Can I use a Quick Acting Fuse for the mains?

[exztinct01]

I recently looked for a slow blow fuse from a local electronics store. I plan to use it for the mains part of the PS for an LM1875 power amp. Well, they don't stock it since it's not commonly purchased by people here. So, can I use a quick acting fuse instead? I bought the 5SF 1-R by BelFuse and plans to put it in the mains and the +/-rails of the amp.
Also, in Rod Elliot's Project 72, the switch comes before the fuse. My IEC Connector already has a fuse housing so can I do it the way where the fuse comes before the switch then?

I'm a little bit worried though since all I have are quick acting fuses.

[DrGonz78]

IMHO you should search some more for a source to buy the slow blow type fuse. However, if your design calls for a 1 amp Slow Blow and you install a 1 amp fast acting then the worst that happens is it blows the fuse. The current surge of the mains is inevitable at start up of an amp as there is a delay in the power supply (think filter caps). Perhaps the fast acting type fuse will not hold up to repeated current surges at start up of the amp over time. The point is that the designer says slow blow and there might be a good reason for this specification. Then again MANY solid state type amps use fast acting fuses on the primary side of the circuit with no trouble. Just don't increase the current rating of the fuse to compensate for the power on current surge, instead get the right kind of fuse.

[J M Fahey]

Not disagreeing with DrGonz78 sage advice, which is correct, I can speak from a practical reality similar to ecztinct's (which *should*  have stated *where*  he lives  :trouble , for better answers ) : outside USA/UK slow blow fuses are not popular, so simply not carried by regular Electronics suppliers, as simple as that.

It's not that they *can't* , simply that "nobody asks for them" so it's wasted capital sitting on the shelves.

Under that reality, first choice is to use the common (fast)  fuse available of rated value and give it a try; if it survives 30 or 40 turn-ons it's approved; if it nuisance blows, then next higher value is used.

Another pet peeve: I do not like +/- rails fuses, if anything I fuse AC secondaries individually: *one* rail fuse blown= guaranteed DC on the speaker 

[exztinct01]

thanks Juan and DrGonz, I was a bit worried I won't get a reply here

[exztinct01]

Oh I forgot, I'm from the Philippines
Not a great place to find parts if you're an electronics hobbyist

[J M Fahey]

Yes, IF you you are going to fuse the secondary, do it between transformer 18V windings and bridge rectifier diodes.
Always the "hot" wires, never the center tap, in general never fuse ground(s).

That said, since all power is being used by the 18+18VAC winding, no need for secondary fuses, the primary one is enough.

European safety rules demand individual fuses in multi secondary transformers, say +V and filaments, sometimes even bias or SS preamp feeding ones, for the very good reason thay a short in a relatively low power winding will not blow the main fuse, only that winding will overheat and eventually may catch fire, all without blowing the main one.

But I guess that is not your case.

As of always stating the Country, it's important because proper answers often depend on that.

Sending you to Craigslist, ordering parts straight from Peavey or Mouser, from Fender or Marshall through a dealer, buying a used amp for peanuts at a garage sale or cheap from Guitar Center, etc, won't exactly help you 

Same here (in Argentina)

[exztinct01]

i just found this out http://ph.rs-online.com/web/p/cartridge-fuses/5413546/
items are a bit pricey though, but for the free delivery, I can't complain

[J M Fahey]

Pricey?? 

10 fuses for *one*  dollar and 25 cents?   :loco


:lmao:

[exztinct01]

[exztinct01]

I know this is very far from the original topic but i don't think a new thread is needed. I've seen dpst switches for the mains of the ps before it go to the traffo, is it better than an spst only in the hot wire?