Question GTX 1080 Ti Not Boot on X470 Ultra Gaming PCIex 3.0 x16 Slot (VGA Red Light on Motherboard)

Mar 18, 2019
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Hi Guys!
I'm writing here this post because i know that this is the best suite forums for solutions and problems like this.
So... a Week ago i built my RIG with new parts and Gigabyte GTX 1080 Ti Gaming Black OC but GPU won't Boot for some reason.

There is my RIG:
MBO - Gigabyte Aorus X470 Ultra Gaming.
PSU - SeaSonic 620W Modular +80.
RAM - GeiL Super Luce RGB 2x8GB 3000Mhz.
CPU - AMD Ryzen 7 2700X + Stock Cooler.
HDD - 1TB WD Blue.
SSD(1) - 120GB WD Green.
SSD(2) - 480GB Team Group.
Case - Antec P8.

I don't think its a PSU problem because the GPU won't boot just in the PCIe 3.0 x16 Slot, if i'm connecting it to the x8 Slot thats works fine.
i tried other GPUs like Palit GTX 1080 and EVGA GTX 980 on the main x16 slot and thats work but just the GTX 1080 Ti (this specific vers) won't boot.
GPU for sure connected to the 6+8 Pin PSU and shows LIVE like fans work and led too.
i have tried to Clear CMOS / Reset Bios and Update BIOS Too.

Red Light comes out from the VGA Light on Motherboard when i'm connecting the GTX 1080 Ti to the main-first slot x16.

What Should i do? (and BTW: Sorry for the Bad English)