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