How to add a speaker cabinet to an amp w/out an existing extension jack?

[Cholo]

What's the best way to add a 2x12 speaker cabinet to my modified Marshall Bluesbreaker? Love my amp but I need more volume. Unfortunately my amp doesn't have an extension speaker jack and I don't want to have to buy a half stack. Surely something can be done. Any ideas?

[bob p]

you're talking about a reissue Marshall 1962 JTM45 with the built-in 2x12, right? and that amp doesn't have an expansion jack? does it have a selectable output impedance?

Being a 30W amp, its not all that powerful and there's a limit to how much volume you're going to get out of it. Adding speakers may help a little -- By doubling the surface area of the speaker array you're going to move more air, which will give you a gain of 3dB from the speakers. that's noticable, but its not that much.

if the amp doesn't have a selectable output impedance then adding speakers will impart a bit of an impedance mismatch on the output transformer, which could work against you in terms of the amp's power output.

that said, the easiest way to expand the amp is by adding another set of identical 2x12 speakers, and to daisy chain them off of the terminals on the back of your speakers, so that the new 2x12 box is running in parallel. this will cut in half the net load impedance that the amplifier sees from the speakers cabs. the sound could get a little flubby.

another option would be to rewire the 4 speakers in series/parallel. this would double the net Z of the load on your amp. the sound may be less flubby with the amp working against a higher load.

i think that those are about the only options that you have when it comes to speakers. If the bluesbreaker isn't loud enough for you, the bottom line is that you may need a find a bigger / louder amp.

personally, i find it hard to imagine a situation where that amp wouldn't be loud enough for me. but then i don't dream of full stacks.

are you really looking for an amp that's louder, or are you looking for an amp that has more gain?

HTH

[Cholo]

you're talking about a reissue Marshall 1962 JTM45 with the built-in 2x12, right? and that amp doesn't have an expansion jack? does it have a selectable output impedance?

Right. It is the reissue with the built in 2x12 and it does not have an expansion jack. It does though allow the user to select the output impedance.

...the easiest way to expand the amp is by adding another set of identical 2x12 speakers, and to daisy chain them off of the terminals on the back of your speakers, so that the new 2x12 box is running in parallel. this will cut in half the net load impedance that the amplifier sees from the speakers cabs. the sound could get a little flubby.

another option would be to rewire the 4 speakers in series/parallel. this would double the net Z of the load on your amp. the sound may be less flubby with the amp working against a higher load.

i think that those are about the only options that you have when it comes to speakers. If the bluesbreaker isn't loud enough for you, the bottom line is that you may need a find a bigger / louder amp.

That's what I am afraid of. However, I love my amp's tone. I have totally had it modded to 1966 specs. Finding a bigger amp may solve my volume issue but it would compromise the tone that I have sought out for so long.

When you talk about rewiring the existing speakers, can this be accomplished by adding an expansion jack to my amp? I'm a newb when it comes to wiring/impedance and such.

are you really looking for an amp that's louder, or are you looking for an amp that has more gain?

HTH

I would just like to increase the volume of my amp. I don't want another one.

Appreciate the help!

[bob p]

If you have selectable output impedance then the upgrade is EASY.

Just find another speaker cab, and make sure the drivers have the same impedance as your current drivers, so the power will be distributed evenly among the drivers.

Once you have the new drivers, you'll need to set the output Z on your amp to match the total Z of the new speaker array. then all that you need to do is to redo some wiring. This will require disconnecting the wires at the speakers and making some changes to the wiring harness.

It would help if you could state the following:

1. what output Z options you have available on the amp selector
2. what output Z your amp is currently set to
3. what the Z is for each speaker
4. how the two existing speakers are wired (series vs. parallel)

i'm guessing that you've got two 16R speakers wired in parallel for an 8R load, and that the amp's Zout is set to 8.

[bob p]

I'm thinking that adding a set of speakers will help -- if you do the mini-stack thing, moving the combo higher up toward your ears will make you perceive the amp as being louder because of proximity effects.

doubling the number of drivers while keeping Zout matched will give you +3dB in SPL. that's sonically equivalent to doubling the amp's output power.

In the big scheme of things, of a JTM45 with 4x12 isn't loud enough, your only other options are:

1) go to an 8x12 to gain another 3db
2) buy another JTM45 and gang them,
3) buy a different amp that's louder,
4) mic your amp through the PA

you never really mentioned why you want the amp louder. do you want it to be louder to project into a bigger room for a larger audience, or do you just want a louder amp for the home? if you want it to be louder just for you, the simple solution is to stand closer to the amp. every time that you cut the distance in half you'll gain 6dB, which is like quadrupling the amp's power. changing the sonic characteristics of the room will also have a HUGE effect on perceived loudness. for example, i've heard acoustic ceiling tiles suck all of the volume out of a very loud amp.