Watts & Ohms questions
 

I have a SWR SM-900 classic and I had a few questions about watts and ohms.

1. In bridge mode it pushes 900w @ 4ohms and 650w @ 8ohms. So if I had two 8ohm cabs does that mean it draws the 4ohm load? And if so does it split the 4ohm load between the two cabs(450w to each cab)?

2. In stereo mode it pushes 400w @ 4ohms and 240w @ 8ohms per side. So if I had a 8ohm cab on each side, is each cab going to pull the full 8ohms? I'm guessing that they would seeing there on diffrent sides.

Read the FAQ for info on Ohms and Watts. These types of questions have been dealt with at length on TB. Easiest way to get a straight answer is to look at FAQ in the Sticky's

Yes, two 8ohm cabs hooked up in parallel will result in a 4ohm total nominal impedance, and will result in your head putting out its maximum 'safe' power level. Be aware that the 900 will get very, very, very hot run that way (i.e., both of the power modules are 'seeing 2ohms' (that is what happens when you bridge a dual mono amp into a total 4ohm nominal impedance cab or cab combination), which means they are running hot. No problem there per se, but you don't want that head in a 2 space rack.

Running the same two 8ohm cabs in 'non bridged mode' will result in the amp putting out its '8ohm per side' spec (i.e., lower wattage). So, you are correct.

Each cab, in either situation, will 'share the power of the amp' equally. It they are two identical cabs, this works out nicely. If they are two different cabs, even though they are getting the same amount of power, they might perform very differently based on sensitivity and general voicing.

If you are running two relatively small 8ohm cabs (like 112's or 210's), you are probably better off running in the lower wattage 'dual mono' mode, since a small cab can't use that much power anyway, and that head will run much cooler. If you are cranking two larger 8ohm cabs (2 x 212 or 2 x 410), the bridged into 4ohms is the way to go, just leave significant space around that head for cooling.

IMO, the optimum cab for that particular head is one large 8ohm cab like a 212 or 410.... nice output bridged at 8ohms, and you don't get into that '2ohm per side' deal that generates all that head.

I gigged the SM900 for a couple years with a single 8ohm Goliath III, and it sounded great.

+1 A very good general source for the OP to read. However, each head has its own special issues, and the SM900 is no exception (i.e., the extreme heat generated, using the case as the heatsink, size of cabs relatively to wattage trade-off with the different ways of running the amp, etc., etc.):)

I have it in a 4 rack space with it at the bottom and a power conditioner at the top. So one space in between free.

That should work fine for bridging into 4ohms if you decide to do that. It will get 'scary' hot though. Supposedly designed to deal with that, but it always made me nervous:D

Made me nervous also running my old one that way! Be very careful of sticking your hands in the rack after running it that way for any lengthy amount of time. Mine would actually give you a nice burn.

Yeah, I think many who have not owned that head read 'it gets hot' and think about the way other heads can get a bit warm when pushed. +1, running hard bridged at 4ohms, the head can literally give you first and even second degree burns if you aren't careful.

Do you think I could move it to a 2 rack space if I was running two 400w @ 8ohms, one on each side in stereo mode? I want to slim down my rack.

Yes. It will get warmer than most heads, but not stupid hot. I ran mine at 8ohms bridged (i.e., the equivalent of 4ohms per side) in a 2 space SKB rack, and the top of the rack got hot enough to get 'soft' by the end of the night. However, I cranked it that way hard for a couple years with no issues. At 8ohms per side, a two space rack will work fine.

Correction....I meant 400w @ 4ohms, one on each side.

That would pretty much be the same as my bridged 8ohm regarding the heat the amp puts out. It will get hot, but as long as you have a 2 space rack that has some space for the amp to cool (like the SKB standard) you should be OK. One thing I would NOT do is go from your 4 space rack with your rack tuner to a 3 space rack with that rack power conditioner right on top of the amp (those things are pretty much not necessary anyway... not good with this particular amp.

A 2 space SKB rack with a tuner pedal, and you should be good to go.

One last question is I bad to run ihe amp in stereo mode and only have one cab hooked up to one side? My friend has an Aggie GS212 for sale. It's 650w @ 4ohms so I would have to hook it up to one side in stereo and it would only be pushing 400w to it.

That is a nice cab. Nothing inherently wrong with that, since solid state power sections to not need a 'load' like all tube amps.

However, it has been a while, but make sure to check the manual (you can download one from the SWR site), since given the 'heat issue' with that amp, I believe they recommend running the power amp closest to the fan if you are only using one side.

That all being said, I would actually try to find a nice, single, 8ohm cab, like a 212 or 410 and bridge that head. Perfect combination of very good power delivery, and not pushing the head to its absolute limits, but also not 'throwing away' power by only using half the amp.

Of course, if you don't play that loud, doesn't matter, and your GS212 4ohm solution should work fine.

I was looking at the Avatar 210, 700w @ 8ohms that would work nice bridge but I'm affraid I won't get enuff low end.

Thats a thermal rating, not a "safe power usage" rating. Anything more than 250-300 watts is going to push a 210 into destruction.

+1 If the OP is looking for a smaller cab solution, two 8ohm 112's, running off each side would probably work great, or an 8ohm 212 or 410 run bridged, to work optimally with that head.

I like a punchy funky sound, I like to dig into my D angle G string and get almost a pop sound from them. but I also need something that will handle my B string. Do you think I'll get that from a 12

Speaker size has nothing to do with it. Its all about the implementation of the entire cabinet design.

What is your budget for a cab?

Want to try and keep under $500. Looking to run 400w @ 4ohms out of the left channel in stereo to it. And maybe get a second down the road to plug into the right channel. I was looking for a two cab setup so I can use one light can for practice then add a second for shows if I need a little more juice.