Definition of busy. busier. ; busiest. 1a : engaged in action : occupied She's busy studying. has enough work to keep him busy for a whileb : being in use found the telephone busy. 2 : full of activity : bustling a busy seaport.
It is considered a high quality tube, and is normally constructed from SAE 1020 or 1026 steel. Note that, technically DOM refers to the process by which the tube is finished after having started as an ERW tube. Technically, DOM is not a type of steel tube, but rather a process. As so often happen though - in common use the term has become accepted to mean a specific type of tubing rather than a process.
If the both the pre- and the power-amp have tubes in them, it's called an all tube amp, even if it uses some SS components. A REAL SVT[SUP]*[/SUP] is all tube because the pre-amp is tube based and the power amp is also tube based. A hybrid is one that has different types of pre- and power-amps.