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Old 14th April 2008, 03:19 PM   #1
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Location: Istanbul
Default Problems while designing a guitar cabinet...

Hello folks,

I currently acquired a vintage Celestion G12-65 (15 ohm) guitar speaker (manufactured in January 25, 1982) in perfect condition.

Here are the reissue version's specs from Celestion
-----------------------------------------------------------------

Fs - 79 Hz (Their main page says 85 Hz? Does it make a big difference)
Re - 11.8 Ohm
Qms - 10.67
Qts - 0.61
Qes - 0.65
Cms - 0.13 mm/N
Vas - 51.4 lt
Le@1kHz - 0.78 mH
Pe: 65 Watts

Now, from what I read, before designing a cab I need to decide the Qtc of the cabinet. It's said that the best value for a guitar cabinet should be between 0.707 and 1.0. Let's say Qtc = 0.71
Also, again from what I read the F3 value should be below the lowest fundamental which is 73 Hz since I play in drop D tuning from time to time (But mostly I use standard E tuning, so should I chose 81 Hz?)

Another thing that I want is a good low frequency response so I choose to go with sealed back design without any ports or damping materials.

The problem occurs when I give all these information to HT Audio software to calculate the box dimensions...
Here are the steps that I took and the given error:
Click the image to open in full size.

I appreciate all the help
Thanks
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Old 14th April 2008, 04:07 PM   #2
sreten is online now sreten  United Kingdom
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Hi,

Your F3 is too low as the program states.

With Vas = nearly 2cuft and Fs ~ 80Hz and Qts = ~ 0.6 you do not
have a driver particularly suited to a sealed box. I'd would say it
will work quite well open backed with a valve guitar amplifier.

THe assumption here is the valve amplifer has significant output
impedance and this will fatten the open backed response.

A closed box is doable with a transistor amplifier, practicality suggests
a stuffed box around 1cuft, though an unstuffed box will be livelier.

Note that reproducing fundamentals accurately is not necessary.

/sreten.
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Old 14th April 2008, 04:17 PM   #3
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That's what I was afraid of I heard that these speakers sound quite bright with an open backed cabinet so I though I might lower it with a sealed box. I purchased the wrong speaker after all

thanks
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Old 14th April 2008, 04:46 PM   #4
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hmm after some try I'm begning to think that open back is not so bad after all. I managed to lower F3 to 78 Hz which I believe is going to give me the fat lower end that I wish. Right? BTW the back openness is around 300mmx100mm

thanks
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Old 14th April 2008, 04:48 PM   #5
sreten is online now sreten  United Kingdom
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http://reviews.harmony-central.com/r...on/G12-65/10/1

Hi,

Open backed or closed I think they will be fine. Most guitar speakers
do not follow "hi-fi" bass alignments, I would not worry about it.
They were originally designed for sealed 4x12's, so go sealed.

/sreten.
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There is nothing so practical as a really good theory - Ludwig Boltzmann
When your only tool is a hammer, every problem looks like a nail - Abraham Maslow
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Old 14th April 2008, 05:02 PM   #6
sreten is online now sreten  United Kingdom
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Quote:
Originally posted by cb951303
hmm after some try I'm begning to think that open back is not so bad after all. I managed to lower F3 to 78 Hz which I believe is goint to give me the fat lower and that I wish. Right? BTW the back openness is around 300mmx100mm

thanks
Hi,

Wrong. Open backed cabinets cancel low bass and do not sound fat.
They work best with the higher Qts guitar drivers that are not suitable
for sealed boxes. I'm no expert on guitar speakers so if they where
originally designed for sealed 4x12's then go with that.

In my first post I should have said not particularly suited to sealed
in terms of PA / hifi speakers. In a sealed box ~ 1cuft they will
sound fat - as in around 200Hz boosted fat.

/sreten.
__________________
There is nothing so practical as a really good theory - Ludwig Boltzmann
When your only tool is a hammer, every problem looks like a nail - Abraham Maslow
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Old 14th April 2008, 05:10 PM   #7
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wow thanks very much sreten!

after some more research I found out that marshall has a 1x12 closed back cabinet loaded with one of these speakers (model no 1933 to be exact) so I'll try to find the dimensions so that I can build one myself

thanks again
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Old 14th April 2008, 06:34 PM   #8
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Join Date: Apr 2007
I started making musical instrument speakers before hi-fi. You have a lot of leeway with a guitar speaker. Don't worry about it. Make it sealed between 1 and 2 cubic feet and it will be fine. Open will have more midrange and be looser. Sealed will be tight sounding for good dropped D chunk. I would just model the dimensions on practicality. Wide enough to hold your amp. 10 to 12 inches deep. 18" high or so. Done.
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