Yea check the inside but be careful as supposedly some components inside carry potentially deadly charges even when the amp is unplugged!! I went poking around inside mine about 2 years ago before I knew about this, then posted about it online and more than 1 person told me that I was lucky to be alive.
most places and people will tell you that amps hold lethal voltages because they don't want to deal with legal repercussions if they get sued. Truth is, you would have to be trying to intentionally electrocute yourself before those voltages would become lethal. For starters, touching the chassis or ground with one hand and probing around with the other hand when you left the amp still plugged into the socket is a quick way to land you in the morgue, but like I said, that is really asking for it.
Keep the amp unplugged, put your non-dominate hand behind your back and only touch the amp with one hand, being sure to not touch a ground with any part of your body and even if you do hit yourself with a live cap, it would take in excess of 2000-3000v before you would have to worry about receiving lethal amounts of current to your heart, way beyond what a tube amp stores. At most it will scare the shit out of you, leaving your arm slightly numb, and you either laughing or pissing yourself.
And a little fun fact with 5150's, if you engage the standby and wait for the green standby LED to go completely dim before you power it down, the caps drain all of their potential and the amp becomes completely safe to poke around with. Still if you question that something is live, test it with a multimeter first, if you get 0v, then you know its safe. I always test caps that are rated over 100v with a multimeter before I start working on anything, because you can never be too safe.
