Well yes back is always /bæk/ in NAE. "Beg" is more variable. Some people have tense-lax neutralization before /g/. According to Wikipedia: "Some examples of neutralization of /ɛ/ to /eɪ/ before /ɡ/ are beg, egg, Greg, keg, leg and peg's coming to rhyme with Craig, Hague, plague and...