In general, use the smallest size fuse that does not blow often to protect your speakers and amplifier.
A good guideline would be no more than 2 amp quick blow fuses for smaller two-way speakers, 3 amp for modest 3 way designs, and 4 amp for rugged 3 way designs or more. Ramp them up one amp at a time if the starting points blow too often in your system and never use a value higher than necessary.
Note that a 5 amp quick blow fuse will pass a sustained 200W. Note also that 200W can be experienced by wiring two 100W light bulbs into a small metal box, running the setup for half an hour or so, and then placing your palms firmly on top of the metal box. While getting your third degree burns treated, you can then contemplate whether your speaker voice coils would be happy with this too.
The speaker fuses must be quick blow type (never slow blow type) or your speakers will blow first, saving you the annoyance and expense of replacing fuses.
Main power supply fuses (those ahead of the power transformer) in general will be slow blow types. This type will stand up under the sudden over-current surge at turn on and still protect the system from sustained overload.
Most amplifiers (such as ours) also have internal fuses between the power supply and the audio circuits. These are kind of a last line of defense in case the speaker fuses are made too large. We in general size these B+ and B- fuses to pass full power into low impendance loads. We use larger fuses for this application so that a normal overload will blow the speaker fuses first to avoid the annoyance of having to remove the cover and replace internal fuses.
Most fuses used in US made equipment will be 3AG type. This is not the fuse value, this describes the size and shape and dimensions of the needed fuse. A typical overall specification would be "4 amp quick blow 3AG series fuse". Most replacement fuses will be available at Radio Shack.
In case you have a problem with your equipment check the fuses first. You just cannot believe how many units have come back here "for repair" just because the owner neglected to check the fuses first or did not call us for help before shipping. Yes I know it is "inconvenient" to check all the fuses, but we would suggest it is a lot more inconvenient to ship the equipment back and forth just to have a fuse replaced.
We also suggest that you acquire and learn to use a simple low cost digital multimeter. Ceramic body fuses do not provide a visual indication that they are blown and often a fuse that has failed very gently will only have a small parted part of the internal element. This can be hard to see. With a meter it is easy to tell a good fuse from a bad one. The good one will read near zero ohms, a blown one will read very high.
I won't get into the issue of good sounding fuses other than to mention that blown ones never sound good.
I hope this helps.
Frank Van Alstine
