Avid DNxHR LB alpha channel missing

Trying to create some lowish quality offline files with a alpha channel.  Tried Avid DNxHR LB (AME CC 12.0.1 windows) but did not get a alpha channel.  Cant work out what I have done wrong;(.  Don't mind using another format but this creates a 500MB file and ciniform on lowest quality created a 1.5GB file which is way to big (for a 50 second offline file).