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Fuse Question

J

Jampeg

Sep 24, 2013
6
0
#1
Hey all,

New to this forum & have what's going to be a stupid question, but here goes...

I have an Ampeg SVT-3 PRO head & have never had any problems. But, I thought I'd buy a few extra fuses, just in case the one in it would happen to blow, sometime. Upon removing the existing fuse, I discovered that it's a ceramic 10A, 250V fast-acting fuse. According to the back of the amp, it takes the following:

100/120V: 125V, T10A Slow Blow
220/240V: 250V, T5A Slow Blow

All of the other amps that I've had use slow blow fuses, so I'm sure that's the way to go, with that.

I called Ampeg support & the guy said that the 10A, 250V fuse would be okay. However, I always plug into "normal" 120V power sources, not into 240V sources. So, I'm thinking that he may have misinformed me & that I need to use a 10A, 125V slow blow fuse.

But, I'm a little tentative to change the type of fuse because I've not run into any problems, thus far.

Anyone have any input for me?

Thanx.
 
#2
Forget the voltage rating. The fuse case/construction is rated to withstand 250 volts and would be perfectly proper on a 120 volt supply. The amperage and how fast the fuse responds IS the most important spec. If the breakdown voltage rating for the fuse case meets or exceeds the supply voltage it is fine.
 
#3
Okay, so if I understand you correctly, a 10A, 250V fuse would be good, right?

As far as fast-acting or slo-blo...slo-blo would be the way to go?

Please, excuse my lack of knowledge on this & I very much appreciate your fast response.
 
#4
You can use a fast acting fuse in place of a slow blow but not the other way around - you run the risk of a nuisance failure but it won't hurt anything. Slow blow are used on the primary (mains) side of the power tranny to keep from popping during startup due to inrush current.
 
#5
Yes a 10A, 250V would be fine. If it calls for slo-blo you must use slo-blo. If it doesn't say slo-blo then it would be a fast acting and using a slo-blo could allow amp damage. Using a fast acting instead of a slo-blo when called out usually causes a blown fuse (when no trouble exists).

Don't use a 12V fuse on a 120V circuit or a 125V fuse on a 220V circuit.
 
#6
It does call for a slo blo fuse & I was very surprised to see that there was actually a fast-acting fuse in the slot. I've never blown a fuse since I bought the head in 2007, so I never pulled the fuse out, before.

I only use the head for a slave. I have a QCS RMX1450 power amp that runs to an Eden D610XLT6 cabinet. I don't know if that helps the life of the fuse or not.
 
#8
The rules of thumb for fuses as I've been told are:
Double the amp capacity of a slow-blo to replace it with a fast blo, and....
If using a 250v fuse, it can be half the amp capacity of it's 125v counterpart.

Judging from these "rules", I would surmise the fuse is acceptable.
 
#15
To build on this...

I, too, need to replace a fuse in my Mesa-Boogie M6 Carbine. The Mesa calls for a 10A, 120V Slo-Blo. I'm having a hell of a time finding those, but 10A, 125V Slo-Blo's are all over the place. Is using a 125V ok? What risk of damage do I run by not using the proper fuse?
 
#16
Fender05 said:
To build on this...

I, too, need to replace a fuse in my Mesa-Boogie M6 Carbine. The Mesa calls for a 10A, 120V Slo-Blo. I'm having a hell of a time finding those, but 10A, 125V Slo-Blo's are all over the place. Is using a 125V ok? What risk of damage do I run by not using the proper fuse?
Click to expand...

Your question has been thoroughly answered in the previous responses. Read.
 
#18
I'm not sure how many more ways we can say it. The voltage rating on a fuse is the maximum voltage it can withstand in use. The one in there could tolerate a maximum line voltage of 120 volts. The plentiful replacement can tolerate 125 volts, so it would be fine. Using a fuse rated 250 volts would also be fine. Going up in voltage rating is perfectly OK, that's merely the maximum allowable voltage. The current rating must stay the same--fuses are intended to open up if a fault condition occurs which makes the amp draw too much current, so we want a fuse that opens at the same amount of current. But the voltage rating is merely the maximum permissible voltage rating the fuse is intended to be used at.

The voltage rating on a fuse is like using wire that's got an insulation rating of 600 volts in a circuit that only carries 12 volts. No problem! But you wouldn't want to use a wire with insulation rated at 12 volts in a circuit carrying 600 volts.

Got that?
 
#19
nashvillebill said:
I'm not sure how many more ways we can say it. The voltage rating on a fuse is the maximum voltage it can withstand in use. The one in there could tolerate a maximum line voltage of 120 volts. The plentiful replacement can tolerate 125 volts, so it would be fine. Using a fuse rated 250 volts would also be fine. Going up in voltage rating is perfectly OK, that's merely the maximum allowable voltage. The current rating must stay the same--fuses are intended to open up if a fault condition occurs which makes the amp draw too much current, so we want a fuse that opens at the same amount of current. But the voltage rating is merely the maximum permissible voltage rating the fuse is intended to be used at.

The voltage rating on a fuse is like using wire that's got an insulation rating of 600 volts in a circuit that only carries 12 volts. No problem! But you wouldn't want to use a wire with insulation rated at 12 volts in a circuit carrying 600 volts.

Got that?
Click to expand...

Just to help clarify for those not familiar with the terminology....current rating in this example means the fuse's amp rating.
 
#20
Fender05 said:
To build on this...

I, too, need to replace a fuse in my Mesa-Boogie M6 Carbine. The Mesa calls for a 10A, 120V Slo-Blo. I'm having a hell of a time finding those, but 10A, 125V Slo-Blo's are all over the place. Is using a 125V ok? What risk of damage do I run by not using the proper fuse?
Click to expand...

Your amp calls for a 10A 250V fuse. The 120V you are looking at is related to mains AC voltage. Thanks!
 

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