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Gallien-Krueger MB 150III with an ext. cab.

Discussion in 'Amps and Cabs [BG]' started by Chris Grzesik, Oct 21, 2009.

  1. Chris Grzesik

    Chris Grzesik

    May 19, 2009
    Storrs CT
    I have a Gallien-Krueger MB 150III, which is a great combo amp for rehearsals and small acoustic gigs. I use a 1970 Fender Jazz bass with a custom make ebony fretless finger board and it sounds great with the MB150. I bought this combo with the idea that I could add my old gray Ampeg B-15 bottom with a JBL K-140 speaker in it and would have a great, larger sounding rig for bigger gigs, punch form the 12" GK and the old, fat sound of the B-15. Well, it sounds awful with both cabinets. If I turn off the GK internal speaker the B-15 sounds good. With both on, it sounds like the cabinets are cancelling out each other. Has anyone had this problem and have any of you used an extension cabinet with the MB150 with success? I asked GK and of course they said that I should buy their MB 150 extension cabinet, which is made precisely for this. But, I don't feel like laying out $300 just to "try" something. Any thoughts? Thanks in advance.
  2. It sounds like that is a phase cancellation issue. Your extension cab is probably wired the 'opposite' of your internal GK speaker. You would either need to switch the '+/-' wires on your Ampeg speaker, or get a speaker cable that has one end wired the 'opposite' of the other end.

    With each cabinet used alone, you would never notice it. However, with both together, what happens is that one woofer is moving out while the other is moving in, resulting in the low end virtually disappearing.

    Easy fix, assuming this is the issue (which I'm almost sure it is).

    Edit: That combination should sound great when you get it working right.
  3. Chris Grzesik

    Chris Grzesik

    May 19, 2009
    Storrs CT
    Wow! That would be an easy fix if it is the problem. I will try it as it is a simple matter of opening up the B-15 top and switching the connections at the speaker.

    I asked a boutique amp maker that I jam with occasionally about this issue and he thought that the cabinets weren't "tuned" together. I like your solution better, obviously as it would save me a lot of money. When I bought the GK I envisioned a very tight upper end with that great B-15 bottom that I loved.

    I started to use the B-15 again with an old Furman pre-amp/parametric EQ and a power amp. Sounds great but I would really like to have the option of using the GK with the B-15 bottom to get the multiple speaker sound.

    Thanks. I will let you know how it turns out.

  4. 62bass


    Apr 3, 2005
    I've used mine with various extension cabs with no problems. I suspect being out of phase is the issue. Easy to check anyhow.
  5. Please do post up. I'm pretty sure you will dig the results.

    'Not tuned together'.... god help us;)
  6. Chris Grzesik

    Chris Grzesik

    May 19, 2009
    Storrs CT
    Success!!! I reversed the speaker wires in the B-15 cabinet and it worked. I now have the GK 1 -12" punch with the B-15 bottom warmth. I now need to work on the tone settings. What else is new.

    Thanks for your help on this. I would have never thought this to be the case.

    I am a little confused though. Isn't it normal for the + to go to + and the - to go to -. Does this mean that the GK was wired backwards? If I go to another head, should I reverse the wires on the B-15 again? This goes against all that I have done in the past. If I didn't have access to the inside of the B-15 cabinet, I would have had to make a speaker wire with reversed ends just to find out if this was the issue.
  7. Excellent! One of your cabs was wired backwards, and it might have been the GK.

    If you are only using one cab, it literally doesn't matter, as far as I know. However, if the Ampeg was wired the 'correct' ++ -- way, then you would probably need to switch it back if you were going to use it with a different cab (which would probably be wired the 'right' way). Using the Ampeg with the wires switched with another head won't be a problem or do any harm.

    It won't hurt anything either way, and you will know immediately if there is a phase problem with different second cab and another head.

    Enjoy that rig. I bet it sounds good. I had a GK full size 112 running below an MB150 combo back in the day, and it sounded great.
  8. Chris Grzesik

    Chris Grzesik

    May 19, 2009
    Storrs CT
    Thanks again.

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