Problem with Thermaltake 750W

My guess is that you probably have insufficient amps available on the +12V rail of your PSU. A PSU rated at 750W doesn't mean an awful lot at face value. It's the number of amps or watts available on the +12V rails that will really matter since that is the rail that your graphics card draws power from. More and more components are using the +12V rail hence the current trend of PSUs to have the majority of their power available to these rails.

If you take a look at the ThermalTake website and scroll down towards the bottom you can see the table that shows how many amps or watts are available on the +12V rails:

http://www.thermaltakeusa.com/Product.aspx?C=1264&ID=19...

You can see that 56A or 672W are available for your particular PSU. By looking at some tests and reviews of your graphics card I can see that at max load your card draws about 285W:

http://www.tomshardware.com/reviews/radeon-hd-4890,2262...

Since you have two that means from your graphics cards alone you are using 570W of power. You also have to bear in mind that the value of 672W available on the +12V rail is a maximum value which means your PSU may not be able to sustain this for long periods of time. When you combine this with the other components in your system that also draw power from the +12V rail such as your CPU etc. and the fact that your PSU may not actually be able to deliver its rated value or it may not be able to sustain that value for extended periods of time, you'll probably find that your PSU is being overloaded and is shutting down to prevent damage to itself and other components.

The official recommended power requirements for your card are as follows:

500 Watt or greater power supply with two 75W 6-pin PCI Express® power connectors recommended (600 Watt and four 6-pin connectors for ATI CrossFireX™ technology in dual mode)

This applies to the entire system not just the card.

As you can see you have exceeded this but these figures depend upon the unit in question. You can see from the Corsair site below that their 650-TX model actually delivers 60A or 720W max on the +12V rails. This is the differences in quality between units that I mentioned.

http://www.corsair.com/products/tx/default.aspx

Corsair, Seasonic and the newer Antec models have a reputation for delivering high quality, solid and reliable PSUs. Unfortunately, Thermaltake don't have that same reputation. This is not to say that you have purchased a bad PSU - it's a million steps ahead of other OEM PSUs. It's just that Thermaltake don't have the reputation of say Corsair.

What I find difficult to explain is why your 600W unit worked ok. The only thing I can think of is that every PSU is different. No two are the same and everyone will have different experiences. I guess this goes back to the manufacturer's reputation since consistency is usually higher. The only other thing is that something else could be causing the issue. I would suggest you check everything over again and failing that, return your unit and hope another of the same will be ok or go for a Corsair unit. Hope this helps and best of luck!

Well , Thanks you for your answer , i will keep everything in mind.

The single 56 amp 12v rail on the TR2 750 should provide more than enough power.
The 285 watt number from above is system power taken at the wall.
What is being shown is the increase in system power between idle ( 178w ) and gpu at full load ( 285w ) a 107w increase.
Take the avarage efficiency of the psu that was used into account ( 87% ) and that 285 watts at the wall becomes a 250 watt load on the psu ( for the entire system )
The 107 watts AC increase ends up as a 93 watt DC increase when the gpu is stressed.
Adding a second card would put power use nowhere near the limit of a 750w power supply.

Here's a review of Crossfire 4890's
http://www.techpowerup.com/reviews/ATI/HD_4890_CrossFir...
420 watts max from the wall for the entire system, around 350w DC with the psu that was used.

My opinion, faulty power supply.

Well Thanks , I will see soon enough if its a Faulty one.Saturday i'm supposed to change it .. If they wont change it i'll make them test it...

Thanks for your opinion.