuned it by gapping it to the corner.

GM

[wws944]

IIRC, all of the plans are missing some information and/or has some way wrong dims, so do require a bit of common sense and/or speaker design knowledge to make them 'good enough' once the vent is properly tuned and the damping done to 'taste'.

Figures...

The 820's vent is hinted at though in the far left 3/4 view where a corner of its framing is shown and its 2-1/4" vertical cutout height is 'hidden' in the section B-B view; so with the implied 5/8" thick side base rails framing the cutout, its width is = to the horn width of ~28-1/8", making the vent area ~63.28"^2 and the 3/4" thick front base rail makes it 3/4" deep. The exposed center brace creates a minor vent divider that combined with the cab's high aspect ratio, multiple vent holes, fiberglass damping and grill cloth shifts tuning lower than typical reflex Helmholtz theory predicts.

If you look way back at the first photo in post #1 of this thread, you can see the 'thin' edge of the vent. In the picture, it runs diagonally from one side of the cabinet to the other. So it is several inches wider than the bass horns. And yes, it has that little wooden block in the middle of it. Funny - I simply thought the wood block was for extra stability, and never knew it was also part of the vent tuning. Learn something new every day!

Hope you’ll publish your cab’s vent dims for posterity though and eventually measure actual tuning of each in a corner.

Will be happy to do so.

Speaking of which, I do have a question: my living room has hardwood flooring in it. Should I make up a triangular-shaped piece of carpeting or similar for them to sit on to help damp any cabinet resonances? We were musing that Altec may have made that assumption.

[Alien_Shore]

perhaps I could bi-amp them. An active crossover would cost about what a pair of decent quality passive crossovers would, and I already have the amps.

A primo idea. I typically will use my active at first to help decide if passives will work on a given setup, and if so, which design parms to shoot for.

[GM]

If you look way back at the first photo...........

Funny - I simply thought the wood block was for extra stability..........

Should I make up a triangular-shaped piece of carpeting.........

Yeah, I posted based on notes I?d transcribed years later from the build to the pics in the B-D book [can?t remember which earlier book that had these same drawings], but after posting I noticed that I?d misinterpreted them when I?d built it, so now I know another reason why it was tuned so low initially!

Oh well, I built it when I was thirteen, so with a testosterone saturated brain combined with being girl/hotrodding/racing ?crazy? it?s surprising it came out as well as it did.

Anyway, the drawing implies a 41? x 2-1/4? vent, but your close-up of the vent cover implies a shorter, narrower slot, I guess to allow plenty of fabric overlap, so what is it actually and are they the same for both cabs?

It seems reasonable to me that the little wood block is strictly for horn support even though they long since knew that it would act as a bit of a vent divider.

Good question. All the vented Altec cabs I?ve seen except the later THX cabs have been pretty lossy due to poor sealing, so vent selection takes a certain amount of production tolerance into account plus typical plush pile carpet doesn?t seal an open bottom very well either, so noticing that the cabs have little furniture ?buttons? on the bottom, I?d say try it both ways if practical to find which sounds best overall since either way it has some air leakage and the carpet will damp the port more than a reflective floor surface.

GM

[wws944]

...Oh well, I built it when I was thirteen, so with a testosterone saturated brain combined with being girl/hotrodding/racing ‘crazy’ it’s surprising it came out as well as it did.

That is more productive than I was, when I was 13...

Do you still have the one (?) you built? Did you ever get it/them tuned to your satisfaction?

Anyway, the drawing implies a 41” x 2-1/4” vent, but your close-up of the vent cover implies a shorter, narrower slot, I guess to allow plenty of fabric overlap, so what is it actually and are they the same for both cabs?

The wood vent panel piece could be as wide as 41". When I took the photo in post #51, we were focused on trying to capture the original grill cloth. Didn't have the presense of mind to measure the vent. I will ask him to measure them and report back ASAP.

Note in the post #1 photos that there are subtle differences in construction between the 1955 and 1957 cabinets on either side of the vent panel. So the panel dimensions could be a little different. Though, of course, the vent itself could be the same size. You can also see at the lower left corner of the 1955 cabinet where the wood got damaged sometime in the distant past. He removed for repair, which is why it was easy for him to bring the vent panel by. He has some 25+ year old 'old stock' wood that he says will be a perfect replacement.

It seems reasonable to me that the little wood block is strictly for horn support even though they long since knew that it would act as a bit of a vent divider.

Good question. All the vented Altec cabs I’ve seen except the later THX cabs have been pretty lossy due to poor sealing, so vent selection takes a certain amount of production tolerance into account plus typical plush pile carpet doesn’t seal an open bottom very well either, so noticing that the cabs have little furniture ‘buttons’ on the bottom, I’d say try it both ways if practical to find which sounds best overall since either way it has some air leakage and the carpet will damp the port more than a reflective floor surface.

So try it both ways. Gotcha.

Thanks!

A primo idea. I typically will use my active at first to help decide if passives will work on a given setup, and if so, which design parms to shoot for.

Any suggestions for reasonably priced active crossovers? There is a local guitar place that has a lot of used gear for sale. Could stop by there and see what they have. Are there ones to definitely avoid?

[wws944]

GM: He sent me the dimensions for the 1955 cabinet vent panel. It measures 39-3/8" wide and 2-7/8" tall. The vent measures 35-3/8" x 1-3/8". As you pointed out, the block of wood divides the vent in the middle. It is 1-5/8" wide. So each half of the vent is 23.5 sq in.

I don't have the dimensions of the panel on the 1957 cabinet yet. If you look at the photos of the bottoms back in post #1, you can see that the panel is a bit wider. Not sure about the vent dimensions - which are, of course, the interesting part. But they should be similar.

[wws944]

Update (finally!):

We had a bit of a heat wave here in Northern California last month, but it has been cooler the past couple of weeks. The fellow who is doing the refinishing reports that it has been cool enough to shoot the laquer on the 'nose' frames. With that done, the grill cloth is being installed on them. He still has a thing or two to touch up before he shoots the main cabinets themselves.

Grill cloth is Newcastle/Acoustone P211. I ordered 6 yards (x 2 yards) directly from Acoustone. There should be plenty for them to work with. We are also going to re-cloth the bass reflex vents, but he has carefully saved the old cloth.

We talked today about the rear of the cabinets, and this is where I would enjoy some opinions from the Altec cognoscenti. (That means US on the forum.) Internally, the factory ran wires from the crossover to a two-terminal barrier strip. If you look back at post #10 in this thread, second photo, you can see it. I guess the intent was that the customer would run the wires under the cabinet and up to the barrier strip.

At some point, my uncle drilled a couple of holes in a back panel of each of the cabinets and installed brass terminals for the speaker wires. We were thinking that maybe we should do something about these terminals since they stick out about an inch. This could potentially lead to accidentally damaging the drywall. Also, they were drilled in different positions on the two cabinets. So there are at least three options:

a.) Leave them alone. It is the way my uncle intended them to be.

b.) Fill in the holes and run the wires under the cabinet. It seems to be the way Altec intended them to be.

c.) Fill in the holes and install a nice pair of vintage (looking?) terminal cups. Lots to choose from around the 'net. Not sure if there were ever any Altec branded ones. Anyone know? Or have good ideas on appropriate ones to use?

[GM]

Short of running the wires straight to the components, the best speaker terminations are cold welds [ideal crimped] with the clamped ring tongue connector being its disconnect equivalent, so cheap as they look, Altec provided the right kind of terminal strip, though probably not of the best materials, but an anti-oxidant paste such as Penetrox will ensure a long term, low loss, electrically stable connection.

GM

[Alien_Shore]

Any suggestions for reasonably priced active crossovers? There is a local guitar place that has a lot of used gear for sale. Could stop by there and see what they have. Are there ones to definitely avoid?

MiniDSP seems like a good answer for your needs.