How do I run an extension cab with my Combo amp?

Discussion in 'Amps and Cabs' started by trumpus, Mar 12, 2008.

  1. trumpus

    trumpus Supporting Member

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    Hey all,

    I was wondering how exactly one would run an extension cab with a combo amp?

    I've got both a Mesa DC-3 and a SF Vibrolux Reverb - both with additional ext. out jacks.

    I am most interested in running a 2X12 or 2X10 cab with the Vibrolux, which has a 4 ohm speaker out and an additional 4 ohm jack. How exactly do I need to connect this to a cab? What should the ohm of the cab be (ie, can I run a 4 ohm cab at the same time as the 4 ohm speakers are pluged in through the combo)?

    Does this make sense?

    Brian
     
  2. Bikedude

    Bikedude Gold Supporting Member

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    I believe YES is the answer your looking for.
     
  3. trumpus

    trumpus Supporting Member

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    ...kind of, but actually, there were a few more questions in there...

    1) What ohm cabinet should I run with my '78 VR? (it's got a 4 ohm jack for the speakers in the combo, and an additional 4 ohm jack that is labeled EXT)?

    2) Can I run this with the speakers int he combo (for a total of 4 10" speakers, or 2 10" and 2 12" speakers).

    Brian
     
  4. big brekfest

    big brekfest Guest

    the ext jack is 4 ohms, so the extension cab should be 4 ohms.

    I don't know about that amp in particular, but a lot of amps i've played mute the internal speakers when an external cab is connected. YMMV. I'm sure if it does, though, there's a mod to change that.
     
  5. meburdick

    meburdick Member

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    I respectfully disagree.

    Your MESA DC-3 has three speaker jacks. One is labeled 8 ohm while the other two are labeled 4 ohm. What you connect and where is *somewhat* up to you.

    You can run an 8 ohm speaker on a 4 ohm jack. But, don't EVER run a 4 ohm speaker on an 8 ohm jack. If you want to connect TWO 8 ohm speakers, they should each be connected to a 4 ohm jack.

    Here are some specific examples (using the CD-3, assuming your DC-3 is a combo, and you external cabinet is rated at 8 ohm):

    Internal speaker only: connect to any jack
    External cabinet only: connect to any jack
    Internal speaker and external cabinet together: connect each to a 4 ohm jack.

    When you connect two 8 ohm loads (the speakers) in parallel (at the same time), the effective resistance of each drops to 4 ohms - that's why you put them in the 4 ohm jacks (remember the part where I said to NEVER put a 4 ohm speaker in an 8 ohm jack? that's why you don't connect anything there).

    Don't connect and disconnect speaker loads while there is voltage applied to the tube plates (when the amp is "on" and not in standby or off).


    DISCLAIMER: My own understanding of electronics allows me to make these statements. However, the comments about running an 8 ohm speaker in the 4 ohm jack also came directly from MESA. Interestingly, they also said that it was actually "good" for the power tubes as it allowed them to run a little cooler (which affects the tone, and is why some people change the jack of the speaker that way).

    I can't give direct comment for the other amp as I'm not familiar.
     
  6. ChrisdFlip

    ChrisdFlip Member

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  7. meburdick

    meburdick Member

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  8. NyteOwl

    NyteOwl Member

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    If there are external speaker jacks available, no value at all, but if there is only one speaker jack it gives one the option of running two cabs. Just remember to keep to the formula: a single 4-ohm out gets two 8-ohm cabs, a single 8-ohm out gets two 16-ohm cabs, and so one...
     
  9. meburdick

    meburdick Member

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    The actual formula for running speakers in parallel is:

    A*B / A+B

    Multiply the two impedances together and divide by their sum.

    8*8 / 8+8 = 64 / 16 = 4
    16*16 / 16+16 = 256 / 32 = 8

    Connect a "calculated" speaker load to a jack with the same impedance rating or LOWER.
     
  10. JimH

    JimH Member

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    The way I learnt at school (which allows for mixed impedances (cabs)):

    1/total = 1/a + 1/b

    so a = 8, b = 16

    1/total = 1/16 + 1/8

    1/total = 3/16

    total = 16/3 = 5.3333
     
  11. meburdick

    meburdick Member

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    Yup. Different formula, same results...

    8*16 / 8+16 = 128/24 = 5.33333333
     
  12. JimH

    JimH Member

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    Until you've got three cabs.....
     
  13. meburdick

    meburdick Member

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    True. Your formula is the "correct" one that is taught in circuits courses. Mine is a sort of "shortcut" one (that is exactly the equivalent of yours when only two values are being used) for use with only two values (which is a very common need).

    I would suppose that solving for two values is done more than half of the time in "the world" - true that people working with circuits may use more values more often... But, I believe that applications for the formula are pretty common in every day life (like this situation, for example), and that pushes the usefulness of the simplified formula I gave up a bit.
     
  14. JimH

    JimH Member

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    Yeah certainly, in this forum for instance! - if people don't know what's what and you tell them that formula which solves their problems and easier to remember then great.
     
  15. drbob1

    drbob1 Silver Supporting Member

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    This question comes up every few weeks and a search on "impedance" will result in infinite threads and much knowledge. However, to summarize:
    1. Any amp has the highest clean overhead at the intended impedance (4 ohms for the VR, 8 or 4 ohms for the Mesa, depending what jack you plug into).
    2. Any Fender and most other amps will tolerate a mismatch of 2:1. So it's safe to plug two 4 ohm cabinets into the Vibrolux (for a total of 2 ohms), or a 4 and an 8 (total of 2.6 ohms, with the 8 ohm speaker cab getting 1/3 of the power-this can work if the 8 ohm cab is MORE efficient than the internal speakers). For the DC30, running an 8 ohm external cab just plug both speakers (internal and external) into the 4 ohm jacks which are parallel, gives you 4 ohms total. You could use the 4 ohm jacks and a 4 ohm external cab, it'd give you 2.6 ohms again, this time with 2/3 of the power going to the external cab. Should still be safe.
    3. If you go beyond a 2:1 mismatch, the result is different depending on which way you go. Too low impedance (less than 2 ohms on either amp) will be the same as biasing the tubes too hot-they wear out quickly and may fail catastrophically. Too high an impedance (more than 8 for the VR or 16 for the Mesa) can result in "flyback voltage" that can cause arcing and destroy your OT (although paradoxically the tubes run cooler). So NEVER go beyond a 2:1 mismatch higher than your amp is rated for. This is why it's much more dangerous to play and amp with no speaker than with a short directly to ground.
     

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