The Question on Everyone's Mind

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 cell 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, who is also a guitar player, 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 shows up, and who
she shows up 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 three.
Is this something I can fix myself or do I need to
take it to a technician?

Thanks,
Very Concerned