Fuse question

Discussion in 'Shock Brother's DIY Amps' started by Entropy, Jul 24, 2021.

  1. Entropy

    Entropy TDPRI Member

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    I had so much fun building the 5E3 that I decided to start the next one.

    I'm planning on doing an 18 Watt build, and have spent ages reading through the 18 Watt forum and looking at schematics.

    A lot of the schematics and layouts I've printed out are really similar. All list 120V wall voltages, but some have 1A fuses, others have 2A fuses, some don't list a value at all. It made me realise I don't understand the fuse value at all.

    After a lot of google-fu I have read all sorts of conflicting advice, including:

    - That for amps under a certain wattage it doesn't matter as long as it's less than a 3A fuse
    - That the swap from 120v to 240V wall voltages should halve the fuse value
    - That the fuse value is circuit dependent so it should be left as listed

    Unsurprisingly, I'm feeling a bit confused. Can anyone explain what I should be doing if (for example) I work from a layout/schematic like the Valvestorm 18W TMB (which has a 1A fuse and 120V primary on the PT)?
     
  2. smuc

    smuc Tele-Meister

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    Read through this:
    http://www.valvewizard.co.uk/fuses.html
     
  3. wangdaning

    wangdaning Tele-Meister

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    Beat me to it. For 18w I am fairly certain you would want to go with 1 or at most 1.5 amp. I think 2 amp or more would not actually have an effect for certain failures.
     
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  4. bebopbrain

    bebopbrain Tele-Meister

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    > doesn't matter as long as it's less than a 3A fuse

    It matters; you don't want a nuisance trip or, worse, a fuse that fails to clear.

    > from 120v to 240V wall voltages should halve the fuse value

    Precisely: P = V*I

    > That the fuse value is circuit dependent so it should be left as listed

    Solid advice! 1A on mains for 240VAC.
     
  5. NSB_Chris

    NSB_Chris Tele-Holic

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    Since you have a cathode follower section, you should consider elevating the heater center tap. This is cathode biased power section so you can just move the center tap to the power tube cathode.
    http://www.valvewizard.co.uk/heater.html
     
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  6. Blrfl

    Blrfl Tele-Afflicted

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    Don't guess. More on that below.

    This is true. Voltage is like water pressure; if there's twice as much, it's going to flow through the same load twice as fast. At the same time, the primary on a transformer for 240V is going to be wound differently than one for 120, so you can't just switch to 240 without changing transformers.

    It is. You can kind of guess and get it right for well-known amp circuits, but it's better not to. If the designer of the circuit has a clue and has specified a fuse value, they've already done the homework to find one that's appropriate. You can install a smaller fuse and it may blow when it shouldn't, which would be annoying.

    Never install a larger fuse. Wiring has resistance that creates heat; the more current you push through it, the hotter it gets. Push too much current through a wire and the insulation will melt, exposing the conductor to it surroundings that might cause a short and a fire. Smaller wire has more resistance and will make more heat, so the fuse has to be selected to prevent the smallest wire in the circuit from having a meltdown.
     
  7. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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  8. Entropy

    Entropy TDPRI Member

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    Thanks everyone, this is really helpful.

    I need to keep thinking about why it gets halved to really understand, but I'm glad I know now. It makes me realise my 5E3 fuse must be too large, so I'll fix that tomorrow (and kick myself for not realising sooner!)

    Reading that Valvewizard page I can see how the calculation requires you to halve the fuse value given on a schematic. Just thinking about some of the 18w schematics I have that don't have a listed value I was wondering if anyone can anyone explain how to calculate the maximum average power used by each of the secondary windings so you can do the calculation yourself?
     
  9. dsutton24

    dsutton24 Doctor of Teleocity Ad Free Member

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    At the risk of starting a manure storm, the answer is that it doesn't really matter, at least at the levels we're talking about.

    I'd bet that an 18 watt amplifier pulls well under half an amp going full bore at worst case operating conditions. If this is so, why are we trying to choose between a two amp or three amp fuse? It seems that half an amp would be plenty. The answer is that a lot of factors go into choosing a fuse. Full load current is one of them, but isn't the most practical factor. Another is that the fuse has to survive the inrush from the transformer, this is a biggie. The fuse has to survive all the stupidity that comes down the power lines.

    The biggest factor? Eliminating nuisance blowing without compromising protection too much. That's the art.

    What exactly are you protecting? You are protecting your house. As a secondary issue, you're protecting the wiring and transformer in your amp. And, you're trying to do it in such a way that the amp isn't an unusable pain in the neck.

    So, choosing a fuse is as much art as engineering. If the choice is between a two or three amp fuse, it really doesn't matter. One amp might hold. Four amps, in my opinion, is probably too big. Bear in mind that my Twin, which is a real fire-breathing amp, has a three amp fuse in it, and it's just fine.

    So there.
     
  10. D'tar

    D'tar Friend of Leo's

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    I've brought this up once upon a time.... I can't Even buy a 120v 2A fuse, i have plenty of 250 1A tho.
     
  11. Blrfl

    Blrfl Tele-Afflicted

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    The voltages stamped on fuses are their maximum working village. Current is current no matter what the voltage.
     
  12. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    My silver face Deluxe Reverb specifies a one amp slow blow with 120v wall voltage. It has never blown. It has more tubes than an 18W or a 5E3 Deluxe. I am pretty sure they do not need two amp fuses in them.;)
     
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