I'm kinda worried and need advice.....

I think my wife is cheating on me.  I am a working musician and, as you would expect, travel a lot.  I have been noticing strange things happening when I get home.  Her mobile phone rings and she steps outside to answer it or she says, "I'll call you back later".  When I ask her who called she gets evasive.  Sometimes she goes out with friends but comes home late, getting dropped off around the corner and walking the rest of the way.  I once picked up the extension while she was on the phone and she got very angry.  A buddy of mine plays guitar in a band.  He told me that my wife and some guy have been to his gigs.  He wanted to borrow my guitar amp.  That's when I got the idea to find out for myself what was really happening.  I said "sure, you can use my amp but I want to hide behind it at the gig and see if she comes into the venue and who she comes in with".  He agreed.  Saturday night came and I slipped behind my Marshall JCM800 half stack to get a good view.  I could feel the heat coming off the back of the amp.  It was at that moment, crouching down behind the amp, that I noticed that one of the tubes was not glowing as bright as the other 3.  Is this something I can fix myself or do I need to take it to a technician?

Thanks!

Signed,

Very Concerned in the Key of G