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The weakest kink in your tube amps power supply... the fuse

Discussion in 'Glowing Bottle Tube Amp Forum' started by DrPepper, Apr 5, 2021.

  1. scooteraz

    scooteraz Friend of Leo's

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    to be fair, I looked up a chart for the chassis wiring allowed ampacity. I am always surprised how small of wire I can use inside of a machine or chassis. But the wire in a fuse is relatively microscopic. I’ll have to do some Thermo calls and self heating calcs to see what kind of sizes we are talking about.
     
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  2. danlad

    danlad Tele-Meister

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  3. Blrfl

    Blrfl Tele-Afflicted

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    It has to be because bare wire can carry more current than the same stuff with insulation on it.

    Electrons passing through a wire create friction and the friction creates heat. Physics says the heat has to go somewhere, which means it's going to end up traveling through the wire itself and then to whatever is next to it. For insulated wire, that's the plastic insulation, which has a much-lower melting point than the metal in the wire. The air next to a bare wire will keep sucking up heat well beyond the point where the metal in the wire gets hot enough to melt, at which point it breaks. (And that, boys and girls, is how fuses work.)

    For comparison, bare 6 AWG copper wire in open air can carry 125A forever without melting. Adding insulation rated for 60°C de-rates that by more than half to 55A.
     
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  4. scooteraz

    scooteraz Friend of Leo's

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    Indeed, insulation ratings (and conduit/cable tray loading) are the limiting factors in cable ampacity.

    However, the wire in a fuse tube is not in “free air”. It is in a confined area that changes the thermodynamics at least some and makes the heat transfer from the space to the world a bit less rapid. In “free air”, the cable relies on free convection for cooling. And as the conductor gets hotter, it has more resistance. In your copper example it is just under 0.4% per °C. So, while the 6 AWG can carry 125 amps in free air, on a really sunny hot AZ day with no wind, that may be slightly derated because the resistance has gone up. Don’t think it would get hot enough to melt, but the power loss would be a bit excessive between the self heating and the lack of available cooling. Indeed, every so often you hear about main transmission lines being derated due to high temperatures.

    Your point that the wire has to be designed to melt at a specific current is correct. So, it has to be large enough to carry the rated current for some amount of time (the earlier reference for the 3AG slow blow was a 4 hour minimum) but still melt at 135% of rated current within one hour. So, that there is some pretty fine hair splitting.
     
  5. scooteraz

    scooteraz Friend of Leo's

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    Well, they are a slightly different construction (the one has a silver conductor) slo-blow. And they’ve been cryogenically treated! And they are made by (or for) a specialty stereo company. So, I’m guessing price matching will still be audiophile expensive....LOL

    @DrPepper, here are those upgraded fuses you are wondering about....And I’m really guessing what you get is a lighter wallet for no better measurable performance.
     
  6. gusfinley

    gusfinley Tele-Afflicted

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    I don't know of many amps that strictly operate from AC voltage. The Power supply in our amps converts the AC to DC voltage.

    You may sneak a few more electrons past the fuse with a "audiophile" fuse, but at the DC power supply, those electrons will be stored up in the reservoir capacitors for later use.

    So if you want to compensate for your "inadequate" non-audiophile fuse, simply use a larger reservoir capacitor.
     
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