The Spider Valve is the first tube amp I've ever owned, and I head the 100w head version. Lately, it's been making this crackling sound, even when not plugged in, and earlier, one power tube was completely glowing orange. Is it just time for new tubes? And also, how difficult is it to retube and rebias the amp yourself?
One last thing: the input jack on the amp head is a tad bit loose, and I don't know if this could be causing it, but I think it's just the tubes.