Spacers and phase plug clarification on a J12L

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fender3x
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Spacers and phase plug clarification on a J12L

#1 Post by fender3x »

First...am I understanding the plans right that the spacer and phase plug is 1/2" thick, and that is why I need to cut two of them for each cab?

I have some good 1/2" plywood. Could I just make them out of that instead?

I also have a piece of 1/2" plywood that I *thought* was BB when I bought it, because the thin ply was so close to the color of the next-to-last ply that I thought it was a single ply. Would the thin veneer be a problem in a spacer or phase plug?
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Seth
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Re: Spacers and phase plug clarification on a J12L

#2 Post by Seth »

Yes, you can use ½" ply for the spacer and phase plug.

The problem with the stuff that looks good on the outside, but is just a pretty veneer, is the glue they use to apply the thin veneer typically isn't very good and the veneer can separate from the base ply it's on. Meaning, anything you glue to the face of it is only held together with the poor quality glue, no matter the strength of the adhesive you use. For anything but braces or other parts that are only joined on their edges, where the integrity of the poorly laminated veneer is unimportant, I'd have to recommend not using it if you think it may be that sort of lumber.
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Seth
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Re: Spacers and phase plug clarification on a J12L

#3 Post by Seth »

Post a photo of the edge of the piece in question, showing the laminations.
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One T39, 16", 3012LF loaded
Tall AutoTuba, 20" wide, 2x 8" MCM 55-2421
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Tom Smit
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Re: Spacers and phase plug clarification on a J12L

#4 Post by Tom Smit »

I would say that since the spacer will be trapped between the baffle and the driver, with screws holding the driver in, there will be no problem. For the phase plug, if you run a screw through the back side of the baffle, and into the phase plug (pre-coated with PL), again, there should not be a problem. In both cases there will be a mechanical fastener that will assist in the task of keeping the parts joined.
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fender3x
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Re: Spacers and phase plug clarification on a J12L

#5 Post by fender3x »

Seth wrote:Post a photo of the edge of the piece in question, showing the laminations.
Ok, posting as an object lesson "kids, be careful, there is a lot of crappy plywood out there."

I think I have enough good half inch plywood to do the job, but if not I'll glue together some of the 1/4" BB. Not worth the risk to use inferior materials.
IMG20220920214054.jpg
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Re: Spacers and phase plug clarification on a J12L

#6 Post by fender3x »

Didn't see Tom's post until I posted. I was sort of thinking the same thing.... But in retrospect I think I'll be happier using the cheap stuff for router templates and such.

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Re: Spacers and phase plug clarification on a J12L

#7 Post by Seth »

Image
Looks like standard 9 ply BB to my eye.
Build in process - 2 WH6, one Alpha 6a loaded, one PRV Audio 6MB250-NDY loaded

Two 2x6 shorty SLA Pro's
One T39, 16", 3012LF loaded
Tall AutoTuba, 20" wide, 2x 8" MCM 55-2421
TruckTuba, 8½" wide, 2x 8" MCM 55-2421

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Re: Spacers and phase plug clarification on a J12L

#8 Post by fender3x »

Clipboard01.png
Clipboard01.png (82.54 KiB) Viewed 1036 times
You can see it better on the bottom than the top. The veneer is almost the same color as the second to last ply so I also thought the "birch" plywood was BB. That is what is so pernicious about some of the cheap veneer plywoods. I didn't know I'd gotten the wrong material until I had it home and got it under sun light. The top side, which is the A side is so thin you almost don't see it in this pic.
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Seth
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Re: Spacers and phase plug clarification on a J12L

#9 Post by Seth »

I'd completely use it for your intended parts and just make it so that surface is toward the driver and the other side gets the glue, to be on the safe side.
Build in process - 2 WH6, one Alpha 6a loaded, one PRV Audio 6MB250-NDY loaded

Two 2x6 shorty SLA Pro's
One T39, 16", 3012LF loaded
Tall AutoTuba, 20" wide, 2x 8" MCM 55-2421
TruckTuba, 8½" wide, 2x 8" MCM 55-2421

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fender3x
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Re: Spacers and phase plug clarification on a J12L

#10 Post by fender3x »

How close is the tolerance on the plug and the spacer. Cut a test phase plug yesterday, and it was too small by about 1/16. It was I was testing out my imporvised circle cutting jig... Same question on the spacer. I have yet to cut those...
One man's magic is another man's engineering. --Heinlein

1 x DR250
1 x T39
1 x Omni 10

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