Mg30cfx

Discussion in 'Marshall Amps' started by Criminal6969, Jun 14, 2019 at 8:39 PM.

  1. Criminal6969

    Criminal6969 New Member

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    I can connect a cab to this amp?
     
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  2. JeffMcLeod

    JeffMcLeod Well-Known Member

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    I just looked at the amp on the Marshall site. It doesn't look like there'a any speaker output jacks on the back.

    I guess you could open it up and do your own wiring if you feel ambitious.
     
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  3. scozz

    scozz Well-Known Member

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    Yup, I agree with @JeffMcLeod,...I don’t see any input jack for a speaker cab.

    Why would you want one?
     
  4. Solid State

    Solid State Well-Known Member

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    You can't but you can get a 10" greenback to put in it and it'll be a whole new world.
     
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  5. SkyMonkey

    SkyMonkey Well-Known Member

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    Those are closed back amps aren't they.

    I run a speaker cable wired in to the back of a Valvestate VS65R (solid state hybrid) which is open backed, and then into a 112 cab.
    I took the connector off one end of a speaker cable (not an instrument cable!), bared the wires and installed spade connectors. The spade connectors are then 'jumped' off the second tabs on the internal speaker connectors. If you then run the cable into a cab, the second speaker would wired be in parallel with the internal speaker.

    Now here are the tricky bits.
    I bet there is no minimum Ohm load instructions with the combo because the MG30CFX was never intended to run with a cab.
    To be safe you shouldn't run the amp with a load less than the Ohm rating of the factory installed speaker.
    Solid state amps can run with loads higher than their minimum load, but not below.
    But if you match the internal speaker rating with a cab speaker in this way (parallel) the load will halve.
    To set my amp up in this way I first changed the original internal speaker from an 8 Ohm to a 16 Ohm speaker and then put another 16 Ohm speaker in the 112 cab, making 8 Ohms again. In parallel, 16 and 16 = 8 Ohms.

    Without changing the internal speaker to a higher Ohm rating you can only drop the load to below the safe load (8 and 8 in parallel = 4 Ohms).

    So if you are determined to run a cab with this amp safely you will have to:

    1. Change the internal speaker to a higher rating (maybe a 10" Greenback as Solid State suggested).
    2. Get a cab.
    3. Make sure the cab speaker matches the load of the new speaker (another Greenback?).
    4. Buy a speaker cable and install it through a hole drilled in the back of the combo.

    I had all this stuff hanging around already, so no extra expense, just a new project to have fun with.
    But if you don't, I would stop at step 1 and enjoy the speaker upgrade until you are sure you want the extra hassle. The total expense could be the same as selling the MG and getting a better amp secondhand.

    Have fun. (I did).
     
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  6. ampmadscientist

    ampmadscientist Well-Known Member

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    Well
    these type amps use a car stereo amp chip for the power amplifier.
    These chips run OK at 8-16 ohms. But not at 4 ohms because it gets too hot.
    So I would recommend you run 16 ohms, just to make the amp last longer and keep it cooler. Never run it at 4 ohms.
     
  7. scozz

    scozz Well-Known Member

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    I’m with @Solid State, replace the stock 10” speaker with a Greenback 10 or a Creamback 10.

    On second thought,....you could put that money aside,....toward buying a better amp.

    The new, better speaker will cost about half of what an Mg30 amp costs new!
     
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  8. pedecamp

    pedecamp Well-Known Member

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    Nope I'm in disagreement with everybody else, the speaker that sounds best with these amps is the speaker it came with and was designed for. Plug and play that amp and move on. You want an amp with a cab buy something else.
     
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  9. Mitchell Pearrow

    Mitchell Pearrow Well-Known Member

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    The way I did it was by soldering a jack into the back panel then we used it to plug a mg 412 in and it has never had an issue doing this.
    Cheers Mitch
    Plus Welcome to the forum.
    Have fun with your amp.
    They are a great stepping stone in the path of a truly fantastic Marshall amp quest, we purchased 2 of them!
    Enjoy your stay
     
    Last edited: Jun 15, 2019 at 1:44 PM
  10. SkyMonkey

    SkyMonkey Well-Known Member

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    I think that is what I was saying.
    With no clear instruction on the amp, use the Ohm rating of the installed speaker as the minimum load guide.
    Without doing this, a parallel load can only drop below the load of the original speaker.
    I was using my own experience as an example, where 8 Ohms is the Min load. Without changing the internal speaker the load would have dropped below 8 Ohms, to a possible 4 Ohms with a second 8 Ohm cab speaker.

    I do not know the rating of the OPs internal speaker (or amp).
    After a short search I could only find a forum thread where the speaker had been changed, and the photo of the old speaker showed a stamp with MG-15CD on the back, no rating in the thread.
    The OP needs to chime in and take a multimeter to the speaker terminals.
     
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  11. SkyMonkey

    SkyMonkey Well-Known Member

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    OK. Turns out that was a Marshall MG15CD amp after all. But the thread starter just put a Celestion Eight 15 in (no Ohms declared) and was super happy.

    There is an interesting sideline in installing a switching jack in the chassis that diverts the signal to a speaker cable (and then a cab) to avoid parallel load issues.

    https://www.seymourduncan.com/forum/showthread.php?293073-Marshall-MG-Upgrade
     

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