Please answer the following question: Given the following context:  To create the material for Highway 61 Revisited, Dylan spent a month writing in his new home in the Byrdcliffe artists' colony of Woodstock in upstate New York. When he returned to Studio A on July 29, he was backed by the same musicians with Harvey Brooks on bass replacing Joe Macho and his producer had changed from Tom Wilson to Bob Johnston.  Their first session together was devoted to three songs. After recording several takes each of "Tombstone Blues", "It Takes a Lot to Laugh" and "Positively 4th Street", masters were successfully recorded. "Tombstone Blues" and "It Takes a Lot to Laugh" were included in the final album, but "Positively 4th Street" was issued as a single-only release. At the close of the July 29 session, Dylan attempted to record "Desolation Row", accompanied by Al Kooper on electric guitar and Harvey Brooks on bass. There was no drummer, as the drummer had gone home. This electric version was eventually released in 2005, on The Bootleg Series Vol. 7.On July 30, Dylan and his band returned to Studio A and recorded three songs. A master take of "From a Buick 6" was recorded and later included on the final album, but most of the session was devoted to "Can You Please Crawl Out Your Window?" Dylan was unsatisfied with the results and set the song aside for a later date; it was eventually re-recorded with the Hawks in October.After Dylan and Kooper spent the weekend in Woodstock writing chord charts for the songs, sessions resumed at Studio A on August 2. "Highway 61 Revisited", "Just Like Tom Thumb's Blues", "Queen Jane Approximately", and "Ballad of a Thin Man" were recorded successfully and masters were selected for the album.One final session was held on August 4, again at Studio A. Most of the session was devoted to completing "Desolation Row". Johnston has related that Nashville musician Charlie McCoy was visiting New York, and he invited McCoy to play guitar at the session. According to some sources, seven takes of "Desolation Row" were recorded, and takes six and seven were spliced...  answer the following question:  Whose producer had changed from Tom Wilson to Bob Johnston?
A:
Dylan