Complex sentence: The amplifier also had a GZ34 tube rectifier, the power amplifier circuit had EL34 tubes, and the preamplifier used an EF86 in the'Normal' channel which, coupled with the AC30's increased output over the AC-15, exposed a reliability issue.
Split into simpler sentences: The amplifier also had a GZ34 tube rectifier, and the power amplifier circuit had EL34 tubes. The preamplifier also carried over the EF86 pentode in its "Normal" channel which, coupled with the AC30's increased output over the AC-15, exposed a reliability issue.