I bought a used blues jr. from guitar center a few weeks ago and it makes a humming noise when the reverb is turned up what can I do to fix that? Also how do I know when I need to change the tubes, and what tubes should I get?
The humming is from a bad reverb circut. Its not just your amp but it was a huge problem with blues jr's before 2003 (completely guesses on the date)

There are websites that show how to fix it. Just google search it.
American Strat:
Area 61 Neck, Area 67 Middle & Bridge
Spretzel Locking Tuners
Graphtech Nut & String Tree
Lots O' Love!

JSX + Orange Cab
Fender Blues Deluxe
Deluxe Memory Man
Fulltone Full Drive 2 & OCD
H2O Chorus/Echo
Oh man, mine does that too! I'd love any more info on how to fix it...
Telecaster - SG - Jaguar
Princeton Reverb, Extra Reverb
P-Bass - Mustang Bass
Apogee Duet 2 - Ableton Suite