HT fuses voltage rating

[lo-fi]

Hi all, I'm thinking about installing a couple of fuses to protect PT's HT secondary.
Amp is an old Dukane PA that I'm respectfully adapting to guitar use, but specific amp doesn't matter I guess.

Original PT spec calls for 350-0-350VAC .100mA
I want to protect it with two distinct fuses before tube rectifier.

I've read a lot of online documentation, including Merlin's pages, but I could not find any clear guidelines about fuses max voltage. It looks like everybody is assuming that standard 250VAC rating will work fine.
I just cannot convince myself about that. Wouldn't it be at least wiser to use .3A@600V fuses (10x38mm) similar to the ones internally placed in DMMs.

Am I missing something?

Marshall's approach does not help since they place a unique fuse after rectifier/diodes but it's sometihng I don't want to do. Anyway, they still use 250V fuses (.5A) that seems to me not a good choice. But if they've been doing that for decades I'm ready to humbly stand corrected.

I've never liked assumtions when it comes to work on power supply section of tube amps.
Any experienced advice here?

Thanks.

[tedmich]

All laudable goals for safety and thems that know will probably chime in soon...

meanwhile fuse tolerance is (to quote Youtuber Ave) a "hotdog down a hallway" see the recent EEVBlog where he found near 1000% differences in response from single lot fuses

precision devices they are NOT

[g1]

There was one larger manufacturer that used higher voltage fuses (600V ?) but I can't recall the exact make.
I posted about it, questioning whether it was necessary and I did get an earful for it. So I think you're on to something.

[Enzo]

I have been soldering for coming up on 70 years, and I ran a pro audio repair shop for about 35 years. 600v fuses are quite rare, 250v fuses are common. All the amp makers as far as I know used common 250v fuses. No problems have resulted. This choice has worked - and passed certifications I would add - all along. You could go find 600v fuses, but really, I think you have a solution in search of a problem to solve.

Just my opinion.

[trobbins]

The first link is to where Littlefuse provides the 'allowance' for using a 250Vac rated fuse for higher voltage levels when fuse is effectively on a secondary side. The other link may also help in weighing up what to do and what fuse to choose.
https://www.littelfuse.com/~/media/e..._guide.pdf.pdf
https://www.dalmura.com.au/static/Va...p%20fusing.pdf

[Helmholtz]

The other link may also help in weighing up what to do and what fuse to choose.

I generally consider your papers outstanding and very helpful. They have more than once saved me lots of time to find my own answers - or they confirmed what I had assumed, which is nice as well.

Thanks for sharing!


Question to native English speakers: Is it correct to say "I find something excellent" or would it be considered a Germanism?

[Enzo]

If you said to me "I find his playing excellent," I wouldn't think anything unusual about it. Seems like acceptable English usage to me. A little formal maybe.

[Helmholtz]

If you said to me "I find his playing excellent," I wouldn't think anything unusual about it. Seems like acceptable English usage to me. A little formal maybe.

Thanks Enzo.

Point is, I originally wrote "I find his papers excellent", thus expressing my personal opinion as well as appreciation. But as that's a verbatim German to English translation, I wasn't sure if it's correct English.

[lo-fi]

Let me first say...I find your support excellent!

Thank you all for the precious knowledge and experience you share here.
That Valve Amp Fusing pdf is a real treasure, sir!

I understand things are actually easier than I thought. No need to worry about that apparent major voltage mismatch.
My concern was to have those HT fuses blown in the middle of the first set during a gig. Not the best situation since I'll have to place them in the circuit with no access from the outside of the chassis.

I've never used HT fuses on more common amps since replcement PTs are largerly available nowadays. Things are different with the old Dukane 460 serie.
I'm really enjoying that old PA restoration and I think it's definitely a keeper. Cathode follower, cathodyne PI, James tone stack, 3 channels, 5881s all in the same amp looks like a tweed era tech parade with a smart tone stack.
I successfully added a 1-tube, transformerless, 3-knob spring reverb. Converted one channel to pentode preamp (GA40 meets DC30) and finally installed a Type-2 PPIMV. Just one last issue to be solved but that's another story and probably will become another thread soon.

[SoulFetish]

Point is, I originally wrote "I find his papers excellent", thus expressing my personal opinion as well as appreciation.

That is a commonly used way of expressing one's admiration or high degree of approval. It wouldn't even register for an American english speaker that you were translating from another language.
Hell, I can't even hear a German accent from you when reading that. That's how nominally English it is.

[Enzo]

I visited a clock shop with a German friend once. All around us were clocks going "tick-tick-tick..." My German friend looked at them and said "Ve haff vays of making you TOCK."

[SoulFetish]

Enzo, I'm just glad you're still up at this ungawdly hour so we could have this chat.
know what I'm sayin?

[Leo_Gnardo]

One thing I haven't seen mentioned here yet, is an answer to "what do voltage ratings on fuses mean?"

Let's look at an ordinary fuse, the kind we often see in guitar amps & other equipment. A glass cylinder, metal end caps, a wire inside connecting the end caps. Your ordinary "fast blo" fuse. So, what happens when the current rating is exceeded? The wire melts, the connection from one end cap to the other is broken, fuse has done its job sacrificing itself for the better good of the equipment surrounding it & maybe preventing a fire. Now down to the fine point "voltage rating." Current, propelled - as it were - by high voltage, can jump over a gap in conductors. In this case the ends of the burnt wire inside the fuse. We call this an "arc." If the fuse wire opens, and the distance between the fuse wire stubs is insufficient, current may continue to pass via this arc phenomenon, then what use was the fuse??? So... higher voltage fuses are supposed to create a large enough gap upon failure (which means success in the fuse world, right?) to prevent an arc from continuing to send current through the circuit which is supposed to be protected by the fuse.

Mostly, in our focus of guitar amps and the like, this means it's best to avoid low voltage fuses like those rated at 32V intended for use in motor vehicles mostly. Many of those were SFE designated, and had different lengths, depending on current rating, so that it wasn't likely to use the wrong fuse in the wrong place. Nonetheless there were 32V fuses made in the standard format 1/4 by 1 1/4 inch, and the point is here, you can't trust those to create enough of a gap when they burn open and may allow an arc to occur. So... avoid those first and foremost. Probably won't find them anyway unless you buy NOS fuses made decades ago - auto fuses have been those plastic blade types since about 1980. Also, SFE fuses were commonly made with ratings of 3 amps and upwards, not much use in a high voltage circuit. So, the question becomes, will a fuse rated 125 or 250V open up in such a way to prevent arcing in an amp that supplies 400 or 500 or 600V in the fused circuit? Well, if you're going to be super fastidious then you better get fuses rated 600V with fuse holders to match. But we have experience to go on as well. I have yet to see a manufacturer use 600V fuses in an HT circuit. It's always 250V. They're cheap and available, and they work. Well... most of the time. I also haven't heard nor seen any 250V fuse arc when used in amplifier HT circuits. But it's possible. So, there's as much of an answer as I can deliver. In AC lines, use 125 or 250 depending on your AC supply, use 250V in your HT, avoid 32V fuses except in your antique cars, and that's that.

If for some odd reason anyone needs SFE fuses, send a PM, I have boat loads of 'em. Well a couple hundred anyway. For your antique motor vehicles of course, not your amps.

Caffeine-fuelled rant thanks to Chock Full 'o Nuts , over & out.

[trobbins]

The key advantage of an AC fuse is that the AC waveform across the fuse passes through 0V, and forces any arc to extinguish. When a fuse filament breaks, some or all of the filament can evaporate, and metal can be sputtered on to the inner wall of the common glass cylinder - I've seen some glass walled fuses that have a nice 'gettering' type coating. It could be that that metal (whether vapour or lining the glass), assists in restarting an arc, although rapid cooling at each 0V crossover seems to allow the fuse to do its job. I'd anticipate that a fuse with say a 10A filament has a lot more metal than say a 1A fuse filament, so typical fuse types used for amps may be less prone to re-arcing than say a 10A fuse that complies with the same safety standard.

I use 5x20mm fuses, and although a filament in that housing would likely have less metal than say for a 3AG fuse, the 3AG fuse offers a longer glass cylinder that could provide peace of mind. However note that the 10mm separation provided by even a small package 5x20mm fuse still complies with the minimum creepage distance requirement for the worst case level of pollution (conductive residue) and worst material grade (glass is the best grade to use) for a 600Vac circuit from the general 60950 safety standard.

One other aspect of a small fuse is the maximum 'interrupt current rating', which is at least 35A for a typical glass fuse. It's unlikely that a B+ type power supply has a low enough impedance to exceed 35A, but that could be possible for a mains side fuse. If the fault current exceeds 35A then other issues could occur, like glass rupture - something that can be experienced, although upstream mains over-current protection is likely to also help in breaking the current (but that depends a lot on the protection device).

All bets are off if such a fuse is added in series with the B+ feed of dc current where the voltage does not periodically get to 0V.

[lo-fi]

Now down to the fine point "voltage rating." Current, propelled - as it were - by high voltage, can jump over a gap in conductors. In this case the ends of the burnt wire inside the fuse. We call this an "arc." If the fuse wire opens, and the distance between the fuse wire stubs is insufficient, current may continue to pass via this arc phenomenon, then what use was the fuse??? So... higher voltage fuses are supposed to create a large enough gap upon failure (which means success in the fuse world, right?) to prevent an arc from continuing to send current through the circuit which is supposed to be protected by the fuse.

Thanks Leo_Gnardo.

Despite caffeine-fuelled rant it ended up being a clear, practical and effective description of the relationship between voltage and fuses.