HT stands for High Tension, the fuse will be in line with the highest B+ supply line. Usually there is a turn on surge of current so they use slow-blow fuses. As they are marked S.B., a fast blow will likely not handle the turn on surge.
Check that fuse you took out of the HT spot, if it is really a 0.5A, I don't think it would power up the amp like it did.
Check that fuse you took out of the HT spot, if it is really a 0.5A, I don't think it would power up the amp like it did.