Hi, I just installed a quad of Sovtek 6L6WGC/5881 tubes in my Mesa Dual Rectifier. I am noticing some blue glow from deep inside the tubes. I am somewhat used to seeing some light blue glow on the plates, which I understand is normal. This time, it is the vertical rods located deep inside the center of the tube that are glowing blue. Is this okay? Any info would be appreciated. Thanks!