Tung-Sol Tube Upgrade Kit For Mesa Boogie F50 Amps 6L6GCSTR 12AX7 This set includes (2) Matched Tung-Sol 6L6GCSTR power tubes (3) Tung-Sol 12AX7 preamp tubes (1) "Gain Balanced" Tung-Sol 12AX7 preamp tube (phase inverter) marked with yellow "Balanced Triodes" Sticker