I purchased a quad of SED EL34. They maybe at the most have 10 hours of playtime on them. Today I noticed that the 2 center tubes are glowing a dark blueish color. 1 tube at the top and the other down near the base. It looks almost like someone is holding a lighter to the tube and you see the reflection of the blue flame. I dont notice any tone loss at all. They sound fine. I also noticed that when I turn the standby switch on the blue glow goes away, but as soon as I turn the switch off the blue come back. Is this normal or do I have a problem with these tubes?