Weldon has been all over this for several years. It is a real threat..and needs far less 'tech' to make it a reality. That said, I think we have a more measures to thwart or otherwise protect against EMP measures than disclosed publicly. Nevertheless, interested folks should read up.
Vacuum tubes are practically insensitive, and so are chemical batteries like those in uninterruptible power supplies. EMP is not the end of the world.