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RAILhead · Jul 10, 2006, 09:17 AM

Quick question:

I need to replace fuse, and the one that blew is 250 volt, 400mA. I know the main thing is that the voltage remains the same, but all I could find were 500mA fuses.

My understanding is that the 500mA fuses will work just fine, since the basic definition of an ampere is just the amount of electricity flow per second. If nothing else, it seems a higher mA would be more efficient.

The guy at the hardware store told me the 500mA would work fine, but I thought I'd post here, too. FWIW, it's going in my guitar amp.

starman · Jul 10, 2006, 09:26 AM

Ever see "Back to the Future"?

"There's a SLIGHT possibility of overload"

Dakar · Jul 10, 2006, 09:27 AM

Great Scott!

vmarks · Jul 10, 2006, 09:52 AM

You -can- but it may not be advisable.

Here's why.

You -WANT- the fuse to blow if there's too much of a load on it. Not blowing would make the failure happen elsewhere, perhaps in the form of an electrical fire.

You're now allowing a 500mA load instead of a 400mA load. This means, what used to blow it will now work for higher loads.

now, milliamps are pretty small. So I have to wonder what application it is that has 250v but that small a milliamp rating?

RAILhead · Jul 10, 2006, 09:58 AM

The power supply for my guitar amp.

vmarks · Jul 10, 2006, 10:02 AM

tube amp, solid state, what?

Tubes can take a lot more in terms of power fluctuation than solid state.

Are you using any kind of surge supressor / something that cleans the noise out of the electricity?

I think I would go with the 500mA and be ok, but I would source a few of the 400mA to have on hand and sub in when I got them.

SVass · Jul 10, 2006, 10:51 AM

A complete short would blow both a 400 and a 500 mA fuse. (The higher ampere rated fuse would be a mite slower.) A slight overload could overheat/overstress the amplifier. Most fuses in portable devices fail from vibration or startup shock instead of overload. Tube amplifiers do have a large inrush at startup that can cause a fast blowing fuse to fail; therefore, sometimes they use what is called a "slow-blow" fuse.

My suggestion is to use whatever the manufacturer recommends (and get several overnight via the internet). sam (retired professional engineer). Note that the voltage rating is a minimum and should not be ignored.

Dopetrackalistic · Jul 10, 2006, 11:18 AM

I think it'd be fine. But then I only went for electrical engineering for a year before I realized how bored I was.

gethigh · Jul 10, 2006, 04:31 PM

Originally Posted by RAILhead

My understanding is that the 500mA fuses will work just fine, since the basic definition of an ampere is just the amount of electricity flow per second. If nothing else, it seems a higher mA would be more efficient.

Power = Voltage * Current (Amps)

More current has nothing to do with efficiency, just how much power is going into the system. Fuses are meant to blow before the maximum tolerance of the circuits. The difference between 0.5Amps and 0.4Amps doesn't seem much, but perhaps the hard limit before damage to the circuit occurs is at 0.4Amps. So if the fuse now breaks at 0.5A it doesn't matter since the circuit is now permanently damaged. That sounds like bad engineering since there should be a margin of safety--either find the data sheet to find the amperage tolerance or take the gamble.

stevesnj · Jul 10, 2006, 04:39 PM

I hope you determined why the fuse blew in the first place.

The Godfather · Jul 10, 2006, 06:30 PM

a 400mA fuse will protect your amp better than a 500mA fuse, by melting before any part of your amp melts. Go with the manufacturer's original design.

If you are too worried about the inconvenience of changing a fuse, go buy 20 fuses today.

RAILhead · Jul 10, 2006, 07:40 PM

I found some fuses that matched the specs online, and they're on the way.

ghporter · Jul 10, 2006, 07:49 PM

You can UNDERFUSE safely, but never overfuse. The 400mA fuse blew because the equipment wanted more current than it was built to handle-an indication of a problem. While putting in a higher-rated fuse might help you find that problem, you probably don't want the rest of the equipment to melt/burn around the smoking hole left by the real culprit. I spent about 25 years working on electronics, and many times I wanted the "real culprit" to blow up spectacularly so I could finally figure out what the problem was, but I never overfused. Fuses are your friend in that they blow to protect you. Don't abuse them.

Doofy · Jul 10, 2006, 07:53 PM

Originally Posted by RAILhead

I found some fuses that matched the specs online, and they're on the way.

We still want to know what amp you're running.

amsalpemkcus · Jul 10, 2006, 08:01 PM

A quick prayer to the almighty makes such decisions very easy, you know.

RAILhead · Jul 10, 2006, 08:45 PM

This is just my Traynor Custom Valve 20 tube amp I'm talking about. It's little (especially compared to my Carvin half-stack), but it sounds like pure butter.

I slapped a 250v 500mA fuse in, turned it on, and it blew — so something must be shorting out inside the amp. I haven't cracked it open to look, so maybe one of the tubes crapped-out and it's causing a short? That doesn't make any sense, though.

By the time I'll be able to drive it to an authorized reseller, the spec fuses will be in — though since the 500mA fuse blew, I think I have bigger problems.

Doofy · Jul 11, 2006, 04:08 AM

Originally Posted by RAILhead

since the 500mA fuse blew, I think I have bigger problems.

Yep. I'd get it looked at.

ghporter · Jul 11, 2006, 06:54 AM

A bad tube almost never causes a short. They are made up of components that are physically isolated from each other by vacuum, so unless you SEE a major part touching another one, it's not likely a tube is at fault.

And since the circuit was originally fused for 400mA, I doubt it is a tube circuit that is to blame here-you need some pretty high voltage stuff to get tubes working, and those power supplies typically take a few amps (though this is just speculation on my part). I'd suspect something in the control circuitry. And whatever it is, unless you have a fully equipped electronics shop at hand, you probably need to have the amp looked at by a pro. First make sure that nothing has slipped into the case and caused a problem (vent holes sometimes allow foreign objects inside the case), and if it looks clean, take it to a good shop.

residentEvil · Jul 11, 2006, 09:26 AM

It could be as simply as a short in your power chord. Check the outlet end and where it enters the enclosure. Might have a loose end.

ghporter · Jul 11, 2006, 10:15 AM

Naah. The power cord being bad would pop the breaker for the outlet. It's an internal problem.

wataru · Jul 11, 2006, 10:33 AM

Originally Posted by vmarks

now, milliamps are pretty small.

I'd say they're not. Amps are one of those units where a little goes a long way. I rarely see any consumer anything that has a need for current that should be expressed as Amps rather than milliAmps.

Doofy · Jul 11, 2006, 10:54 AM

Originally Posted by wataru

I'd say they're not. Amps are one of those units where a little goes a long way. I rarely see any consumer anything that has a need for current that should be expressed as Amps rather than milliAmps.

Kettle? Toaster? Heater? Microwave? Cooker?

Do ya live in a cave or what?

SVass · Jul 11, 2006, 11:01 AM

400 milliamps time 120 volts = 48 watts and at 50% efficiency would be about a 25 watt amplifier. (Actually, the efficiency is probably a lot lower than 50%.) Vacuum tube circuits normally have a high voltage power supply using a rectifier, choke, and electrolytic capacitors. In the old days, the capacitor(s) would fail eventually due to heat or old age creating a short circuit blowing the fuse.

By the way, the voltage is high enough to cause bodily injury to those poking around inside. sam