So I've had my fender bassman 500 head for about 6 months now. (Purchased new) I was practicing with my band last night and suddenly about 45 minutes or so in to playing, my volume cut in about half! Never had any issues like this before. I turned the amp off for a minute and then fired it back up and all was fine for the next 20 minutes or so of practice. Could this be a tube issue? I know it's under warranty but I don't know any local places that I can't take it to get serviced. And I'm fine with replacing tubes if need be. Any educated opinions? I haven't really heard of anyone having issues with these amps so I'm curious what I can find out from my fellow tb'ers.