Fuse values ???

[peterjh85]

I blew a fuse in my AC30 last night, and I couldn't find/buy a 4 A, 250 V fuse to replace it with. All I could get my hands on were some 4 A, 125 V fuses. Would this be fine or should I try and find a 250 V? Thanks!

[strat a various]

As far as I know, amperage burns fuses, not voltage. 125V ought to be fine, that's what I have. If I'm wrong after all these years, I wish one of these electricians around here would tell us.

[WaltC]

Since I=E/R (I = amps, E=volts, R= ohms resistance) a 4A 125V fuse will have a difference resistance than a 4A 250V fuse. So they're not the same. That said, all the fuses that I use for guitar amps (and pretty much anything electronic), and all that I've seen at Radio Shack <G>, are 250V fuses.

So get a 4A/250V fuse to replace your blown one. And, find out what blew the last one...

[donnyjaguar]

Walt, with all due respect that has nothing to do with it. The voltage rating is only for the voltage that would appear across the fuse after it blew. Look at it from an extreme, for instance, a 1A 12V fuse mounted to a PC board wouldn't be a good choice for a 1A, 1,000Vdc circuit. So if Peter's amplifier is used with 220Vac mains then it would be a good idea to find a fuse rated for that (or higher). If he's running from 120Vac then its perfectly fine.

[peterjh85]

the amp will only be seeing 110 V. I won't ever be plugging it in outside the US. So it sounds like it should be okay with the 125 V fuses.

[mike80]

Walt, with all due respect that has nothing to do with it. The voltage rating is only for the voltage that would appear across the fuse after it blew. Look at it from an extreme, for instance, a 1A 12V fuse mounted to a PC board wouldn't be a good choice for a 1A, 1,000Vdc circuit. So if Peter's amplifier is used with 220Vac mains then it would be a good idea to find a fuse rated for that (or higher). If he's running from 120Vac then its perfectly fine.

I think what Walt is trying to say is that a 4A 125V fuse will have lower resistance than a 4A 250V fuse, which might not cause the fuse to blow in case of a future component failure.

[schmidlin]

AC30 fuses are WAY overrated (especially for US). If it blew it you have a shorted tube or trannie IMHO.

And yes, current is what matters for fuses.

Keep us posted!

[thilton59]

If a 250volt was in it, and is what is supposed to be in it, then keep that value in there. And find what caused it to blow.

[donnyjaguar]

I think what Walt is trying to say is that a 4A 125V fuse will have lower resistance than a 4A 250V fuse, which might not cause the fuse to blow in case of a future component failure.No, Ohm's law doesn't factor into it. The fuse is only concerned with current. The voltage passing through it is of no consequence.

AC30 fuses are WAY overrated (especially for US). If it blew it you have a shorted tube or trannie IMHO. I agree. 4A @ 120Vac = 480Watts. For a 30Watt amplifier that puts the conversion efficiency in the area of a steam engine. The fuse should be half that value.

[WaltC]

hmmm...... if ohm's law didn't have anything to do with it, then how'd 4A @ 120VAC get to be 480watts.....<g>

amperage is directly related to resistance and voltage, whether we like it or not. Plus, most fuses available for electronics gear are 250V fuses anyway...

[sidneymg]

I have the opposite problem. My amp calls for a 2.5 A slo blow 125 volt. All I can find is a 2.5A slow Blow 250V. Will that cause damage to my amp?

[davemccarthy707]

I have the opposite problem. My amp calls for a 2.5 A slo blow 125 volt. All I can find is a 2.5A slow Blow 250V. Will that cause damage to my amp?

Maybe. a 2.5A 250v = 5A 125v bottom line

[donnyjaguar]

hmmm...... if ohm's law didn't have anything to do with it, then how'd 4A @ 120VAC get to be 480watts.....<g>

amperage is directly related to resistance and voltage, whether we like it or not. Plus, most fuses available for electronics gear are 250V fuses anyway...

I of course agree with the second statement. :)

The 480Watts is what the amplifier would need to exceed in dissipation before the fuse blew. If the amplifier was 220Vac, that would be 880watts. Just citing this to compare with the output power the amplifier is producing. :)

There are some slo-blo, and low current, style of fuses where the voltage rating is relevant during normal operation. These have internal resistance that should be considered. That doesn't describe a typical tube amp though.

[SatelliteAmps]

The fuse rating has little to do with the actual output of the amplifier. The fuse has to do with power consumption from the wall, and the main power supply of the amplifier. Extremely efficient solid state amplifiers can get almost 80% of their consumption into output power. Tube amps aren't anywhere close to that.

Looking up the data off the manufacturers data sheets, I see the main resistance difference between the 125v and 250v fuses as a difference from .031Ω to .038Ω (slight differences in different manufactureres.)

The main thing I have noticed in working on amps that were made for 240v is that the fuse ratings are always halved for the 240v, or doubled for the 120v side (depending on how you want to look at it). Which means the voltage HAS to have some effect on which fuse to use. A lot of people mistake those ratings and put in the wrong voltage rated fuse for the application. If an amp says 240v 2A or 120v 4A, then the 4A fuse needs to be a 120V fuse for the amp to have the proper level of protection.



I have the opposite problem. My amp calls for a 2.5 A slo blow 125 volt. All I can find is a 2.5A slow Blow 250V. Will that cause damage to my amp?

You should be fine.

[ClinchFX]

From fuse manufacturer Littelfuse http://www.littelfuse.com.hk/data/zh/Product_Selection_Guides/Fuseology.pdf under "Voltage Rating" on the third page

As mentioned previously (See RERATING), fuses are
sensitive to changes in current, not voltage, maintaining
their “status quo” at any voltage up to the maximum rating
of the fuse. It is not until the fuse element melts and
arcing occurs that the circuit voltage and available power
become an issue.and

To summarize, a fuse may be used at any voltage that is
less than its voltage rating without detriment to its fusing
characteristics.Ohms law does apply to fuses, otherwise the fuse would never blow - However, under normal operation, the voltage across the fuse will never be more that a few volts. If the whole mains voltage was dropped across the fuse, there would be nothing left to power the amplifier.

Therefore, it's quite safe to use a 125V or 250V rated fuse with a mains voltage of less than 125V. Both will blow at the rated current.

The reason for differing fuse current ratings for 120V and 240V equipment is that 240V operated equipment requires only half the current to achieve the same power as a 120V circuit - P=VI. Therefore, the fuse current rating for 240V gear will be half of the current rating for a fuse in 120V gear.

Peter.

[peterjh85]

Thanks for your replies very helpful. I think US voltage is 110-120, so I should be good it seems. But at the same time, I'd like to stay on the safe side and replace the fuse with a 4 A, 250 V fuse which was originally in the ac30 tb reissue. My local radio shacks do not carry them. Is there anywhere else I can find these?

[ClinchFX]

Thanks for your replies very helpful. I think US voltage is 110-120, so I should be good it seems. But at the same time, I'd like to stay on the safe side and replace the fuse with a 4 A, 250 V fuse which was originally in the ac30 tb reissue. My local radio shacks do not carry them. Is there anywhere else I can find these?

Yes , you're roughly correct about US line voltage.

I don't know too much about local stores in the US, but I sometimes buy components from Digi-Key (http://www.digikey.com/) in the US. Their service is great and they do have 4A 250V fuses in stock. You need to know the physical size of the fuse - it's most likely 1/4" diameter by 1-1/4" long with a glass tube between the end caps, often called a "3AG" size, nothing to do with current rating. If the fuse is marked "4A 250V" it is probably a normal fast blow fuse. If it is marked T4A 250V, it will be a slow blow fuse. There are a heap of other markings that specify fast or slow, but these are the most common. It's not a great idea to replace a fast fuse with a slow one.

As someone else suggested, a blown fuse usually indicates that there is a fault somewhere in the amp, however, I have known fuses to fail in their own right. The fuse is there to protect things from going up in smoke when a fault happens. If the first replacement fuse blows, don't keep feeding fuses to the amp - take it to a tech.

I hope this helps.

Peter.

[peterjh85]

Thanks Peter for your advice. I've got to find a T4E 250 V fuses, as that's what originally was in the mains. The fuse blew because I was stupidly messing around with one of the ground wires while the amp was on - idiot move. Thanks for any additional help on tracking that fuse down.

[ClinchFX]

OK Peter, that'll do it every time:p - just kidding.

The "T" indicates that the fuse is a slow blow type. Without knowing who manufactured the fuse, I can't figure what the "E" means, but it may be a further definition of how long the delay is.

Digi-Key stock two types of 4A Slow Blow 250V 3AG Glass fuses, Part number F2546-ND by Littelfuse and 283-2751-ND by Cooper Bussmann.

Peter.

[WaltC]

"T4E 250 V fuses"

try here:
http://www.hoffmanamps.com/MyStore/perlshop.cgi?ACTION=thispage&thispage=parts14.htm&ORDER_ID=289232879

use the pop down menu to select 4A for the fuse value and enter a quantity. Doug's a great guy and sells good stuff...

[peterjh85]

Thanks WaltC for the link. Those are a bit too big. I was looking for the 5x20 mm size. I went to my local radio shack and found a T4A 250 V fuse. Do you guys have any information regarding the "A" vs "E" rating on the two fuses. Thanks again!

[WaltC]

the "A" in that rating stands for Amps and means it's a 4 A (T for slo-blo). So that looks like the right fuse for your amp.

[zzmoore]

We have a lot of different fuses with many different amp ratings. All of them are rated for 250 volts. I do not see one that is stamped 120. That aside, I don't understand how it would make a difference. My understanding is that a fuse will open when its amp rating is exceeded. I would assume the volt rating concerns arcing. It would not factor into the impedance of the fuse. The resistance of the fuse is not based on voltage.

[peterjh85]

the "A" in that rating stands for Amps and means it's a 4 A (T for slo-blo). So that looks like the right fuse for your amp.

Maybe the "E" in the T4E that came out of my Vox reissue is an UK code for
Amp. Either way, thanks for the information. Have a great weekend you guys.