Hey, I'm planning on making an extension 2x12 cabinet for my brand new Mesa combo for a more versatile setup. Before anyone asks, yes I have searched, and searched, and searched to no avail. I want to be able to use it for open cleans when needed, or be able to convert it for the tightest metal tones. I also have some questions about dimensions, so here goes:

1. If I recess the back of the cab, could a secured, tolexed piece of MDF or plywood be adequate for making an open back into closed back cab? (ie. is a complete seal necessary?)

2. I want it to resemble my combo in external size and I measured it to be 11"x21"x29" externally so what design and measurements should I follow when building? I've drawn up diagrams, but I'd like to compare it to a solid design for reference.

3. I'd like it to be sturdy enough to hold the amp (~95lbs) but if thats not possible, its ok, I can just toss it on top.

1, yes, for a closed back to be effective, the seal must be tight'
2, depends on which way you go, open or closed back
3, thats a really freaking heavy amp, i wouldnt put in on top, cuz if it falls, its gunna **** shit up, but, if you add solid bracing inside the cabinet, im aure it would support it. all depends on your carpentry/ wood working skills
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Wow, that was fast. Thanks!

I think I'll be able to pull of a decent seal, I can always add layers of tolex on top of the back panel until I get a tight seal.

I'm going to try the dual function cab, so I'm thinking a basic prism made out of 2x4's should suffice with bracing at all right angles and butt joints. right? (screws + wood glue to secure). I'll wrap that in 1/4" ply and then tolex. I need an odd inside dimension though, I'll get that if I go with the Mesa dimensions or similar. right?

Scrap the idea of it holding the combo, I'll put them next to each other. I don't want to risk dropping my most expensive and most delicate piece of gear.

1 more question, I was thinking, would it be a good idea to put a cooling fan in the back of the cab, not to cool it since it doesn't produce appreciable heat, but to promote airflow behind the speakers?
A cooling fan totally defeats the purpose of having a closed back.

To make a good seal on the removable back, line the edges with rubber piping, which you can probably find the the hardware store (for sealing doors, windows, etc.)

If you want it to support much weight, you should run a brace down the center back, top, and maybe front.

The dimensions will be determined by what speakers you use. Interior volume (as in cubic cm) is important for a closed back cab.
I thought of using the rubber lining for sealing doors but since my amp already has an open-ish back (open with the bottom half covered), should I just go ahead and make a closed back cab to tighten up OD and leave cleans to the combo speakers?
Also, you said the interior volume matters, but does it go further than "use odd dimensions" for the inside?
use odd numbered dimensions that arent even multiples of each other to keep resonance down.

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yeah you should at least have some pillar/brace to hold weight. what if u decide to sit on it? make sure you can.
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I'm thinking of doing the same thing as OP. How would I go about calculating the dimensions for a closed back. I just read this guide (http://www.duncanamps.com/technical/speaker_cab.html) which suggested to use a specific book for the resonance calculations. Anyone have any ideas?


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