Fender Groove Tubes Gold Series GT EL84 Duet (White) amp Vacuum fuse - 2 PACK A fresh replacement tube to boost your amp's tone! Groove Tubes have improved harmonic balance, longer life, matched gain to frequency response ratios and require no rebiasing when replaced in color-coded sets Red-clean; blue-dirty; white-medium Groove Tubes have improved harmonic balance, longer life, matched gain to frequency response ratios and require no rebiasing when replaced in color-coded sets Red-clean; blue-dirty; white-medium