SUBSTI-TUBE 1 6SL7 to 12AX7 (includes GT-12AX7 Tube)                 SUBSTI-TUBE 1A 6SN7 to 12AT7 (includes GT-12AT7 tube)                 SUBSTI-TUBE 2 6SC7 to 12AX7 (includes GT-12AX7 tube)                 SUBSTI-TUBE 3 6SJ7 to 12AX7 (includes GT-12AX7 tube)                 SUBSTI-TUBE 928 Octal to 9-pin adapter for the use of EL-84 tubes in an octal socket. NOTE: This is only for use in Class A amplifiers such as the GT Solo Single, THD Univalve, etc.                 SUBSTI-TUBE 928-PR Duet Power Reducer Class A converter Octal to 9-pin adapter for the use of EL-84 tubes in an octal socket. This converts class A/B amplifiers to class A for lower power. Set of two with tubes included.                 SUBSTI-TUBE 928-PR Quartet Power Reducer Class A converter Octal to 9-pin adapter for the use of EL-84 tubes in an octal socket. This converts class A/B amplifiers to class A for lower power. Set of FOUR with tubes included.