Join Date: Jun 2002
Location: Victoria, BC, Canada
Well this depends on how much power you have the amp set up for and what it'll draw at idle. Generally, you should pick a value that will blow the fuse under a serious condition but also that will facilitate the start-up and then idle currents of the unit.
Lets say you have an amplifier that draws an inrush current of 6amps for a few milliseconds and then idles at an amp or so. At full power output it draws say 3amps. If you use a slow-blow fuse, you can use maybe 4 or 5 amp fuse. The inrush will exceed the current of the fuse, but being a slow-blow, the inrush will be too fast to blow it. However, if the amplifier shorts out, it will run long enough to kill the slowblow.
In a tube amplifier, you don't really want a slow-blow unless it's a really big amp.
In your case, find out even a ballpark value for the amps maximum current draw and use a fuse just above that. I believe there are some mathematical formulae you can use so we'll see if somebody more knowledgeable about fuses comes with ideas.