This paper investigates the use of 'ain't' in past tense contexts in African American English (AAE) using a corpus of recorded speech collected in Philadelphia in the early 1980s. A study of 42 speakers' rates of use of 'ain't' in past tense contexts finds increase toward 'ain't' in both real and apparent time. This increase is stronger among speakers born and raised in Philadelphia compared to those who migrated there from the South, supporting previous work linking innovation in AAE to linguistic segregation in the urban North during the Great Migration. Finally, this paper uses data from the morphological form of verbs following 'ain't' in past and perfect contexts to argue that the use of 'ain't' for 'didn't' resulted from the reanalysis of present perfect constructions containing 'ain't'.