BillFitzmaurice.info

I've got an early 90s GK MB 150 amp (similar to http://www.gallien-krueger.com/products_mb_series.html) that I like to use for small gigs or for gigs with FOH support when I don't feel like schlepping my full rig. For the small gigs this amp is either ideal or overkill, and for larger gigs it's either adequate as backline or drowned out by the PA. What I'd like to do is come up with some sort of a small 8-ohm extension cabinet that I can use to bump up my stage level a bit on the larger gigs. GK sells a matching 12" box for $300 that would obviously be perfect, but at that price it seems much more cost effective to build my own (besides, it's more fun to build, right?). Does anyone have any suggestions for a 12" driver (8 ohms, ~100 watts) that would work well for bass in a lightweight sealed or ported box with roughly the same form factor? I know there are no BFM designs in this ballpark aside from the Jack, and I'm just about finished building two of those to use on gigs which require me to haul a larger rig.

Thanks in advance....

The only thing that makes sense IMO is a sub, probably T39. A second full range cab that's not identical to the first isn't going to match well. A sub, OTOH, will give you an octave plus of lows that the G-K is incapable of delivering. Refer to Gauss's review of his combo/T39.

The only thing that makes sense IMO is a sub, probably T39.

No doubt that would be very cool. The problem is that even a 14" T39 would be more than I'd want to carry to a gig where my full rig isn't necessary, and that's why I was looking into ideas for a small and simple 12" box. To be honest, now that I'm upgrading my main rig to Jacks I'm finding it difficult to justify keeping my MB 150 (it's served me well all these years, so it's hard to let go). On top of that, I also have the head version of this amp and using that to power a single Jack might just obviate the MB 150 completely. I guess I'm just trying to keep this trusty piece of gear useful.

Would any of your other designs fit the bill? The T18 looks to be about the right size, but I realize that it's not intended for stage use.

jbass wrote:On top of that, I also have the head version of this amp and using that to power a single Jack might just obviate the MB 150 completely.

Gear collector! Who needs three bass rigs? Sell the combo and buy your wife something nice.

Gear collector! Who needs three bass rigs? Sell the combo and buy your wife something nice.

Hmmm...good thought. Do you think she'd like a Rickenbacker 4001?

jbass wrote:

Gear collector! Who needs three bass rigs? Sell the combo and buy your wife something nice.

Hmmm...good thought. Do you think she'd like a Rickenbacker 4001?

She'd love it, and probably would let you borrow it anytime you wanted to.

and the T18??

Gregory East wrote:and the T18??

Inappropriate.

One of the main principles of sound that gets ignored by the majority is that two sources of the same sound interfere with each other. This means when you are playing with two speakers, the sound on the left is different from the sound on the right, because the soundwaves from the two separate sources add or subtract from each other when they meet. The frequencies that get affected change depending on where you sit, so one table says "bass is really clanky" and the other says "bass is tubby."

There are two ways to get around this - the most common is to have two identical speakers, and stack them vertically. If they're not identical, the soundwaves won't be the same (because of the color of each speaker and cabinet). So if you want a standard extension speaker, it must be the same as the first to keep your sound the same across the room.

The other method is to "bi-amp". This means that the low frequencies (usually 100hz and down) go to a subwoofer (via an electronic crossover), and the rest (above 100hz) go to the main or top speaker. No frequencies add or subtract because the two speakers are playing separate parts of the overall sound. This is what I did - I built a T39 as a subwoofer with the built-in crossover. Not only does this give me a speaker designed to actually reproduce the low bass notes, it frees up the top speaker to play mids and highs louder.

So I take my GenzBenz combo, with the tone I really like, to most of my gigs. When there's no PA, or I just want to knock people on their butts, I pull out the T39. It keeps the tone of my combo, but gives me the range down low I never had, and enough volume to shake the rafters in the gym.

The other method is to "bi-amp".

I'm starting to like this idea a lot. Would a 14" T39 do the trick? If so, do the height and depth remain the same as the wider versions? That looks to be the case from the photo at the bottom of http://billfitzmaurice.info/forum/viewt ... =26&t=9819.

jbass wrote:If so, do the height and depth remain the same as the wider versions? That looks to be the case from the photo at the bottom of http://billfitzmaurice.info/forum/viewt ... =26&t=9819.

With all of Bill's Subs the height and depth remain the same, only the width varies.

jbass wrote:

The other method is to "bi-amp".

I'm starting to like this idea a lot. Would a 14" T39 do the trick?

Are you talking about building a T39 to put under your combo amp? I'm not sure how Gauss does it with his GenzBenz, but I doubt there is a simple way to do with your GK. I doubt there is a separate output with a crossover from the power amp to power the sub. It may require some fancy wiring and an external crossover of some sort. Not saying you can't do it, just saying you won't just plug it in to the extension cab output on your combo amp.

If you aren't talking about building it to use with your combo amp, then ignore above comments and proceed forthwith!!

The amp is capable of handling a total load of 4 ohms and has two options for using external speakers. The first is to use the internal driver with a external 8-ohm cabinet, which was my original idea. The second is to disable the internal driver and use either one 4-ohm cabinet or two 8-ohm cabinets. Since there's no way to tap into the connection between the amp and the internal speaker like Gauss did, I guess the only option would be to build a crossover into the sub and run it along with the internal speaker. Doing so, however, doesn't sound very efficient to me.

With the Jacks as your main rig, you just bring fewer of them to a smaller gig and have no reason to haul around a combo amp. Plus, adding a sub to your combo will just make your combo amp bigger and more difficult to haul than your main rig. If you want to build a sub to add to your main rig, go for it.

I would add a +1 to the selling the combo amp idea. Or just keep it as a practice amp so you don't have to haul your main rig to practices. I keep an old combo amp that I loan out if I have a friend who have a kid who is thinking of playing bass on the agreement that they give it back if the kid doesn't stick with it or sticks with it and upgrades to bigger amp. I've done that a couple times now. It's an old Traynor amp where I won't care that much if it doesn't come back.

My combo has two output jacks, one of which has the internal speaker plugged in. I run the amp to the sub (which takes the lows and puts the highs into an output jack), from the sub's output jack, I plug in the combo's internal speaker. In your case, if the speaker is plugged right into the head, you might have to do some surgery to make the cord into a jack-side and plug-side. I bought speaker wire and stereo jacks (along with a male-to-male stereo connector) to make it happen.


You should know that bi-amping doesn't add the impedances of the two speakers. The ohm-rating is an average of how the speaker responds throughout the full-range, and actually fluctuates from it's named rating at many points. When you bi-amp, the ratings are separate for the highs and lows, giving the lower impedance speaker more juice. For example, if your combo has a 4 ohm speaker, and the sub has 10 ohms (8 ohm speaker with the additional resistance of moving air through a horn), the highs will be louder.