Video cart and power supply

Anyway it will not work with molex. Molex supplies up to 40W power. 6 pin PCIE supplies 75W.
Adapter cables are good only when very little additional power is needed (for weaker cards) and 770 is not such case.
For 770 to work you need at least one normal 6pin cable from the PSU, second one may be molex to 6pin.
But one direct from PSU is required. Decent new PSUs have 2x6pin and do not need converter cables.