Fuse question for jcm800 2204 type build

[Champ_49]

Hi all.
Sorry. I edited this again as I realized my last posting was really confusing.
So on the jcm800 2204 layouts I see this.
2A - 240V
3A - 120V

I misinterpreted it but can someone tell me my interpretation is right?

So if my amp is operating at 240V i'd use a 2A fuse rated for 250V
If my amp is operating at 120V I'd use a 3A fuse rated for 250V

Sorry.  I confused myself before.  But is this right?

My amp is in 120V operation so would it be safe to use a fuse at as low as 2A for better safety? Would this blow the fuse?  It's better to use a fuse at lower rating vs one at a higher rating in amperage. Is this right?  If it blows no harm I guess.

My next question  would be why do some people use slow blo  fuses.
Are these used for the mains only? Or for the HT as well.  Wouldn't a fast blo fuse be better to protect the components vs a slo blo fuse?

If anyone can enlighten me in this please.

Jay

[jjasilli]

Please post a site to the layout.


Slo Blo:  From Wikipedia: "A fuse interrupts excessive current ("blows") so that further damage by overheating or fire is prevented.  .  . Overcurrent protection devices are essential in electrical systems to limit threats to human life and property damage. The time and current operating characteristics of fuses are chosen to provide adequate protection without needless interruption. Slow blow fuses are designed to allow harmless short term currents over their rating while still interrupting a sustained overload."

Tube amps have normal over-current situations at start-up: 

*  For a fraction of a second until the filter caps "charge",  they are a short circuit to ground.  A short circuit = near Zero resistance which causes Infinite current (amperage) to flow. 

*  Also tubes have "heaters" = filaments just like in a light bulb.  The filament offers resistance = Load, which prevents a short circuit.  But the filament, like all resistors, has much more resistance when Hot than at room temperature.   I.e., a cold filament, until it gets hot & glows, has too little resistance.  Hence it allows too much current to flow (Ohm's Law) until it gets hot & glows, and rises to the proper amount of resistance.

A slo blo fuse is needed to allow these normal over-current conditions to briefly occur and pass.  A fast blo fuse of the correct current rating would blow before the amp could achieve normal operation at turn-on.

[alerich]

My amp is in 120V operation so would it be safe to use a fuse at as low as 2A for better safety?

It wouldn't technically be unsafe but it might cause you to waste 2A fuses unneccessarily. This design calls for a 3A mains fuse at 120VAC input. You should use the correct fuse. There are literally probably millions of this design (both Marshall amps and clones) in use in the field and operate without issue with a 3A mains fuse.

It's better to use a fuse at lower rating vs one at a higher rating in amperage. Is this right?  If it blows no harm I guess.

Unless it blows in the middle of a live set or an important once in a lifetime take in the studio. It's always best to use the size and type fuse that the design calls for. I don't know that blowing an under-rated fuse will do any more harm to the amp than simply shutting the amp off. It can be a hassle real quick, though.

My next question would be why do some people use slow blo  fuses.

For the same reason some people wrap a blown fuse with tin foil and stuff that into the fuse holder. They either don't realize how dangerous it is or they do realize how dangerous it is and they are idiots. They likely have an issue with the amp that is causing a properly rated fast blow fuse to blow so they insert a slow blow and risk doing more damage. *Always* carry spare fuses in your gig bag of the correct rating for the gear you are using. Sometimes a fuse can blow in a properly working piece of gear for no reason. Doesn't happen often but it can happen. That's why the first troubleshooting measure is almost always to try another fuse... unless you smelled smoke or something.
 

Wouldn't a fast blo fuse be better to protect the components vs a slo blo fuse?

Yes. There are situations and designs where a slow blow fuse is the correct type... just not in this case.

[Champ_49]

Hey thanks everyone for the replies. I forgot to click the notify box for any replies and didn't realize I got any replies.

Yup. My  amp (based on a jcm800)

calls for 3A fuses at 120V. I was actually very confused because I based the amp on a triode electronics layout which said I need 4A fuses. But I noticed on all other layouts it says 3A at 120V.

http://www.triodeelectronics.com/50wmarshkit.html

This is the layout that says 4A. Maybe it's a typo?

Hence the confusion. Should I just follow the 3A?

If it is a typo it should be fixed. 

[sluckey]

The old wiring layout at the triode link you provided calls for 120V/3A mains fuse.