One tube doesn't glow blue

I have an Electra Dyne 1x12 combo. I noticed a while back that all of the power tubes were glowing blue except for one. Looking at the back of the amp, it is the one third from the left (one of the inner pair). I just assumed it was a problem with the tube, and since I always play the amp in 45 watt mode I didn't worry about it. The amp functions properly and sounds fine in both full 90 watt mode and half power 45 watt mode. I just recently bought some new power tubes and when I looked at them running, the same tube wasn't glowing blue. So I switched tubes around and still the third from the left tube wasn't glowing blue. Again, the amp sounds ok, although there is some crackling type noise emanating from the speaker. I've had this noise pretty much since I bought the amplifier. I bought it used from Guitar Center. I have registered it with Mesa/Boogie, but I'm sure it's out of warranty by now.
Is there an easy fix for this, shorted tube socket, resistor, bad solder joint, etc.?
Also, I got to thinking since this is a Simul-Class amp it might even be normal operation. Any ideas?
Thanks!!

Normally I would say that is probably normal. But after switching tubes and seeing the same effect, I wonder if that socket has a bad connection or if the screen resistor has gone. Should be a quite inexpensive fix for any tech to look at it and repair.

Thanks for the reply. That's what I'm thinking too. No matter what tube is in that slot, I've swapped em around, put in EL34's, etc., it's always the tube in 3rd socket that doesn't glow like the others. The same tube will glow in any other socket.

Check with the lights out and a dark room. If you can see a blue halo or glow near the center of the tube it is working (you may need to remove the protective cage to get a good look between the plates of the tube in question. If you had a tube overload or red-plate, it is possible a screen resistor may have been damaged (this would have also taken out the fuse too.) I believe the ElectraDyne is a Simul-class circuit. One pair run in Class A/B and the other is a form of class A. If you can see a hint of blue around the cathode at the center of the tube it is fine. If all you see is the heater glow and nothing else, the tube may be off.

One easy way to find out if that tube (location) is not working, the tube in the suspect socket location will be much colder than the others. It will be warm due to the heater but not so hot that you cannot touch it. Use caution and some common sense and just grab the glass! If it is cold, its mate will be extremely hot.

I would have the amp looked at by a Mesa Tech.