Do reformed theologians call "God the Father" Jehovah? "God the Son" is generally agreed to be Jesus.

Do reformed theologians agree that "God the Father" is Jehovah, or is this only something claimed by the denomination calling themselves Jehovah's Witnesses?
Jehovah is an English rendition of the proper name of God from the Hebrew Bible, also rendered "Yahweh" or YHWH, which means I AM and is referred to as "The Tetragrammaton". It isn't specific to Jehovah's Witnesses, and can be found in many older English translations of the Bible.

In the Hebrew Bible (Old Testament) the doctrine of the Trinity isn't really formulated (Jews would say it isn't there at all) so there is no concept that the name for "God" applies to only one person of the Trinity. It is usually thought that the name(s) refer to "God as a whole". 

Jesus in fact equates himself with Yahweh (Jehovah) when he says "Before Abraham was, I AM" (John 8:58).