I suggest following the manufacturer's guidelines.
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The heat dissipation for tubes is important to ensure a normal lifespan for them. If it cannot be ventilated by natural convection currents, the very hot tube becomes hotter and this is detrimental to the filament etc.. In most applications the damage occurs over time.
IMHO it should not take that much space, my preamp for instance, uses 6 12Ax7S running at full voltage and has a highly ventilated cover. It is an ARC which uses these covers. If you have seen most any ARC tubed gear then you recognize this method of ventilation. It requires about 4 inches of shelf space just for the air.
The proof of the pudding is in the eating (not "in the pudding" ) All of the original tubes are strong and functioning perfectly, this is an old preamp.
I have owned my C2300 since January and think it is very good. I am using Mapleshade Triplepoints underneath it and there is just an inch above it, but my rack is open on all 4 sides and heat build up is not a problem. In fact, the top barely gets warm after extended listening sessions. Now my Mc275 is a different story...
I really like this preamp and its built in MC phono stage. I have replaced the stock tubes with Telefunken smooth plates in the line stage and the MC phono stage. Overall it is extremely quiet (even the phono stage) and has a rich character with nice detail and dynamics.
Personally I find the blue meters distracting and as a result they stay off in my system. But I do appreciate the easy user interface including changing the phono settings on the fly with the remote.
If you remove the top/any covers(not the bottom plate) from the pre-amp: it'll stay cooler. I've always done that with tubed gear. Never have had a problem with tube longevity, even though my gear has always resided in an enclosed, rack type cabinet(not the power amps). Pearl Tube Coolers also will help. Page 16 in this catalog: (http://www.percyaudio.com/Catalog.pdf) If the pieces are very close together RFI/EMI shielding is a good idea(See page 14 for Texas Instruments shielding). There's a sheet of it somewhere, between every piece of my equipment.
Thanks for the ideas.
any comments on the C2300 sonics for linestage use (not phono)? Does it have that nice (to my ears) warm, liquid, relaxed Mac flavor but still definitive with no part of the spectrum exaggerated or supressed? (Even though it's not the last word in dynamics or slam.) This is my take on the C46 preamp I now own.
Keep in mind that the style of rack matters a lot. If you have a rack that's open on all sides, I do, then top clearance isn't as crucial. If the rack is enclosed or partially enclosed then it' very important. As someone said you can use a fan, and as always a little common sense goes a long way. Happy listening!
Go over to Audiokarma and do a search on the McIntosh Discussion Forum. You will find a lot of (mostly) useful information on the C2300 including comparisons with other McIntosh preamps.
Here is one link to a review by TonePub:
I listen to vinyl 90%+ of the time, but the line stage seems good on its own and most of my CD's are very listenable through it. I can't compare to your C46 because I have never heard it.