Building a Guitar Amp Cabinet

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Hi, I'm looking to build an amp cabinet but all the information I find online just tells me how to physically construct the wooden enclosure, which I can handle quite easily!

I have a Fender Super Champ XD which is a 15W tube amp with an 8 ohm output. I want to use the tube and effects in the amp but would like it louder.

Is it possible to do this by adding a 12" speaker cabinet below? (the current one is a 10") Would a double 12" be louder? Also, what does the cabinet consist of, is it just a speaker and input or is there a driver which I've heard about but have no idea what it is!

Any help is much appreciated!

Thanks, Ed
 
Hi, I'm looking to build an amp cabinet but all the information I find online just tells me how to physically construct the wooden enclosure, which I can handle quite easily!

I have a Fender Super Champ XD which is a 15W tube amp with an 8 ohm output. I want to use the tube and effects in the amp but would like it louder.

Is it possible to do this by adding a 12" speaker cabinet below? (the current one is a 10") Would a double 12" be louder? Also, what does the cabinet consist of, is it just a speaker and input or is there a driver which I've heard about but have no idea what it is!

Any help is much appreciated!

Thanks, Ed

Usually one just adds another "driver" which IS the speaker in inside the enclosure (the box).

So adding the speaker usually means adding a box (wood) and a driver (speaker).

As a result, you have more speaker-surface area pushing air,
and more power-handling capability.

This won't add more "watts" to your amp,
but it might allow it to drive two speakers,
at a higher volume than only one speaker would allow.

That is, if the first speaker can only realistically handle 10 watts,
but you drove it at 15 watts, it would eventually overheat,
and the voicecoil could melt-down or fry.
Also, if the speaker is pushed so it moves more than its designed "excursion" allows (usually only a few mm for a guitar-speaker),
then the speaker could crumple, rip, or otherwise become damaged.

On the other hand,
there may be a problem hooking up two speakers:

Each speaker is usually a nominal "ohms" rating (impedance),
Impedance (Z) is the resistance to A.C. (music) that an equivalent resistor would present to D.C.

Two add a speaker to an existing speaker circuit
usually requires that both speakers match in impedance:

Thus if you already have an 8 ohm speaker,
to add another speaker it should also be 8 ohms in impedance.
(this is not power-handling, just resistance).
Equal resistance ensures that each speaker shares an
equal LOAD or share of the music power, spreading it fairly.

In any case, you must also decide how to hook up the 2nd speaker:

You can either hook it up in series, or in parallel:

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parallel-vs-series-speaker-connections.gif


In series, 8 ohm + 8 ohm = 16 ohms, which presents 16 ohms to the amplifier output.

In parallel, 8 ohm || 8 ohm = 4 ohms, which looks like a 4 ohm speaker to the amp.



In the second case, it will seem louder, but you risk allowing too much current to flow to the speakers.
If this happens with a Transistor Amp (not tubes),
it might fry the output transistors and kill the amp.

In the first case, two much resistance chokes off the current, but is better handled by the amplifier.


In neither case can you get the same impedance (8 ohms) with just two speakers.
To keep the impedance the same, you must add 3 speakers to your existing speaker,
and wire it in Series/parallel, which means both series and parallel:

An externally hosted image should be here but it was not working when we last tested it.



The first thing to do is to try to find out what impedance your existing speaker is.

For this you should look for markings on it,
or else consult the circuit-diagram for your amp.
Alternately, you can take a D.C. resistance measurement,
and try to estimate the impedance from that.

I hope this helps.
Nazaroo
 
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Hi, I'm looking to build an amp cabinet but all the information I find online just tells me how to physically construct the wooden enclosure, which I can handle quite easily!

I have a Fender Super Champ XD which is a 15W tube amp with an 8 ohm output. I want to use the tube and effects in the amp but would like it louder.

Is it possible to do this by adding a 12" speaker cabinet below? (the current one is a 10") Would a double 12" be louder? Also, what does the cabinet consist of, is it just a speaker and input or is there a driver which I've heard about but have no idea what it is!

Any help is much appreciated!

Thanks, Ed

Yes two identical speakers are 3dB louder then just one speaker. But 3dB is just only barely noticeable.

Speaker are rated in dB/Watt. One 12" speaker might be 96 dB/W and another might be as high as 103 dB/W. So simple swapping out your 96 for a 103 makes it louder then having two 96s.

For a noticeable increase you need to do "all of the above" and build a 4x12 cab. You can work out the math to see what you need to get an 18dB boost. Most peole think 18dB is a bunch louder while 6dB is just "a little louder".

Yes the speaker cab is just a box. many of them have open backs or at least partially open backs. I'd use 3/4" x 12" lumber food the sides and 1/2" baltic birch for the from baffle. Then you wire all the spears to one input jack.

You need to worry about getting the impeaches correct or match to the amp.


One tool to buy: Go to Radio Shack and pick up a sound level meter for $50. The RS one is a kind of standard that is well used. These measure sound level in db. Measure your rig and one that you think is loud enough. Then you cn see how many more dBs you need. Takes lots of measurements while playing different kinds of songs and kin of average it.
 
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In regards to wiring the speakers to match the ohms:

Surely I could then match two 16ohm speakers in parallel to match the heads 8ohm? But would the head accept this or would the current be too much? As the head is 8ohm, 15watt.


In regards to volume:

Is it the wattage that has the biggest effect on the volume?

For my amp, a 10", 15w combo they've recently released a head/cab sister amp which is a 15w head with a single 12" cab rated at 80w, both 8ohm. This seems like a good idea to replicate considering my amp as it is is the same as the head. But to me it looks like 65w of the speaker is being wasted as the head only provides 15w?

Thanks!
 
In regards to wiring the speakers to match the ohms:

Surely I could then match two 16ohm speakers in parallel to match the heads 8ohm? But would the head accept this or would the current be too much? As the head is 8ohm, 15watt.

Yes two 16 ohms would be perfect, retaining the impedance match. Don't worry about current, if the impedance is right.


In regards to volume:

Is it the wattage that has the biggest effect on the volume?

For my amp, a 10", 15w combo they've recently released a head/cab sister amp which is a 15w head with a single 12" cab rated at 80w, both 8ohm. This seems like a good idea to replicate considering my amp as it is is the same as the head. But to me it looks like 65w of the speaker is being wasted as the head only provides 15w?

Thanks!
The biggest effect on volume (loudness) is efficiency:

Thus a 100 db speaker would be much louder than a 90 db speaker, same ohms, same wattage.

65 watts is not being "wasted" in the new combo.

First of all, speakers are overrated.
They say 80 watts, but mean more like 40-50 sustained.
This is important, because for bass, its more like a sustained sinewave signal, than a much lighter random music signal (i.e., elevator muzac).

Second, you want your speaker to be able to handle more power than the amp puts out. That way, the amp can be turned up without danger to the speaker.
Also, sometimes a pedal or active pickup can overdrive an amp, pumping out more watts than its rating.

Thus a 15 watt amp driven by a power-pedal might push 20-25 watts of power into the speaker!

Also, when an amp clips (running out of headroom) it makes the signal wave 'square'.
This actually adds more harmonics to the note (other notes at higher frequencies). The other harmonics contain additional POWER, which translates as motion and heat in the speaker.
So a 15 watt amp that is clipping might be punching the speaker with far more watts of power than just the 'root-note' you're playing.

Finally, you don't want to run a speaker at its maximum wattage.
Thats the wattage that the coils will overheat and fry at,
and the wattage that the speaker-maker will no longer warrantee it.
 
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Joined 2005
the 'highish' power rating of guitar speakers would be due to not playing bass on a guitar :D

worth to note that the Fs/'SPL'/Xmax relationship may be rather large 'variables', between the different driver sizes

if you want it really loud, I would choose a driver with higher Fs, and low Qts, small VAS, and put it in a closed box

but for easy transport, think small, and 'modular'
you can always ad more later on;)
 
I think I'm starting to get it!

I'm going to take it easy first, make a 1x12, then if that goes well attempt the 2x12.

Is 16 x 17.5 x 9 (40.7x44.5x23.3cm) big enough for a 1x12 closed back cab? That's the size of Fender's corresponding model but a lot of forum threads rally behind size and depth like nobody's business.

Harking back to GCSE physics: is it possible to wire 2 8ohm speakers in parallel, then in series with a 4ohm resistor to present 8ohms to the head? Or in series then in parallel with a 16ohm resistor? Or does this have a detrimental effect on the sound?
 
You're basically wasting power as heat in the resistor. Not useful if you want it louder.

For guitar cabinets, just go by "that looks about right", or make it the right size for your amplifier to sit on and it be pleasing to the eye.

Some might claim there's some science behind them, but for guitar use, you're after "tone", not HiFi, so often the science usually used in speaker design gets thrown out in favour of subjectively better sound - remember, the guitar amplifier is part of the instrument, so should add its own character to the sound.

Chris
 
FWIW you can safely run less impedance than optimal on a tube amp but don't go much higher- You can run 4 ohms or even two ohms no prob but dont try hooking up a 4x12 cab in series for 32 ohm. The opposite is true for solid state.

The load is reflected back to the tube, if you have 5k output transformer with 8 ohm speaker then you hook up 16 ohm speaker (two eight ohm in series) the output tube will see a 10k load instead. Alternately if you hook up a 4 ohm speaker (two eights in parallel) then you will load the output tube with 2500 ohms instead. This (less ohm load) usually results in more second harmonic distortion which imo sounds good for guitar.

Not hooking any speaker up at all with the amp on can hurt the output tube and make it arc inside. However completely shorting the wires is harmless.
 
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