JTM45, two 2x12's and a hotplate?

Discussion in 'Cabinets & Speakers' started by nobbydog, Nov 21, 2020.

  1. nobbydog

    nobbydog New Member

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    So I have some understanding of the usual ohms setting for speakers questions. I read a post here recently about two 2x12's into one output from the JTM45 but my 'query' is a little different. I have a JTM45 and want to run it into two JVMC212 2X12 cabs. Each cab has two 8ohm speakers in series. So - if I run one cab I set the JTM output to 16ohms to match one cab? And if I run 2 cabs I set the JTM to 8ohms on the basis that the two outputs from the JTM put the cabs in parallel?
    And finally - what if I want to add a hotplate into the 2x12 setup for gigs? Do I need 2 hotplates, one for each speaker output? Or can I do the Y cable thing mentioned in the other post - then into one hotplate, then back out to the 2 cabs?
    Sorry some of this has been said before but I cant find a thread that specifically deals with the hotplate with 2 cabs setting on the amp.
     
  2. houseofrock

    houseofrock Well-Known Member

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    Last edited: Nov 21, 2020
  3. nobbydog

    nobbydog New Member

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    Thanks for the quick reply. Yes the hotplate does have 2 speaker outputs So I guess the question I still have is what the ohm setting on the JTM needs to be if I use only one output from the amp to the hotplate and then the hotplate splits to the 2 cabs. I'm 'guessing' 8ohms on the amp - ie. goes to an 8ohm hotplate that is running two 2x12's of 16 ohms each, but parallel out of the hotplate, hence the 8ohm hotplate?
     
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  4. neikeel

    neikeel Well-Known Member

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    Normally you match your head setting to the attenuator (that is the 'load' that the amp sees). So, assuming your Hotplate is a 16ohm (Blue) one then the single head and cab is perfect match. If you use both cabs and only have the 16ohm Hotplate then THD suggest you set your amp to 16ohms plug that into the Hotplate and then each cab into the speaker outs of the Hotplate. Then the Hotplate buffers but your head sees the correct load.

    Of course if you go for 8ohm Hotplate you set the amp for 8ohms etc. and then you have perfect match but will not get perfect match with one cab/Hotplate/head combination.
     
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  5. RLW59

    RLW59 Well-Known Member

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    Why a Hot Plate? I get the impression you don't already have one (or else you'd be asking how to hook up whichever impedance you already have).

    Once upon a time the Hot Plate was one of the best, and it's still good. But instead of hunting for a used one (or two, if you want the option of using one cab or both), why not get a current-production attenuator with switchable impedance? Then you can use 1 or both of your cabs, and be covered if someday you get different cabs.
     
  6. nobbydog

    nobbydog New Member

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    OK thanks, so single cab - set the amp to 16ohms, hot plate is 16ohms - sorted. And the same for 2 cabs which could work with the 16ohms hotplate too subject to the buffering (I don't know what that means)? But if I had the 8ohm hotplate I'd set the amp to 8 ohms as the 2 cabs are in parallel out of the hotplate, hence 8 ohms? Apologies for repeating some of this but I don't want to ruin the amp.
     
  7. nobbydog

    nobbydog New Member

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    I do have a 16ohm hotplate, just bought the second cab and JTM so trying to figure out my options.
     
  8. houseofrock

    houseofrock Well-Known Member

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    What is the ohm rating of the two cabs? Couldn't find it in previous posts.
     
  9. JohnH

    JohnH Well-Known Member

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    It sounds like, as noted above: 16 Ohm Hotplate goes into 16 ohm amp tap and then run your cabs in parallel out of that which will be an 8 Ohm load.

    This works OK because once you have a few dbs of attenuation, the amp only sees a small fraction of the speaker load and mainly what it sees is the attenuator load. But if you ever need to engage a full bypass switch = full volume on the attenuator (if it has one), the amp will then see the speakers directly and you should then move to the 8 ohm amp tap.

    Hot Plates were the beez kneez 15 years ago and are solid units. But we know a lot more about attenuator design now and we know how to build a better one that will maintain the tone and load the amp more consistently (based on 'bedrock science' - useful for lots of things!) - check out the Workshop section if you ever feel like a weekend project.
     

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