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cooling fan suggestions

9/1/2004 2:15 PM
cooling fan suggestions
I've put together a prototype 4-EL84 amp and it sounds great but runs pretty hot.I've never used cooling fans before but think this amp may dictate one.  
Any suggestions on fans and where to tap the power for one.My amps primarilly are in hardwood head cabinets so looks are an issue.These are Marshall style heads with the tubes on top of the chassis.  
9/1/2004 4:18 PM

I like the 12v DC fans. You can get some that are only 2 inches in diameter. I added a bridge rectifier to the heater lines and one low voltage 470u across the plus and minus leads. This gave me around 8.5v. Just enough to run the fan quietly. Just aim it to flow across the power tubes.  
9/1/2004 8:36 PM
Dr. Photon

I would use a 12 volt computer-style "muffin" fan. I'd just use a bridge recto on the 6.3 volts AC. Just wire up a capacitor (470uF, 1000uF, whatever...) accross the output of he recto and you've got aroudn 9 volts. 9 volts will run the fan fairly quietly. If yopu need the fan quieter, just use a resistor in series w/ it to slow it down or get a "silent" PC colling fan with a thermister/pot that varies it's speed. In the PC world, there are all sorts of fans, from the Vantec "Tornado" screaching ultra high power fan (actually a relabeled delta?) to temperature controlled fans to "silent" fans. a 3 inch (80mm) or a 4 inch (90 mm) fan would be fine.  
12 volt "computer" fans are easier speed controlled than 120 volts AC fans.  
If you have a unique cooling challenge, you could try a unique fan, such as small 12 volt Squierl cage blowers (nidec gamma 28 comes to mind) and various other oddities
9/1/2004 9:43 PM

Thanks guys.Used to build several computers so I'm surprised I didn't think of that.Still know the suppliers so I'll check it out.
9/2/2004 10:09 AM
Greg Simon
Could I use the same approack in my MTI era SVT to change the noisy AC fan into something quieter that would still move enough air?  
9/3/2004 12:33 AM
Don Symes

Certainly cheap enough to be worth a try. Keep all the wiring AWAY from the preamp.
9/3/2004 5:28 AM
Dr. Photon

good point that I forgot to bring up. Sometimes these brushless DC motors can be noisy, electrically and magnetically. It shouldn't be a problem, as long as you keep the thing away from the inputs. As long as it's positioned to where it can blow on the output tubes and stuff, it shouldn't be causing preamp problems. I'd still keep it away from any low-level input signals or anything that's high gain. But that would mostly be a problem if the fan were stuck under the chassis and blew UP at the tubes through some vents or something.  
Also, try to grab some ball bearing fans. ball bearing fans last longer than the cheaper sleeve bearing fans. If the fan doesn't say it's ball, it's a sleaver. A sleave unit will work OK, but sometimes they seize up after about a year or so in a computer that's on 24/7. While an amp typically isn't running as long as a computer is, they are expected to work in crappier conditions (any cigarette smokers at the gig???). While a sleave bearing unit is definitely good enough, paying a little extra for a quality ball bearing fan is worth the extra money due to the increase security and reliablility, and you won't have to worry about it, save blowing the dust out of it every once in a while.  
Fans usually don't just die right then and there, they typically slow down first, and start to make noises untill they finally stall out. The fan is less critical to the amplifier's operation that some fans are to the operation of a computer. When a fan dies like this, you can usually bring it back by peeling off the sticker and applying some light machine oil like 3-in-1 or roller blade oil or something to it.
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