I have noticed that some of my tube amp power tubes are glowing slightly blueish. Why? It has been explained to me that this only means that the tube is ok. However, I find it fascinating! Because what is this blueish glow, really? To me it sounds like secondary emission of either the anode or the glass envelope. It is either this or some residual gas that exists inside the tube. But, with regard to what has been explained to me, I do not believe in this last part. Do you physics experts perhaps have any explanation of this amazing phenomena?