Complex sentence: Harrison tried to salvage the recording on 9 August with a six-and-a-half-hour session of lead guitar overdubs, during which he sat in the control room playing the guitar, while the amplifier was recorded from an echo chamber.
Split into simpler sentences: Harrison tried to salvage the recording on 9 August with a six-and-a-half-hour session of lead guitar overdubs. During the session, he sat in the control room playing his guitar, while the amplifier was recorded from an echo chamber.