Have a tube that gets really bright when turned on and then dims quickly

I have two 6922 tubes on the top of my preamp, and one of the tubes gets very bright at the base of the tube when I turn on the preamp. It doesn't light up the whole tube just the lower part of it.  It dims fairly quickly and then it looks just like the other tube.  I switch the position and that sudden brightness follows the tube.

The tube sounds OK and it's fairly new. 

It just seems kind of weird, wonder why this is happening. Anyone have any ideas?