Is there supposed to be a hiss from a tube amp without anything plugged in?

I think one of my tubes might be blown, im not sure. I have yet to test it out again, i opened it up and checked the large gl tubes, and i noticed that the prongs werent pushed in all the way on both of them so maybe that was the problem. I tried turning it on and there is a fairly audible hiss coming from the speakers, is this normal for a tube amp? its a peavey 5150 combo