The Downbeat in Yokohama is one of the classic jazz spots in the entire Kanto (Tokyo Metropolitan) area. It opened in 1956 and has only once changed owners. It's in the rather seedy block neighborhood of Hanasakicho, across from the main Sakuragicho-station front plaza. Lots of little drinking dens, old karaoke pubs and some other jazz joints to visit around here if you have a night.
Best thing about the Downbeat besides it's extensive album collection is the sound system. While the seats and decor are all older than most of the customers, the speakres are top-notch! Live music events every once-in-a-while, though coffee time here is the best option I think.