Fuse problem:cannot understand what i'm missing


Hello fellow Audiogoners.I need your help.
A year ago i replaced a stock fuse on my Krell SACD mk3 (slow blow 500mA 20 mm (small)) with Hi-Fi Tuning fuse.
I threw away that Krell fuse thinking that i can always buy one at Radio Shack.I did some changes in my system and decided to try how Krell would sound with a generic fuse through my system.I went to Radio Shack and bought a 4 pack of Slow Blow fuses (T 500mA 250V).They all burned down one by one as soon as turned the power switch on.Everything is OK when i put back Hi-Fi Tuning fuse with the same specs.The fuses that burned down were slow blow fuses,500mA .I cannot understand why they are failing but a Hi Fi Tuning fuse keeps working without a problem.Are there any other specs that i'm missing?Please,help me solve this dilemma.Thanks a lot!
overhang
Overhang, I buy my fuses from Automation Direct. I am sure that they carry whatever you need. If anything , it's another source.

http://web4.automationdirect.com/adc/Home/Home
Are you sure you have the correct fuse? Perhaps the package is mislabeled, are the fuses marked 500mA SB? If it's a SB it should be a coil of wire and not just a straight piece of wire between the 2 ends.

I've bought a lot of fuses at RS over the years and never had a problem. They don't make them so it is highly unlikely that their definition of slow blow is different than others. Something else is going on.
The "T" instead of "SB" probably means "Time Delay".

The delay characteristic may be different from the fuse supplied by Krell.
I went through the same thing with Radio Shack "slow blow" fuses. They are not slow blow. The construction looks like regular fast blow fuses and "fast blow" they do!!

I ended up ordering from Mouser.