Its common these days to use flameproof resistors in certain locations, usually the screen and cathode circuits), to prevent damage to the output transformer. If the tube conducts too hard, it can exceed the wattage limit of the resistor, causing it to fail just like a fuse.
Of course, why not just use a fuse, which has the advantage of being user-replaceable? There is the argument that a fuse might cause the circuit to not sound as good, but IME I've not seen that argument hold up in practice. Our Novacron amplifier has individual fuses for each power tube, and in studies we have done we don't find that the fuses or lack of them is any issue. But reliability certainly **is**.
Of course, why not just use a fuse, which has the advantage of being user-replaceable? There is the argument that a fuse might cause the circuit to not sound as good, but IME I've not seen that argument hold up in practice. Our Novacron amplifier has individual fuses for each power tube, and in studies we have done we don't find that the fuses or lack of them is any issue. But reliability certainly **is**.