Selecting output transformers

[Heavenamps]

I haven't been able to find much info on how to go about selecting an OT. I notice that OT's have power ratings which seems pretty straight forward but I've also noticed that they come with an impedance rating as well. What exactly is your goal with the primary impedance? I'm assuming it's about matching the impedance of your power tubes. Is that correct? Please enlighten me

[PRR]

Read tube datasheets.
http://www.mif.pg.gda.pl/homepages/frank/vs.html