I pulled a dead fuse from a box I recently picked up. It's rated 125v, 2.5A, 20x5mm. These are easy to find but I have a choice between fast or slow blow. Which type would have been OEM on an 80's boombox?

My 2 cents worth...:

First and foremost - a blown fuse means you HAVE A PROBLEM - so check and eliminate the problem and hope that the next fuse will blow as well instead of the circuitry it protects ...if the problem recurs.

Normally as far as amplifiers are concerned old tube based amps had start up surges that would knock off normal fast blow fuses and they had slow blow fuses instead.

Slow blow fuses are normally used when you think you'll have surges/load differences or if your fast blow fuses blow 'too often' under normal 'safe operating parameters' where a slight load tolerance is required.

A fast blow offers instantaneous protection 'come what may' whereas a slow blow fuse 'understands' that there will be a few surges/load and only blows after probably 2 dozen such surges - it has to be fatigued to death as against the instant kill of a fast blow.

I don't know if really blasting music at PMPO counts as a 'surge' worthy load on your amp unless you have some odd habit of disturbing your neighbors by rapidly twisting your volume knob from zero to max in cycles

More important would be to use one of those (belkin/similar) surge protector power strips to safeguard against incoming voltage faults.

Hope this helps...don't blow your fuse