Popping sound

Is the popping sound loud? If so, then it also could be that your power amp gets too hot after a while. Years ago I had that problem. I could solve it by removing the top plate of the amp, so it had some extra cooling.

Hopefully its something simple and/or external to your system. Tube starting to fail would be simplest and most easily resolved issue if issue is in system. If popping doesn't resolve itself via any of the methods suggested above I'd not continue using tube pre or amp, these things don't go away by themselves, continued popping could lead to much larger failure. 

I finally switched right and left tubes in the preamp and the popping seems to have gone away all together.  Maybe just toggling them was all that was needed.  
 Thanks to all of you who suggested this.