One head, two cabs, confusion

Discussion in 'Amps and Cabs [BG]' started by Buzzo, Apr 6, 2014.

  1. Buzzo


    Nov 15, 2009
    Not sure where this belongs but I'll ask it here;

    So I've got a Sunn model T head with a switch for 2, 4, 8, and 16 ohms on the back, and two speaker outputs. (see pic)
    The first cabinet I've got is a Marshall jcm900 and it has two back inputs, and a switch (see pic).
    The other cabinet is just some 8ohm marshall with one input jack, no switches or anything.

    How do I set it up so it's safe to run these two cabinets with my one head?


    Attached Files:

  2. How powerful is the head and what is the 15 cab?

    Your guitar 4x12 isn't up to much bass. Old bandmate had one which we gingerly tried out with bass, pftt, not a happening thing. I'm thinking it may be safe enough to switch it to 16ohm and run parallel with the 15 if it's a bass cab. Amp on 8 ohm pushing 5.something ohms is less than ideal but rumoured better than overloading a 4ohm tap.
  3. walterw

    walterw Supportive Fender Gold Supporting Member Commercial User

    Feb 20, 2009
    what is the 8Ω cab? i didn't see where it was a 15.

    anyway, you've got two cabs of different impedance and a tube head; that means no, it ain't gonna happen.

    you end up with a total impedance that's not 4, 8 or 16, which means you're stressing the amp by mis-matching it. some amps can take it and others can't, but no way would i ever do that to my tube amp!
  4. The problems are the 1960A is either 4Ω or 16Ω and the other cabinet is 8Ω. This means they will not get the same power but, depending on what the speakers are, that could actually be a good thing. In either setting, one cabinet will get one third of the power and the other will get two thirds. The other problem is the resulting impedance will not match the options on the amp's switch - it will be either 2.7Ω or 5.3Ω. However, I have read several times at several places that most tube amps can handle running at a lower impedance than the switch is set for. Running at a higher impedance than the switch setting can be hazardous to the amp so do NOT do that.

    What this all means is use either the 4 or 8Ω setting on the amp, depending on which setting you use on the 1960A, 4 if the 1960A is set for 4Ω and 8 if it is set for 16Ω. First you have to decide which setting to use on the 1960A. The lower impedance will mean it gets more power than the other cabinet and the higher one will mean it gets less power. You will have to decide which works best with your cabinets.
  5. Yeah, the crucial thing is the amp may or may not operate ok with less cab impedance than the selected tap. The guys that know all about that are around more in the early mornings.

    The 4x12 will have a big sensitivity advantage over any 15, but can't handle much power. Putting it on 16ohm could be advantageous, knocking it's power from the lions share to the hyenas.
  6. el murdoque

    el murdoque

    Mar 10, 2013
    there is one safe way.
    Use only half of the 4x12, by setting it to stereo and use either left or right. Connect both cabs to the head and run on 4 ohms. it's not ideal, but on the other hand it's not ideal to use a guitar cab on bass anyway.
  7. That might also have a major pitfall. Somehow I doubt the 12 pairs have discrete sealed chambers. If it's the one box, the unused pair turn into 180° misphased passive radiators.
  8. gumtown


    May 7, 2007
    New Zealand
    Yep, 1960a has no partition chamber, it' all one big box of 4 x 75 watt celestions.

    The Sunn T are typically only 100watts output.
    You might get away with using the 1960a at 4 ohm (mono), plus the 8 ohm extension cab with the amp set to 4 ohm.
  9. Mike in Chicago

    Mike in Chicago Supporting Member

    Apr 3, 2011
    Or, you can get some real bass cabinets. Never understood the Marshalls for bass thing. Unless of course it's a "looks" thing.

    And before I get the "Lemmy", "tell that to Lemmy" Lemmy this Lemmy that guano...Lemmys a rhythm guitar player.
  10. You need get the attention of some Sunn guys. Thread title is a "should have read the stickies" yawn for them.

  11. Webtroll

    Webtroll Rolling for initiative

    Apr 23, 2006
    Austin, TX
    I'm thinking the Marshall cab set to full @ 16 ohms, the GK cab is only 8 ohms, and run the head at 4 ohms. Not ideal, but within tolerance I believe. I've run a rig like that before (4 ohm head, 8 and 16 ohm cabs in parallel) with solid state and tube heads, never had a problem. I'm no expert and it's possible I was just lucky...
  12. gumtown


    May 7, 2007
    New Zealand
    No mention of it being a GK cabinet.
    I could assume it is a model 1982 4x12 or something like that?

    Either way you don't have an ideal situation, if the 8 ohm cab is a 2x12, you could then run the 4x12 at 4 ohm.
    Having the 1960A cab @ 4 ohm will take 2/3rds of the head's power, and the 8 ohm will use the other 3rd. (head set to 4 ohm)

    If you switch it to 16 ohm, it will use 1/3rd and the 8 ohm will use 2/3rds.
    (head set to 8 ohm)

    Those are the two current options, but still leaves the amp load in a unbalanced loading situation, but shouldn't hurt the amp.