I've seen 250V fuses explode with 800V when they opened and the arc made the glass explode. There are other ways of dealing with this problem. 1. a fuse in the center tap of the filament transformer. 2. a 25 or so ohm glitch resistor in the B+. 3. low wattage low value resistors from the grid to ground bypassed with a cap that is rated for the B+. Not only is what you did bad engineering, if that fuse should fail while the cover is off someone could be blinded by flying glass. It is also going to leave scorch marks on the surfaces around the fuse holder.