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
All Fender® Tube Amps come Factory Equipped with Groove Tubes. They are much easier to replace (no re-biasing needed when replaced with color coded sets). They have improved harmonic balance, longer lifespan, matched gain to distortion ratio, plus a wider frequency response.