Slow and Fast blow Fuses

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Slow and Fast blow Fuses

Postby John Martin » Thu Jul 26, 2007 10:03 pm

Whats the difference between the fast and slow blow fuses? Where do you apply it in the circuit? Where is it recommended? Amps, preamps, sources?
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Re: Slow and Fast blow Fuses

Postby dannyb » Thu Jul 26, 2007 10:59 pm

John Martin wrote:Whats the difference between the fast and slow blow fuses? Where do you apply it in the circuit? Where is it recommended? Amps, preamps, sources?

slow blow fuses are usually used in tube amps were current draw can momentarily exceed the current rating while solid state circuits need fast blow fuses to protect them against accidental shorts, this is as far as i've observed in different tube & solid state amps
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Re: Slow and Fast blow Fuses

Postby tony » Fri Jul 27, 2007 6:08 am

John Martin wrote:Whats the difference between the fast and slow blow fuses? Where do you apply it in the circuit? Where is it recommended? Amps, preamps, sources?


the answer is there, "fast, and slow", :D

a "fast blow" fuse rated 4 amps, could be a "slow blow" rated for 8 amps....

fuses are used as a means of preventing fires in case of equipment malfunctions...

a fuse rated 1 amp may pass current several times that amount, all depends on the time duration, say 2cycles of 60hz current.

fuses are deployed in series with the primary of the power transformer, and sized according the anticipated power draw of the appliance...

for example, let us say we have a stereo amp rated 250 watts per channel, therefore transformer primary va vould be in the vicinity of about 700, so therefore needs around 4amps fast blow, but since the same amp have filter caps that need about 200 milisecs to chage up, then a 6amp slo-blow fuse can be used. :D
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Postby joe3rp » Fri Jul 27, 2007 7:50 am

As per my experience

slo-blow/slow acting/time delay fuses - used mostly in the power supply side. This is because the huge turn on surge to get the power supply capacitor charge. Though this only takes milli-seconds, this is characterized by your light flickering when electrical equipment are turned on.

quick acting or "ordinary" fuses are usually connected at the output side or the speaker side.

Quick acting fuses though may be used in the power supply side, but may need to be uprated(increased in value) to compensate for large surge.

One will note that sometimes when one turns on an equipment (which normally flickers the light) that there is no flickering. It is because the perfect switching has occurred, this is when the switch makes in the zero cycle of an AC wave.
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Postby JackD201 » Fri Jul 27, 2007 10:40 am

I love this place! :)

Mods please move to Technical Archives for members' future reference.

Thanks :)
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Postby arnoldc » Fri Jul 27, 2007 10:45 am

moved as requested...
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Postby 6a3fan » Sat Jul 28, 2007 8:05 am

My question is where can I buy slo-blo fuses? My tube amps require slo-blo and I couldn't find them at DEECO and Raon.

I don't quite agree that an 8 amp quick blo fuse is a 4 amp slo blo. Perhaps momentarily, but a constant current of 5 amps will blow a real 4 amp slo-blo, but will be dangerously passed by the 8 amp quick blo.
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Postby John Martin » Sat Jul 28, 2007 8:40 am

you can buy specialty fuses at www.rsphilippines.com, or walk in to their ofc at 21/f Multinational Bancorp bldg. Ayala Ave. 3days delivery coz the stock comes from Singapore.

Or www.farnell.com, ofc at Perea St. near greenbelt mkti
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Postby tony » Sat Jul 28, 2007 9:05 am

6a3fan wrote:My question is where can I buy slo-blo fuses? My tube amps require slo-blo and I couldn't find them at DEECO and Raon.

I don't quite agree that an 8 amp quick blo fuse is a 4 amp slo blo. Perhaps momentarily, but a constant current of 5 amps will blow a real 4 amp slo-blo, but will be dangerously passed by the 8 amp quick blo.


you don't have to agree, your statement itself says so... :D

slo-blow or fast blow, the current is never constant, but varies as the ouput to the speakers varies....

in my 40 years of dealing with fuses, i have found them not such a big deal, for a fuse originaly rated 1 amp, replacing it with 2amps, still the fuse can blow... more so with solid state gears.

yet there are times when a fuse rated 3amps could be replaced with a 2amp fuse with no detrimental effect, you can get away with it...


fuses as i have said earlier is there to prevent fires in case of malfunctions, for SS gears, they can not be relied upon to protect the semicon devices in your amps specially the power transistors, even with fuses they can still fail.
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Postby rascal101 » Wed Jan 02, 2008 6:34 pm

If you are using a 2A fuse on a 220V outlet you then theoretically use a 4.4KW appliance. What this simply means is that you can get away using a 2A fuse as long as your amp is rated 4.4KW or less.
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Postby rascal101 » Wed Jan 02, 2008 6:48 pm

Just don't power up your 4.4KW amp instantaneously. You need slowly power up using a variac. This way you will not exceed rated inrush currrent of the 2A fuse.
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