In 1967, Kellgren was a recording engineer working at several New York City studios including Mayfair on 701 Seventh Avenue at the edge of Times Square, a drab upstairs office, a single room which held the only 8-track mixing system in New York.[2] There, Kellgren worked with artists such as The Velvet Underground who recorded Sunday Morning in November 1966, Frank Zappa and Jimi Hendrix, engineer