Multiple mains voltage selection and fuse rating

FleshOnGear

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Just double checking here - the amp panel on my JMP 50 says 2 amp fuse, but I should be using a 4 amp fuse since my mains voltage is 120v, correct?
 

XTRXTR

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what model you are talking about?

If you are talking about an actual JMP from before the 80s you should use a 2A for a 1987 per schematic. The Unicord says 3A for the Mark II LEAD version which runs the 6550s

Fuses are current limit protectors not voltage protectors.
 

XTRXTR

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All of the fuses 30mm and 20mm types I have seen are rated for 250V I think that covers all wall input voltages on the planet. But I am only one among 8 Billion - I have a high probability of being incorrect 🙂
 

Matthews Guitars

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People don't understand the voltage rating of fuses. That is only referring to the ability of the fuse to prevent arcing after the fuse element has blown due to an overcurrent condition. A 32 volt fuse rated at 3 amps will work in a Marshall but it may very well arc internally after it blows. A 240 volt fuse rated at 3 amps is designed to NOT arc over after it blows.
 

chocol8

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What's the voltage rating for the 2A fuse? Typically if the voltage doubles the fuse rating is halved.

It has nothing to do with the voltage rating. A fully functioning amp will draw half as much current off the mains if they are 240v vs 120v. Therefore, to get the same protection, you need a fuse that has half the CURRENT rating for 240 volt mains vs 120v mains.

You could use the same fuse, but there will be less protection provided. Going the other way, from 240v to 120 you have to double the fuse current rating or it will blow on you. It's best to use the proper current rated fuse for the mains voltage you are using. The things are cheap, so do it right!

The voltage rating can be anything higher than the wall voltage you are using. So, at least 125 in the US and at least 250 in much of the rest of the world.
 

XTRXTR

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It is interesting the varying schematics. The 1970 Unicord drawings show the ac input switch at the 120V with specified fuse rating in place. The Marshall drawing 78331-2 shows the ac input switched to 240V and where your fuses may be located along with a chart in the upper left corner with choices depending on your wall voltage. I think the Marshall drawing is also showing differences in MFR models depending on the import country electronics safety requirements.

So the OP said he is 120V wall input which indicates that his cord to the wall fits a US/Mexico/Canada type grounded plug spec. The ink on the panel says 2A so again we can deduce he needs a 2A.
 

FleshOnGear

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Pete Farrington

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Bear in mind that IEC fuse types are specified differently to UL.
Being in North American, I expect you’ll find UL fuses a lot easier than IEC, the latter being what Marshall will probably have used.
Hence 2A sloblo may not cope, you may need to increase the value to 2.5A or 3A sloblo.
See https://dalmura.com.au/static/Valve amp fusing.pdf
 

KraftyBob

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It has nothing to do with the voltage rating. A fully functioning amp will draw half as much current off the mains if they are 240v vs 120v. Therefore, to get the same protection, you need a fuse that has half the CURRENT rating for 240 volt mains vs 120v mains.

You could use the same fuse, but there will be less protection provided. Going the other way, from 240v to 120 you have to double the fuse current rating or it will blow on you. It's best to use the proper current rated fuse for the mains voltage you are using. The things are cheap, so do it right!

The voltage rating can be anything higher than the wall voltage you are using. So, at least 125 in the US and at least 250 in much of the rest of the world.
Exactly. What I meant to say, and admittedly wasn't clear, was what is the voltage rating on the amp, not the fuse. I understand that the fuse rating is max for the fuse, and that amp amp running 120v is pulling 2x the current as an amp from Europe running 240v. My bad.
 

FleshOnGear

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Bear in mind that IEC fuse types are specified differently to UL.
Being in North American, I expect you’ll find UL fuses a lot easier than IEC, the latter being what Marshall will probably have used.
Hence 2A sloblo may not cope, you may need to increase the value to 2.5A or 3A sloblo.
See https://dalmura.com.au/static/Valve amp fusing.pdf
I just got done working out the amp for 30 minutes at full blast with a 2A fuse. No issues so far. I wonder why the tech that went over this amp before I bought it had it fitted with a 4A fuse?
 

Pete Farrington

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It looks really bad on a tech if the customer takes the amp away and then pops a fuse in short order.
Plus different EL34 will achieve different peak currents, which will reduce as they wear out.
And if the mains voltage is up at its upper limit, the amp will draw more current.
So theres a few variables, and there’s a balance between protecting the amp and spurious fusing.
 

william vogel

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Installing a Littlefuse branded 2 amp 250 volt Slow Blow fuse is the correct and adequate protection for your amp. If this mains fuse blows, there is a fault. I have experienced cases where the 500mA HT fuse blew because current spikes occurred within what I call normal operation. I’ve never experienced the 2 amp mains fuse blow without a fault situation as long as a quality brand fuse was fitted. I have seen inferior import fuse failures that included endcap twist leading to filament breakage and open circuit occurrence although over current probably never occurred. Point is that it’s important to purchase quality parts.
 

william vogel

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Sure. My main concern is that I’m not sure the Bussmans I have are slow blow. The package they came in didn’t say so.
The slow blow fuses filament is usually wound around a fiberglass rod. This is slow blow. Fast blow have a single wire filament that is straight not wound.
 

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