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Even "non-audiophile" fuses give up sometimes after a period of use. I've had Buss and Littelfuses pop for no apparent reason. Replaced them, and things were fine forever after. I certainly wouldn't worry about it so long as a replacement fuse doesn't blow quickly (which might then indicate an issue with the component).
I'm now wondering if it may have been caused by static? We have low humidity here in Colorado, and frequently I get a spark when grabbing the volume control of the line stage. More so, than my other components. I'll try grounding the unit, because the power cord is a built-in two prong job. A humidifier may also help.
A fast blow fuse as a AC mains fuse feeding a presumably linear power supply (Transformer/ Diode Bridge/ Capacitor) is highly unlikely, much more common is a slow blow for this application, your original fuse will have some letter stamped into the end cap identifying if its a slow blow or a fast blow.
Kenny, Noromance may be referring to the ongoing "Synergistic Red Fuse" thread, in which several people reported experiencing false blows (fuses blowing unnecessarily) with some SR fuses in some equipment. I and others therefore suggested using SR fuses having a current rating one or possibly two rating increments higher than the current rating of the stock fuse. I have no idea as to whether or not that may be applicable in the case of HiFi Tuning fuses.
The voltage rating difference you referred to is a different matter. And everything else being equal a rating of 500V is actually better than a rating of 250V, although as a practical matter it is unlikely to make any difference in a 120V application. The significance of the voltage rating is that when the fuse **should** blow if that rating is lower than the applied voltage the fuse might not open the circuit properly (i.e., it might continue to conduct current). Or it might rupture or explode.
Also, the only time a fuse will "see" those kinds of high voltages will be when a mains fuse (or a fuse that is in some other high voltage circuit) has blown or is in the process of blowing. Under normal operating conditions the voltage between the two terminals of a fuse will be a small fraction of a volt, regardless of the voltage that is applied to the circuit.
Different manufactures have different codes for fast blow and slow blow, the letters F and T does not automatically mean Fast or Slow.
As stated earlier it is most likely a slow blow fuse thats employed as the mains fuse in you Boulder preamp but you should consult with them. FWIW I'd forget all about any aftermarket "hifi" fuses and go with a industrial grade fuse readily available form reputable suppliers like Mouser for an example.
Solved - The "T" designation is correct for slow blow from HiFi Tuning, and confirmed when I found my sales receipt. The "T" actually means timed, or the technical meaning for slow blow. I contacted Boulder Amplifiers who told me the fuse should be slow blow, so the fast replacement fuse I'm using now is not correct, even though it's been working.
Considering, that the HiFi Tuning fuse was in there for at least a year before it blew, and a fast replacement tube has been working just fine for over a week, it makes sense for me to conclude what stringreen and others have suggested, that the fuse may simply be sub-standard.
Time to order a new fuse.