The Sovtek 5881WXT is the same as the 5881 but with a full size base for easy installation in vintage amp designs. Used by Fender, Crate, Carvin, etc.
These Sovtek 5881WXT's are in platinum matched sets. Their platinum matching process consists of a 24-hour tube burn-in period followed by a bias (plate) current measurement in milliamps.
Order tubes BEFORE they need to be replaced, and save the gig.