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HELP! I seem to have no idea how amps work....

Discussion in 'Amps and Cabs [BG]' started by JoZac21, Nov 3, 2012.

  1. JoZac21


    Nov 30, 2009
    Brooklyn, NY
    I currently have a GK 400RB-III combo (15") that I acquired from a friend through a trade about a week ago because I needed an amp for a gig, quick.

    I am currently in a band that is looking to go on tour very soon, and I need a touring rig...

    Today, completely out of the blue, I remembered that my brother had left some sort of speaker cabinet here at my house, not knowing what it was... I took a look, and lo-and-behold, it was a Kustom cab... 12" w. tweeters!!

    Here's my problem/question:

    If I can hook up this cab w. my GK combo, I know I will have a workable touring rig... I just have no knowledge of impedance and the like...

    Here's the info I have:

    The amp head runs at 240W: 150W @ 8 Ohms, 240W @ 4 Ohms... that's what the owners manual says.

    The extension cab says this on the side:
    8 Ohms
    100 Watts RPM
    200 Watts Peak

    As I didn't study at MIT, I have no idea what this all means...

    For gits and shiggles, I hooked them up w. an instrument cable, and it worked at a low volume just fine...

    My Question:


    Thanks so much!! I've never had to deal w. this kind of thing before.

    If you need more information, I'd be happy to give it, assuming I can figure it out... Thanks!
  2. That cab is very likely a guitar cab, or maybea PA cab, you risk damaging the speaker in it if you try to crank the bass up loud.

    If your combo has an internal 8 ohm speaker built-in (which I suspect it it) and you plug an additional 8 ohm cab into it, the resulting load on your amp will be 4 ohms. Which your amp will handle no problem.

    However, as I started out with, that cab you have is not great for bass. Get another 1x15 cab (a REAL bass cab) and then you will be ready to tour!!
  3. 5port


    Oct 14, 2009
    LI,new yawk
    From what your telling me the internal speaker in the combo will be getting 125W and the extension speaker will be getting 125W (close enough). It should work. Give it a try. You may have to turn-off the tweeter in the extension.
  4. Growly Lytes

    Growly Lytes

    Dec 4, 2009
    Downunder Oz
    Bass player
    The combo speaker what impendence is it ?
    If its 8ohm well you should be able to hook them up together two 8ohm cabs makes 4ohm load so the amp will put out 240w splitting the watts to each cab.
    If one of the speakers is 4 ohm & the other 8ohm you cant do.
  5. JoZac21


    Nov 30, 2009
    Brooklyn, NY
    It may very well be a PA cab, but does that really matter? Seriously, I have no idea...

    Can do if needed.

    According to the Owners manual, the 15" in the combo is at 8 ohms, and the cab I found is 8 ohms as well...

    It seems from what everyone is saying, the worst out come is that I damage the speaker I didn't even know i had, and got for free, may get damaged... this sounds pretty okay w. me...;)
  6. Sid Fang

    Sid Fang Reformed Fusion Player Supporting Member

    Jun 12, 2008
    The GK manual says:

    "Maximum power is achieved by connecting one 8 Ohm extension cabinet (such as the 210RBS, 410RBS, or 115RBS) in addition to the combo’s internal 8 Ohm speaker load."

    Because when the amp is seeing both the extension cab and the internal speaker, the combined load is 4 Ohms, so you nominally get 240W instead of 150. Those 240W should be distributed approximately equally across the two speakers, so the outboard cab will get a maximum of about 120W. The speaker is rated to handle peaks higher than that, but only 100W sustained input. What that means in practical terms is that you should be fine using the rig you describe, so long as you don't turn the amp all the way up and crank your bass. I'd use such a rig without hesitation for an informal jam or a one-off gig where I needed more sound, but it's a little risky to go on tour if you're depending on it. You should first of all plug it in (it certainly won't explode on contact) and see what it sounds like at moderate volume levels. If it sounds like crap, the power handling is moot.
  7. JoZac21


    Nov 30, 2009
    Brooklyn, NY
    I just don't want to blow anything up...

    If this can actually work, and everything is all kosher, my best friend is a carpenter, as well as a bass player, and I will employ his talents to build me a custom cab for the speakers...
  8. JoZac21


    Nov 30, 2009
    Brooklyn, NY
    I apologize, but I'm a little confused... if it says that using two 8ohm speakers is ideal, and I'm adding a second 8ohm speaker... wouldn't that make it "ideal"...

    I will certainly take your advice as given and give it a shot...
  9. JTE


    Mar 12, 2008
    Central Illinois, USA
    The 8Ω cabinet is the right impedance to match with your combo. The only issue is what the extension cabinet actually is. Find a model number on it somewhere to determine if it's a bass cabinet or a guitar cabinet. If it's a guitar cabinet the actual speaker MIGHT or may not be useful if moved to another cabinet. But just making a wooden box and stuffing a speaker in it isn't such a good idea either. The wrong box dimensions and porting could cause the speaker to die in the new cabinet too.

  10. JoZac21


    Nov 30, 2009
    Brooklyn, NY

    Good thinking... i found the model #... this is what I have:


    It does seem to be a Monitor... does that matter?

    What if I get rid of the horn... or the tweeters as well... and just use the 12" speaker?
  11. Marginal Tom

    Marginal Tom

    Apr 28, 2010
    O'Fallon, IL
    The Marshall cab probably isn't a bass cab. If you push it hard or pump a lot of lows through it, it will probably die on you in the middle of your first set on tour. You'd be much better of with a 15RBS extension cab. A 15" and 12" cab receiving equal wattage wouldn't sound as good, even if the 12" was a bass cab.
  12. tbirdsp

    tbirdsp Supporting Member

    Sep 18, 2012
    Omaha, NE
    In my opinion - that cheap monitor cab will be next to useless for bass guitar. Why are you so certain that the combo won't be fine as is?
  13. JoZac21


    Nov 30, 2009
    Brooklyn, NY
    What Marshall cab?

    The combo's awesome... but needs a bit more power for the kind of gig's I'll be playing... not much, but a little.
  14. tbirdsp

    tbirdsp Supporting Member

    Sep 18, 2012
    Omaha, NE
    Not to belabor the point - but how do you know? It wasn't enough for your last gig? What size venues will you be playing? Will there be PA support?
  15. JoZac21


    Nov 30, 2009
    Brooklyn, NY
    It was enough for my last gig, although that was a smaller venue... PA support is always noted, it's the reason I'm not running to go buy a 410 at the moment...

    As far as venue sizes... it's going to vary... which is the point... I'd like something for small, medium and large gigs... I know that, especially w. large venues, bass amps are really just stage monitors... so I take that into consideration...

    All I'm asking is, for the time being, until I can get some thing better (I am BROKE, with a capital BROKE)... will this WORK?

    I don't mean to sound snippy... I appreciate all the help!
  16. tbirdsp

    tbirdsp Supporting Member

    Sep 18, 2012
    Omaha, NE
    Well - you can try it (but use a speaker cable) and I think the worst you'll do is blow the speaker in the Kustom monitor cab.
  17. Pilgrim

    Pilgrim Supporting Member

    The cab you linked is rated 100 Watts RMS, which basically means that it will handle 100 watts at a consistent playing level. If you push the cab by playing with great impact or really push notes, that causes peak wattage, which is higher. The cab doesn't list a peak wattage rating. The combo shows 180W output @ 8 ohms (280W @ 4 ohms w/ext. cab), so you can easily put more power into that Kustom speaker than it's designed for.

    When you add an extension speaker, you lower the resistance to the amp, which allows it to produce more power. So if you connect the extension, your amp can put out up to 280 watts, which is a lot more than that Kustom was designed for. Incidentally, 280 watts can get really loud with good speakers.

    If you can play your combo with this as an additional cab, and you are NOT pushing the combo to max volume (let's say you're playing with the volume knob at 50%), you should be OK. That Kustom cab may or may not sound very good, but adding the additional speaker area should result in higher perceived volume for the audience. If you push the volume on the combo high, you will almost certainly damage the speaker in the Kustom cab before long.

    BTW - you NEED to learn this stuff about watts, ohms and speakers. You cannot be a competent electric bass player who can make decisions about equipment until you understand it.
  18. DethByDoom


    Sep 23, 2012
    Well said. Man… if you're going on tour and think you need more speaker… just go buy a used fridge. They're all over Craigslist.
  19. JoZac21


    Nov 30, 2009
    Brooklyn, NY
    Thanks so much... from what I heard testing it out at a low volume, the extention cab sounds excellent... but I'll play around w. it...

    I agree that I need to learn about watts, ohms, etc. Do you know any good articles/sources on that?

  20. two fingers

    two fingers Opinionated blowhard. But not mad about it. Gold Supporting Member

    Feb 7, 2005
    Eastern NC USA
    A $129.00 monitor will be virtually useless as a bass extension cab. And the speaker from that monitor will be virtually useless in another cab all stuck together with your rig. I have done gigs with mid level JBL stuff and was able to throw barely enough bass in it to bother with. Before going on tour, search for a used 8 Ohm 15" BASS cab. It will sound (at least) 10 times better and won't be useless as far as volume goes. I understand that the idea of a "free" speaker cab is exciting, but it isn't worth taking with you. All you will do is blow the speaker the first time you crank it up. Another thing you need to learn about speakers is that just because it says 100W on the back, that does NOT mean you can crank your amp all the way up and it will take the abuse. In fact, watt ratings on speaker (in my view) are almost useless. You can out a 100W speaker in a 20W guitar amp and blow the crap out of it.

    To the 2nd point. A "master carpenter" might be able to build a pretty box. But there's a lot more to building a speaker cab than nice joints. There is a lot of physics involved. Speaker cabs (good ones anyway) are tuned to a particular speaker and frequency range. If the cab is designed wrong, it will make your crappy speaker sound even worse or perform even less efficiently than it already does.

    I wish you all the best on your tour. But if you are going to tour, you need BASS gear. Not a thrown together "wishful thinking" rig.