I've always wondered about the T type fuses with seemingly tight Quick
type but diagonal fuse-wire and a blob in the middle, compared to the
more obviously forgiving ones with a spiral wind over a core or a spring
like part of the wire.
Laney VC30 valve amp of 1996 with a blown HT fuse. I cannot confirm with
the owner until tomorow whether it failed at switch on and have so far
not tested the valves. Barely noticable small white patch on the glass
barrel of the fuse, so soft blown. At leat 2 years of useage most days
and all valves no fading of the markings . 4 x EL84 on the HT. The
manual specifies T315mA spiral wound but the failed one is the straight
form unless there was a tiny springy bit in the middle , evapoated,
rather than the usual springy bit at one end. Remnant wire is still
diagonal , just 2mm missing in the middle.
So replace with a T315mA spiral one and not go up to 500mA? assuming
the valves test ok