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So i got a blown fuse the other day on an amp head, thought it would be a quick fix as I have a box of new fuses laying about, only to discover I needed a much smaller fuse to fit into the holder - almost like a car fuse. Is this the norm? Where can I purchase these from?
Secondly what is the most likely cause of the blown fuse in an amp? - I appreciate the reasons for this could be endless without testing in person, but what signs should I be looking for?
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The most likely cause is a faulty valve, if it's a valve amp. These can sometimes be intermittent, so you might replace the fuse and all appears well, and put it down to a 'bad fuse', but then it will blow again sooner rather than later. Actual bad fuses are *very* rare, although not totally unknown. If the blown fuse is blackened inside it's a pretty sure sign of a fault in the amp, if it's just clear with the wire melted it *may* be a bad fuse, a power spike in the supply or something else.
As merlin said, Maplins have fuses if you can't find what you need cheaper online.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
(keep a fire extinguisher handy...)
*An Official Foo-Approved guitarist since Sept 2023.
Cheers @ICBM - how do i know if its an F or T fuse?
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
"Trage" is certainly correct. Not so sure about 'Fink'? I have always assumed 'Fast'.
I have in fact some reservations about fitting F fuses, especially in 'knock about' valve gear? The vast majority of fuses found in equipment will be of the T (sometimes called 'delay' or 'anti-surge') type. The result is if an F type blows the punter is unlikely to have one handy and will fit a T. If the amp still has a fault that NEEDS a fast acting fuse to protect it, it won't BE protected.
My experience with a certain 200W amp was that in the event of an OP valve flash over (The Purple death of a crap batch of KT88s) the FAST anode supply fuse failed but also the T in the traff primary AND the 13A in the plug! What use then a special fuse? Fortunately a set of good OP valves and things were all happy again (but you would be buggered for a round at the pub for a while!)
But it has to be said, 'fusing electronic equipment is NOT an exact science and a crude form of protection at best'. While our mains CAN go up and down by 10% 'twill ever be so!
Dave.
Perhaps I should add that you should ALWAYS replace a fuse with the EXACT type and rating!
If you still get random, nuisance popping, get onto the equipment manfctrs, there might be an uprate that has been officially tested and approved.
Dave.
I'm fairly sure that making fuses with an exact current failure rating is quite tricky too, and the tolerance is quite wide, something like +20% or maybe even higher - but -0%, if I remember correctly... in other words the fuse should theoretically never blow at *less* than its rated current.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
The regs stipulate that the mains fuse should blow within a certain time if a secondary winding shorts, else the secondary winding should be fused.
The problem with valves amps is the 6V filament supply, as shorting this will not blow an appropriately rated time delay fuse, so Fender us a fast blow fuse which also requires a MOV in rush current limiter otherwise the inrush current will blow the mains fuse on turn on.
What's utterly ridiculous is that on some amps, they fit a completely unnecessary filament fuse, but *still* not an HT fuse… with this result… (which I think you've seen before)
When I fixed this I re-wired the filament fuse holder as an HT fuse so this will not happen again.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
Quite. The LT fuse is there only because the mains fuse doesn't blow fast enough when the LT is shorted in that design.
Regs are regs though......