I have a 100 watt 1975 traynor that seems to have lost some clean headroom and when I dig into the strings I notice the tubes change to a blue kind of spark inside the tube. All power tubes do this at the same time. I just had the amp in to Gar gillies of Garnet amplifiers before he died R.I.P. and he checked the bias. Is the change of color power tube saturation or a bias problem or the fact my tubes are about 6 years old?