Setting up a gig rack is all about heat management, and this is especially important for a guitarist, bassist, or keyboard player who may play outdoor gigs where the ambient temperature can go above 100 degrees F (Working in the Desert Southwest as I do, I encounter this all the time). Problem is, most newbies copy what they see in recording studios via Mix magazine, or whatever. Recording studios are climate controlled environments! Therefore, the ambient temperatures therein are going to be set with keeping the amps, effects units, and computer equipment happy in mind.
So, in a climate controlled recording studio, putting the power amplifier on the bottom of the rack makes perfect sense: It won't overheat, you never need to fiddle with it, and putting preamps and other gear above the amp makes accessing them for adjustments easier. Putting the power amp on the bottom of a gigging rack is, however, idiotic.
What does heat do? It rises. If you put a big, badass, MESA/Boogie Stereo Simul-Class 2: Ninety on the bottom of your guitar rig, those eight 6L6 output tubes, three 12AX7 input tubes, and two gargantuan output transformers will bake everything above them! However, if you put the power amp on the top, the rising heat from it will actually aid the cooling of the lower units through convection: The rising hot air on the top will pull in cooler air from down below.
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Here are two of my racks to demonstrate the principle:
ALL YOUR GEAR ARE BELONG TO US!
One of the reasons that power conditioners with light modules make exactly zero sense for gig racks is because the wretched, blasted power conditioner belongs on the bottom of the balsted rack! Power conditioners - even ones like the Furman AR-1215, which has isolation transformers - generate very little heat: Putting them on the bottom gives a relatively open area for air circulation. Just above the Furman units I have my Lexicon MPX-G2's, which I use in stand-alone mode as the preamps and effects units. These generate a significant amount of heat, they are very deep units, and they have a vent on the top - about six inches back - which should never be blocked by a unit above them. I usually recommend an open space above Lexicon MPX units, but the Beheringer Racktuner generates almost no heat (I let one run twenty-four hours with the lights on, and there was a barely noticable warm spot above the internal power supply), and it is only four inches deep, so the Lexicon's vent is not blocked. Basically, the Behringer is almost as good as an empty space.
Then, the amps are on the top. The top rack is my day-to-day dinner club/backround music rig. The Bryston 2B-LP there is a single space, sixty watt per channel unit, and the heat sinks are on the front, outside of the rack. Nonetheless, it has some upper vents that need to breathe on the top a few inches back, and it can get so hot the sinks will burn your fingers if you try to hold onto them. Nice thing about the SBK racks I use is that they are light as a feather, rugged beyond belief, and they have about an inch of space all around the gear: The rack "breathes" perfectly.
The lower rack is my high end gig rack with the incredible Lexicon Signature 284 All Tube Class "A" Stereo Recording Amplifier and Direct Source: Its +4 db direct outs allow me to plug its pristine class A EL84 output right into the house PA. Yeah, it rocks.
Not pictured here is my large venue outdoor rig, which is the same except it is six spaces to accomodate a Bryston 3B-NPB in the top slot. That amp has massive heat sinks on the inside of the rack, so it really needs to breathe. Setting the rack up with convection cooling in mind makes it relatively compact, and allows me to play folk festivals and whatnot out in the heat with noooooo problemo.
So, don't ever set up a gig rack like a studio rack, OK?
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BTW: You can easily see how the Behringers will light up a dark stage.
Cool, huh?
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Think she's really afraid of the dark? Me neither.
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