Mesa Boogie 5881 STR 425 tubes in pair. The MESA 5881 STR-425 is a military grade 6L6 that was designed with an emphasis on rugged durability and consistency, capable of handling high voltage requirements. It produces a thick, rich tone with tapered highs that lends itself to styles of jazz, fusion, some blues or any style where tapered highs and a full round tone are desired.