Normally, power tubes can last many years at daily use, but over time they lose performance. Signs of worn-down tubes: Muddy bass response Lack of treble Loss of power Changes in volume Higher noise level Rattling noise (mechanically unstable/microphonic tubes) Note, that correct bias adjustment for power tubes are essential to their life-span and the sound of your amp. Bias too hot: Power tubes wear down quickly. Bias too cold: Cross-over distortion and muddy bass response, but less wear on the power tubes. Adjusting the bias might be adequate, so you don’t need to replace the power tubes. If a power tube fails, it often shorts the plate to the cathode and blow the HT fuse. In this case, it is recommended to replace all power tubes. The ROAR – Quad 15 is built with a short circuit-proof power supply, so you don’t need to replace any fuse if a power tube shorts.