Is this a joke?

[Graylion]

An oldie but goldie I think - sorry to those who have heard it many times before!

"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 vintage Marshall JMP MKII 100W 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?"
Cheers, Lionel