Source Alpha not working

[zizikko]

Hi. 

I was trying to recreate this texture from this tutorial, but there is a part at 4:13, when they used Alpha Source Mode, but that function is disabled for me. What can I do?
Thanks!
https://www.youtube.com/watch?v=PBkFaeKzhGE&t=298s

That function is enabled when it's not connected to other nodes. Once I add a connection that option is disabled. In the attachment you can see both.

[davescm]

Source Alpha is not applicable for blending single channel greyscale inputs, so should grey out. It is designed to set how the alpha channel of colour RGBA inputs is used.

Dave