"Letter From A Guitarist" joke

Letter from a Guitarist to the "Dear Abby" help column in a newspaper.

Dear Abby
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 need to take it to a technician?
Thanks - Very Concerned.