Amp fuses

SimpleSimon

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?

merlin

Not sure about tech help and why the fuse has blown in the first place, but try Maplins for a replacement. 

ICBM

You need 20mm x 5 mm fuses for most modern amps. Be sure to check whether it's an F or a T fuse you need, as well as the value. Don't use the wrong value or the wrong type.

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.

Gassage

No worries- just wrap it in silver foil and carry on.

(keep a fire extinguisher handy...)

SimpleSimon

ICBM said:

You need 20mm x 5 mm fuses for most modern amps. Be sure to check whether it's an F or a T fuse you need, as well as the value. Don't use the wrong value or the wrong type.

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.

Cheers @ICBM ;- how do i know if its an F or T fuse?

ICBM

SimpleSimon said:

Cheers @ICBM ;- how do i know if its an F or T fuse?

It will say on the amp next to the fuse holder, along with the value. If for any reason you can't find it, have a close look at the old fuse - it will be written/engraved around the side of one of the end caps.

DJH83004

A bit of useless info regarding fuse designations - probably my poor German, but F type are fast acting (from the German Flink = nimble), and T are slow acting (from the German Trage = sluggish), I was taught this many years ago, when working with Siemens - but it may be absolute rubbish and they may have been winding me up   

ecc83

DJH83004 said:

A bit of useless info regarding fuse designations - probably my poor German, but F type are fast acting (from the German Flink = nimble), and T are slow acting (from the German Trage = sluggish), I was taught this many years ago, when working with Siemens - but it may be absolute rubbish and they may have been winding me up   

"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.

ecc83

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.

ICBM

ecc83 said:

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.

I agree, fast-blow fuses are often a problem. One reason they're sometimes used is in things like the Fender Hotrods, which have no secondary fusing, so the primary fuse has to do both jobs - not sure how they get away with that under EU regulations, and it's bad practice anyway! If the user fits a T fuse the amp will often blow the screen resistor if a valve fails, before the fuse goes.

ecc83 said:

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!

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.

jpfamps

ICBM said:

ecc83 said:

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.

I agree, fast-blow fuses are often a problem. One reason they're sometimes used is in things like the Fender Hotrods, which have no secondary fusing, so the primary fuse has to do both jobs - not sure how they get away with that under EU regulations, and it's bad practice anyway! If the user fits a T fuse the amp will often blow the screen resistor if a valve fails, before the fuse goes.

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.

ICBM

jpfamps said:

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.

It's just such a bodge to save to cost of an HT fuse, which would not only solve all those problems, it would protect the amp much better.

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.

jpfamps

ICBM said:

jpfamps said:

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.

It's just such a bodge to save to cost of an HT fuse, which would not only solve all those problems, it would protect the amp much better.

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.

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......