What made the fuses blow is too much current flowing through them for a long enough period of time to melt a portion of the fuse element (or vaporize part of it).

In addition to that true but useless reply, I also offer:

Beings I don't have a schematic of your amp, I will make some generalizations.

The power line fuse, is in series with the power cord, power switch and the primary of the transformer(s)

There are usually fuses for the main positive and negative DC voltage supplies to the output stage (and maybe other stages) of the amp. Often, there is a pair of fuses per channel.

Sometimes there is a fuse in series with the output of the amp. This is not always present.

If you are playing the sustem realy loud and into a low impedance for a long time, the output fuse may blow. Sometimes, a DC voltage fuse may blow instead especially is the wrong value, and/or type, of fuses are installed.

If an amp component fails, this may cause fuses to blow. A shorted output transistor will tend to take out a DC voltage fuse. A bad main supply filtering capacitor could cause the AC line fuse to blow, as would a short in a transformer.

Old fuses will tend to fail at lower current values that what they are rated for, expecially if they have had numerous current "surges" that came close to, but not quite, blowing the fuse.

If a previous (ab)user of the amp had replaced blown fuses with any old value that was on hand that did not immediatly blow, then many of the fuses and/or amplifier circuit components may be abnormally stressed and weakened.

As to what could make several fuses blow, I'm guessing here even more. A AC line power surge (including lightening strike nearby) might do it, as would an EMP from a "nearby" nuclear explosion.

In regards to fuses themselves, in addition to the rated current and voltage ragings, there is another fuse rating: fast blow, slow blow and medium blow. This refers to how fast the fuse will react to an overload of current.

AC line fuses often are "slow-blow" to allow for a brief current surge when the amp is turned on (transformer inrush current and current surge due to the initial charging of the DC voltage supply filtering capacitors).

Let us know what happens when you replace the blown fuses and turn on the amp.

Paul

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the 1derful1

[This message has been edited by Paul J. Stiles (edited April 18, 2003).]