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Plug 2 heads into 1 cab?

Discussion in 'Amps and Cabs [BG]' started by StuartV, Oct 22, 2010.

  1. StuartV

    StuartV Finally figuring out what I really like Supporting Member

    Jul 27, 2006
    Manassas, VA
    Question: Is there any risk of damaging anything if I plug 2 bass amps into 1 cabinet?

    The reason I'm asking is, tonight is my first gig on bass. I am a suspenders and belt kinda guy. I'm told that the place we're playing has questionable wiring. Also, I bought my SVT2 used and I do not know if it has the bypass mod to keep it from shutting down if it gets low voltage.

    I'm going to take a circuit tester and do my best to make sure I'm plugged into good power, but if the wiring is questionable, then I reckon even a plug that tests good could still end up exhibiting problems at some point. And the circuit tester won't alert me to low voltage anyway, I don't think.

    My main amp is an SVT2Pro. My backup is a Promethean. Ideally, I would like to have both amps on and plugged into my cabinet so that, if the SVT shuts down, I can just pull my instrument cable out, plug it into the Promethean and resume playing.

    It seems like it should be okay to have 2 amps both running one cab, as long as the combined input did not exceed the cabinet's power rating. But, it occurs to me that if both amps are plugged into the same cabinet, the juice from the SVT will be going into the cab and then also out the other speaker cable to the Promethean. So, maybe it would damage it?

    Does it matter if the Promethean is on or off in this scenario?

    Thanks for any help!
  2. i believe you will damage both heads if plugged into one cabinet
  3. PaulNYC


    Apr 2, 2009
    New York, NY
    i got paid for a gig once.
    don't do it. just unplug from the SVT and switch to the promethean.
  4. randall p

    randall p

    Aug 14, 2009
    norwich, ct
    Ted Weber makes a box that allows you to switch between 2 heads and one cab. Although, I just checked their site, and it doesn't handle high power.
  5. StuartV

    StuartV Finally figuring out what I really like Supporting Member

    Jul 27, 2006
    Manassas, VA
    Sounds like I need to set it up so that I move my instrument cable over from one amp to the other AND I move the speaker cables over from one amp to the other, too.

    Oh, well. A few extra seconds (most likely mid-song, if it happens) to change over. I don't mind bringing the Promethean head as a backup. It's small and light. But, I'm not bringing a whole extra cab, just so I can have 2 complete rigs there, with one just being a backup. If the SVT takes a crap, I'll just deal with it.

    Thanks for the responses.
  6. JulienJeff


    Mar 1, 2009
    I'd say leave the SVT at home and take your backup for that gig. Seems safer to me
  7. JTE

    JTE Supporting Member

    Mar 12, 2008
    Central Illinois, USA
    You WILL damage both heads running the way you described. The power amp outs will be connected to each other and that'll cause failure. Might just trip the amps' fuses/breakers, but Moe likely to release the magic smoke that smells of money flowing outta your pocket.

  8. StuartV

    StuartV Finally figuring out what I really like Supporting Member

    Jul 27, 2006
    Manassas, VA
    I hear ya'. But, a friend of mine played the same place last week and used his SVT CL and it was fine. I'm pretty sure I'll be fine. Like I said, I'm just a suspenders and belt kind of guy and want to be prepared for the worst, even though I don't think it's very likely at all to come to it.
  9. StuartV

    StuartV Finally figuring out what I really like Supporting Member

    Jul 27, 2006
    Manassas, VA
    ps. and not only is it my first gig, but we've sold out the venue (oversold, actually). And the singer's former record producer from when she was active in Nashville is coming down to hear us. I really want to use my best stuff (no solid state backup amp, if I can help it)!
  10. AwkwardLoudness


    Jan 11, 2010
    MI USA
    The likely impedance mis-match would be more of a concern than the maximum power handling being exceeded. Just bring the Promethean as a backup. Connect it to the cab only after the SVT2 is disconnected.

    I sometimes play two amps through an 8x10 cab. One amp drives the top 4 speakers, and the other amp drives the bottom four speakers. I don't know what cab you have, but if it's not specifically equipped to handle two amps through separate speakers/jacks then definitely don't do it. The possible result would be two fried amps.

    Have fun with your first bass gig.
  11. StuartV

    StuartV Finally figuring out what I really like Supporting Member

    Jul 27, 2006
    Manassas, VA
    Thanks, AL. My cabs are Aguilar - a GS210 and a GS115. No, they're not setup for bi-amping, and I never had any thought of running both amps at the same time.

    But, yes, with all this input from everybody, I will make sure that I only have 1 amp hooked to the cabs at a time.

    Thanks again, everybody! T - 10 hours and counting. Woohooo!!!
  12. herndonbassist

    herndonbassist Low Down Thumper Supporting Member

    Apr 7, 2005
    Herndon, VA - NoVa
    The advice that you've gotten is correct. Connecting both will definitely release the magic smoke.

    Good luck on the gig this evening!
  13. Bassamatic

    Bassamatic keepin' the beat since the 60's Supporting Member

    Yes - you will absolutely smoke one or both of the amps.

    The reason is that the output impedance of an amp is just a fraction of an Ohm. Each amp will be driving each other (essentially a short circuit) as well as the speakers. Poof!
  14. eyeballkid

    eyeballkid Supporting Member

    Jul 19, 2009
    the amp that is NOT plugged into a cabinet should be left OFF until it is connected to a cab.
  15. If presumed problem happens, then simply do a quick switchover (have it ready), should take about 10 seconds. Do not plug both into cab at once!
  16. jgroh


    Sep 14, 2007
    Ok, heres a thought (that may not work but just thinking out loud...) What if he ran one line out of both amps and into a Boss LS-2? Then he could switch one off if it goes bad and use the other? Again, I have no idea if that would work so dont take my word for it!
  17. Fuzzbass

    Fuzzbass P5 with overdrive Gold Supporting Member

    This cabinet is an exception to the rule. Most speaker cabinets have two or more speaker jacks, but for the vast majority of cabs, only one jack should be used as input (i.e. connected to the amp). Multiple jacks are there for flexibility: to (A) allow the choice of Speakon or 1/4", or (B) the ability to "daisy chain": power two or more speaker cabinets from a single amp. Running two amps into one cab is almost always a bad idea (as already stated).
  18. StuartV

    StuartV Finally figuring out what I really like Supporting Member

    Jul 27, 2006
    Manassas, VA
    Well, it turns out it was a good thing I took my suspenders and belt.

    The power at the venue was so crappy their own PA amp (yes, I think they only had 1) was cutting out. Initially, I had my SVT plugged into the same circuit with their PA amp. Since they only had 1, I thought that would be safe. I let the opening act use my rig and it cut out and came back twice during their first song or two. So then we moved my power plug to an extension cord that ran to a different circuit. Also, I did use a circuit tester ahead of time and I at least verified that the wiring was correct before I plugged anything in.

    It was okay after that until about half way through their set and then my SVT cut out and they couldn't get it going again. I had stepped outside and somebody came and got me. I didn't mess around with trying to get it going again. I immmediately just pulled his instrument cable out and plugged it into my Sansamp BDDI which was already hooked up to my Promethean, pulled the speaker cable out of the back and plugged it into the Promethean and the show went on.

    I decided not to risk my SVT cutting out again, so I played my whole show using the Promethean. It still sounded awesome. When I was listening to the first band, I couldn't really tell any difference between when he was using the SVT and when he was using the SABDDI/Promethean.

    And the Promethean into the Aggie 210 + 115 cabs was PLENTY loud. I had the input gain set to somewhere around 10 or 11 o'clock and if I turned the Master up to 12:00 it was TOO loud (for the venue - capacity around 100 and pretty full).

    Thanks again for the help. It would have really sucked to smoke both amps.

    Oh, and I have re-setup my rig at home and the SVT is working just fine. I'm glad it wasn't damaged by their fudged up electrical.
  19. knigel


    Apr 20, 2009
    Hopedale, MA
    I'm the Squier of Fenderbirds
    "How can you trust a man who wears both a belt and suspenders? The man can't even trust his own pants. " --Henry Fonda, Once Upon a Time in the West, 1968

    Of course, it's a good thing you are....
  20. StuartV

    StuartV Finally figuring out what I really like Supporting Member

    Jul 27, 2006
    Manassas, VA
    Henry Fonda obviously didn't have to deal with people who would pants you.


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