[Gibson GA-50]

Just got an old Gibson Ga-50 Amp from my grampa with two 6l6g tunes one 5v4g rectifier and two 6sj7 with two 6j5 preamp tunes.  The Amp only makes a loud humming sound.  Any idea how I can fix this up or any info at all would be great . Do I have to bias the tube replacements ? Are there any more reliable modern replacements I can use ? The thing is old as super dusty. Thanks 

[Gibson GA-50]

My grandpa had an old gibson ga-50 amplifier from his childhood. His brother was the original owner. The thing squeals a bit when plugged in. Do I just start to replace power tubes trying to get it back to playing?  Thanks !