The Groove Tubes GT-6C4 Rectifier Tube is a common 9 -in rectifier for smaller amps like the Silverstone guitar/amp case combos of the 50's.
90 day warranty on power tubes, includes replacement of whole set if one tube goes bad. 180 day warranty for preamp tubes. 3 year warranty on SubstiTubes and Solid State Rectifier. Check with M-Audio for warranty on GT microphones.