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[Acejam]

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[Strongbow]

Q1. 750w will definitely be enough for 1 6990, and will definitely not be enough for 2. I would say 950 at least if you're running 2 6990's.
Q2. Nah, gigabyte brand will be fine, go for it

Side note: i would say don't go with 2 6990's, better purchases can be made for pretty much any use you have.

[mattpker]

As previously said by another user, I would not buy 6990's. A better price per performance are 6950's that are unlocked to 6970's.

The following card can be unlocked:

http://www.newegg.com/Product/Product.aspx?Item=N82E16814127555&cm_re=6950-_-14-127-555-_-Product

You can unlock it by following these directions:

http://www.techpowerup.com/articles/overclocking/vidcard/159


These can do 400Mhash/s per card for $270. 6990 can do 800Mhash/s per card for $750+. As you can see, building multiple machines with 6950's is a much smarter option.

[Strongbow]

If you intend to run two 6990s, my understanding is that you'll need to run Linux.

Nope. Windows can do 4 GPUs, 2 in each of the 6990's. It would work fine.

[padrino]

Another option for those looking for a good deal, the XFX HD-695X-ZNFC can be had for $209 after rebate and a number of people also unlocked them, not reference cards and 1GB of RAM but for BTC it may be a great option.

[xen82]

I had a 650WCorsair PSU and could only run the card on 830MHz (but then again my CPU is a i7-2600. I recommend getting a stronger PSU if you want to run two cards, I got a Thermaltake 1200W for my system running 2 cards @ 880MHz.

On a side note, when people get 800MHash/s on the 6990, how do they go about that?
I average about 310-320MHash/s per 880MHz GPU core (running Ubuntu 10.04 x64, Catalyst 11.5, 2.4 drivers, and Phoenix 1.48). On my Windows 7 x64 box I get 343 MHash/sec on Phoenix 1.48 and the latest drivers/SDK. Suggestions?

[accod]

Try running a single core on the 6990 and see if you get closer to 400MHash/s.  Also check to see if your core speeds stay at 880MHz or are jumping about a bit.  I found the power throttling kicked in on mine and needs to be raised before you can get 400MH/s on both cores at the same time.

[mjhorn]

Try running a single core on the 6990 and see if you get closer to 400MHash/s.  Also check to see if your core speeds stay at 880MHz or are jumping about a bit.  I found the power throttling kicked in on mine and needs to be raised before you can get 400MH/s on both cores at the same time.

Are you running Windows or linux with on your setup? I'm looking for more info on the power throttling under linux, but so far have only found anything useful for Windows.

[itsagas]

750W for two 6990 is not enough.  I have two 6990 running and they are using 880W while mining under full load (with a 1000W PSU).  Also, they throw off tons of heat and suck more power than my 4x9650 setup.  The 4x9650 draw under 800W under full mining load, run cooler and get a better hash rate than my 2x6990.  And the 6950s were way cheaper to buy. 

Hope it helps. 

[accod]

Try running a single core on the 6990 and see if you get closer to 400MHash/s.  Also check to see if your core speeds stay at 880MHz or are jumping about a bit.  I found the power throttling kicked in on mine and needs to be raised before you can get 400MH/s on both cores at the same time.

Are you running Windows or linux with on your setup? I'm looking for more info on the power throttling under linux, but so far have only found anything useful for Windows.

Windows for now until I decide on the volts, clock, and fan speed I want to run and then I'll modify the bios and run linux.  Afaik if you switch to the second bios it will disable power throttling but I'm running on a 520W PSU so don't want to risk this (my rig uses 360W at 300MH/s x2 (750MHz iirc)).

[AmpEater]

Lots of good info/suggestions in here - thanks guys!

Yeah for 2 cards my 750W PSU isn't enough. I think I've found 2 PowerColor 6990's for sale that are lightly used, so I'm going to try and pick those up, along with a 1200 watt Corsair PSU. Is 1200 *too* much? I want to perhaps add a 3rd card in the future... (but I'd need to run Linux for that, when the time comes)

1200 corsair PSU is great.  No such thing as too much capacity.  That will allow you to run 3 cards in the future

[Jack of Diamonds]

Please feel free to critique.  

Thanks,
Josh

IMO 5850's are the best reference card, costing slightly over $100 dollars and yielding from 280 to 330 mhash/s.
In comparison a $500-$600 dual GPU card fetching merely ~650-700mhash/s at stock speeds seems like a bad hardware investment.

The only problem is availability, you would be hard pressed to find many in new or mint condition. However there are plenty of used ones on the open market.

[AmpEater]

Please feel free to critique.  

Thanks,
Josh

IMO 5850's are the best reference card, costing slightly over $100 dollars and yielding from 280 to 330 mhash/s.
In comparison a $500-$600 dual GPU card fetching merely ~650-700mhash/s at stock speeds seems like a bad hardware investment.

The only problem is availability, you would be hard pressed to find many in new or mint condition. However there are plenty of used ones on the open market.

This comparison is meaningless without Mobo/Ram/CPU/PSU/Case/HD costs factored in.  In my case (dual 5970 rigs) GPUs make up 65% of the cost of a mining rig.  For 5850s hardware costs will be over 50%

[EPiSKiNG]

Cards should be arriving later today.

Installing this as well later tonight:

Windows can be wierd about crossfire.  I have a 6990 and 2 5870 but no crossfire enabled, then I have another box with 2 5970 that requires the crossfire cable to be installed in order for opencl to recognize all 4 cores.  It used to be (stream 2.1) that crossfire degraded performance, but I've seen that with sdk 2.4 that problem has been fixed.

EP

[Atroxes]

2x Stock 6990's use roughly 700W combined.

The rest of your setup demands atleast 200W.

Your PSU *should* be able to handle that. Could be something wrong with your 2nd PCI-E port.

Try putting only 1 card in that slot into your motherboard, and see what happens.

[PcChip]

If that PSU is not a single-rail system, then make sure to look up which modular slots correspond to which rail, and load them equally.

Also, without using dummy plugs, phoenix will only see the GPU that the monitor is currently plugged into at the moment; What I do is plug the monitor into my 5830 and run Phoenix/PhatK, then unplug it and swap to the 5850 and run the 5850.bat, then unplug the monitor alltogether to get a slight 2 MH/s increase (seen by logging in remotely via TightVNC)

[Wepp]

I am running a dual 6990 setup. It's drawing about 1.12kW of power, with spikes up to 1.18kW.
These suckers are hot, and hungry.
And hot.
And loud.
And hot.

I hope you have a 3-slot or 4-slot motherboard, because sandwiching two 6990's together with only 1/4 inch of room between would be a huge mistake. Even separated by an empty slot, and cooled by TWO 200mm 166CFM fans (one from front and one from side), they still run around 95ºC with the GPU fans at 70%.

Can't complain about getting 380MH/s x4, though.

Probably could overclock and get 400MH/s with watercooling.


I should also note that my power draw is higher than yours because I also use this as my database server. Four hard drives and an Intel i7 950 will do that.