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Converting open back cab to closed back without Thiele/Small?


blindopher

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Hi,

 

These are my cab's dimensions:

 

W24"(61cm) X H20.75"(53cm) X D10"(28cm). It's an open back cab made of 18mm birch plywood. Single 30W 12" speaker (Weber Alnico Blue Dog).

 

I would like if possible to close it to tighten the low end. I play a 5W SE class A amp.

 

So I was hoping I can I avoid the dreaded Thiele/Small calculations by asking your advise. Out of your own experience perhaps, are these dimensions suitable? Could I harm the driver in any way?

 

Anything else I need to know or watch out for?

 

Thanx!

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I wouldn't worry about Thiele/Small calculations much. A Guitar is a low fidelity midrange instrument that only produces frequencies between 150 to 5Khz.

 

Put a piece of wood across the back and duct tape it in place and see what it sounds like. If it doesn't get you what you wanted, you avoided allot of work and damage to the cab.

 

My guess is, its the head that's voiced for a smaller speaker like a 8 or 10" that doesn't produce as much bass and not a problem with the cab. I'd also try something like a lower wattage Vintage Alnico speaker like Jensen or Eminence. The SPL level wouldn't be as high but they would tighten up the bass allot.

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When it comes to guitar cab designs there is almost zero thought about T.S parameters. The 4X12 half stack was made in such a way that it utilized less than one standard piece of 4X8' plywood. There are a few designs that utilize T.S parameters but are very expensive and don't necessarily sound godly compared to others that don't. I have a pair of Seymore Duncan 1X12" boxes that must be from the 80'-90's that are closed back, ported and sized like they were meant for a specific speaker? That original speaker is unknown to me and having used about 4-5 different ones in it, I can say they all sound different no more no less. I ended up with G12-75T's in them simply cause I liked the sound of them.

 

The general rule is that bigger cabs sound bassy and bigger while smaller boxes sound smaller and more focused. You can use a box design that will tune the cab to a certain frequency, but that doesn't mean you have to use a speaker designed to resonate at that frequency. Closed back speakers generally sound tighter and more focused and can achieve high spl and can produce lots of tight bass. Ported designs can have lots of bass too at the ports tuning and open back speakers generally sound bigger and more room filling. The open back design is probably the worst in terms of performance as it does very little to control sound. I prefer closed back ported designs. I have two different ones which I like a lot. I have a 90's H&K 2X12 model that sounds as big as a 4X12" and is half the weight. It produces lots of low end and is well focused. The Duncans I have work well with any speaker and sound big for a single 12" box.

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