Complex sentence: Jackson never considered himself a great singer, partly because the group chose poor keys for his vocal range, but his bass playing, with heavy use of a plectrum, was a distinctive part of the band's overall sound.
Split into simpler sentences: Jackson never considered himself a great singer, partly because the group chose poor keys for his vocal range. However, his bass playing, with heavy use of a plectrum, was a distinctive part of the band's overall sound.