SWR SM-900

swrbass5 · #1 03-13-2011, 06:52 PM

Recently bought an older sm900. Always played SWR amps, and never had any major problems out of them , but this 900 is getting so hot, that I'm worried about thermal shutdown. Anyone else have the same problems?
I'm using a 2U rack that's been the home of my sm400s for the last 10 or so years, but I've just ordered a 4u rack, so I can add a rackmounted fan from ProCool (rackfans.com)
We'll be playing outside at the House of Blues for most of the summer, (and it's gonna be freekin hot enough as it is!)
Anyone got suggestions? I really love the sound of this amp, and would like to keep it that way!

rough house · #2 03-14-2011, 01:10 AM

I just purchased a older pre fender sm-900 and from everything i have read, they run hot. I mounted mine in a 4u skb case and i drilled holes in the side of the case so the hot air blowing out the side of the amp can exit the case. Im am also looking into getting the rackmount fan from procool also.

KJung · #3 03-14-2011, 05:20 AM

It is unfortunately the nature of that amp. SWR says it is designed that way. It always made me nervous back in the day. Mine got literally too hot to touch when bridged at 8ohms, much less 4ohms, but never failed. However, these are not known to be at the 'high reliability end of the reliability continuum. I view it as a design flaw.

Just give it enough room to breathe (i.e., put it in a 3 space rack, not a 2) and you should be fine.

Plays_For_Dog · #4 03-14-2011, 05:33 AM

+1 The SM-900 is characteristically hot. It's their nature. I've read on this forum of guys putting better fans in them, cutting the case for more airflow, etc to help with the problem. I have the SM-500 and have never had it overheat. If it's hot in the room/outside, the fan will kick in high gear, but that's a rareity. I've also seen people mount additional fans inside their rack case to push more air all around the unit. You could also mount the amp in the rack case without the top casing, but then you make the amp more vulnerable to foriegn object damage. Great amp, just the temp control is quirky.

mellohawk · #5 03-14-2011, 05:33 AM

I play through a pre-fender SM500 (about 12 years old) and it's gotten hot enough to fry eggs. Did a few outdoor summer gigs where I thought for sure it would shut down from the heat, but never had a problem.

The chassis on the SM series is aluminum and is designed to act as the heat sink for the amp, so naturally, the amp gets very hot. It's just doing what it was designed to do - dissipate heat away from the components. If your fan is working properly, you should have no issues whatsoever.

parsons · #6 03-14-2011, 07:41 AM

the manual states that you must leave a 1U rack space empty above the amp to allow it to keep cool. I believe the sm400 manual says to use a minimum 3U rack just for this reason...

ihixulu · #7 03-14-2011, 07:44 AM

My 1996 SM900 has pushed 4 ohm loads for hours in 90+ degree weather for many summers now. The key is to give the amp room to breathe - I think putting any SWR head in a 2 space rack is a bad idea.

I played with supplemental fans for while and found that a single, small 12v fan (which I dialed down to 9v) pushing air front to back across the top of the amp was enough to keep the internal fan at low speed on the hottest days.

Oh, yeah, make sure the amp is biased properly. When the bias is out of spec on SWR heads the results are not pretty.