I've got an entry level HPE SRV 778640R-B21 which supports dual CPU but comes with an E5-2603v3 (LGA2011-3 / DDR4). It has also 550W power supply.
I want to remove the current cpu and add a pair of E5-2690v4 if it's possible. If the power will be problem I can chose another pair of cpus like 2650 which consume lower watts.
I've googled but there is no clear info/datasheet. Is it possible to upgrade cpu or what would be your choice?
The E5-2690v4 isn't supported in that server at all, probably due to the TDP, but the 2660v4 is supported and is probably the closest to the 2690v4 that will work. You MAY be able to get away with that 550W PSU but if I were spending the money on the extra CPUs then I'd be strongly-tempted to add in the 900W redundant PSU option (778640R-B21) too.
Oh and if you ever want to find HP/HPE datasheets the term to google for is 'quickspecs' ok :)