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Gots a question about my power cord

Discussion in 'Shock Brother's DIY Amps' started by dhodgeh, Sep 15, 2020.

  1. aging_rocker

    aging_rocker Tele-Afflicted

    May 8, 2019
    The upsidedown
    Tin foil would be just dumb. The foil from a cigarette packet works much better. Ask me how I know....
    Blue Bill, ronzhd and Nickfl like this.
  2. Nickfl

    Nickfl Friend of Leo's

    May 24, 2016
    I think we may be getting away from the point here.

    Ron suggested the OP use a much heavier power cord and I pointed out reasons I saw that would make that unnecessary and impractical.

    You chimed in to tell me that isn't how fuses work and that its there to protect the amp.

    I said that the fuse prevents the amp from drawing more than the fuse is rated for so the power cord will never see more current than that.

    You have now pointed out that if the power cord shorts internally the fuse won't protect anything. So are you saying that means that no amp should have a power cord that is rated for less than 20 amps? As you also point out your scenario describes a dead short that will trip the circuit breaker at the electrical panel and even though that will momentarily exceed the current rating of the power cord (and maybe even the house wiring) that is built into the safety margins.

    Ok... so I think we may have both misunderstood each other here.

    When I say the fuse is to prevent your house from burning down, I mean it is to prevent a fire occurring in the internal wiring of the amp. It protects against shorts that occur inside the amp itself, which are overwhelmingly the most likely place for them to occur in such a way that they would fail to trip the breaker in the house. I in no way meant to imply that it protects against shorts elsewhere in the mains power wiring, but I have a hard time imagining a fault that does not blow the amp fuse and manages to draw more than the 7amp rating of an 18ga wire through the power cord but doesn't draw enough current to trip the breaker in the house panel. I'm sure we could come up with something, but it would be a stretch.

    When you said the fuse protects the amp, I heard that as "protect from damage", but as anyone who repairs amps knows, that fuse will not necessarily prevent power transformer windings from burning out before it blows and if you want to protect the amp itself you need fuses on the PT secondaries. I now realize that you may well have meant "protect from bursting into flame", which is accurate.

    I'm going to chalk this up as one of the pitfalls of internet communication rather than either of us trying to be difficult.

    I assume we can agree that it is ok to use an 18ga power cord in some applications and that a guitar amp meant to draw less than 1 amp in normal operation would be among them?
  3. Peegoo

    Peegoo Poster Extraordinaire

    Oct 11, 2019
    Between the Raindrops
    There's a Devil in every circuit, and if you're not careful he will beat you with a stick:


    I snipped ^that image^ from an ancient Magnatone amp schematic.

    I've encountered miswired power cables more than once, so always check continuity from each plug contact through the cable to confirm live, neutral, and ground colors.
    JohnnyJumpUp likes this.
  4. Ronkirn

    Ronkirn Doctor of Teleocity

    May 1, 2003
    Jacksonville, FL
    fuses. . there are basically two kinds that find their way into our amps.. a "normal" fuse .. these guys blow instantly when stressed .. allowing for no margin of error. The other, and the kind many amp manufacturers recommend is a Slo-blow .. it will tolerate a momentary surge, a transient spike in the current flow... lunging beyond the rated amperage for a brief interval ... Note the amps recommendations and use the correct type.. You can still get them at most better hardware and DIY stores.

    Remember, if a circuit has a published rating at a specific value.. it is generally engineered for us klutzes that will abuse it and push it beyond... but only momentarily, before the safety provisions "cut in".. Some amps have an internal overload circuit that will "kill" the signal before the maximum is reached... (this is to protect the electronics from dolts that will actually do what I mentioned above, wrap aluminum foil around a blown fuse to keep things going... DO NOT DO That. I only mentioned it above to poke fun at those whackos that actually did so back when I was a kid...) so if your amp is dead and the fuse is still good... that's probably what's going on... Some protection circuits automatically reset, others require ya open 'er up and punch a "circuit breaker" somewhere.. and some do not have the protection circuits at all... AND.. Know what you're doing.. nasty surprises can be lurking in open amp circuitry, even if off, and unplugged. Capacitors, specially the "big" caps store serious voltage at low current but it can seriously zap your ass faster 'n you can say, "cack! I hate it when that happens". If you wanna know what it feels like, go pee on a spark plug while the engine is running... you'll only do it once.. :eek: < don't do that either... :rolleyes:

    now why would ya want one kinda fuse over the other.... well in "rock", it's not unusual for the guitarists to get a bit rambunctious.. with extended segments that can actually exceed the limits of the amp ..

    In such moments the amp can "draw" a higher amperage than under more normal moments.. If you have a "normal" fuse, it will not tolerate such and "POP".. it's gone... a slo-blow fuse is designed to allow for a momentary surge... so if the "pig tail" is marginally rated... it can over heat during those instances, when you're all crankin' at 11, straining the current handling capacity of everything in the chain.. never a good thing...

    So if you push it, and it gets warm.. then you do so again, it's now already warm, so it gets warmer.. then do so again.. and now the insulation is darn near the melting point... softened considerably... so. someone steps on it... and the insulation fails and you have 117 VAC short (in the US) ..

    So if you have selected an 18 AWG "pig tail" rated at a marginal 15 amps... you may find it getting warm to the touch, whereas something more robust eliminates that from the thought process..

    I'm never afraid to "over engineer" where the "mains" supply is involved as long as I know a fuse of the correct rating is protecting things between the Mains supply, and the sound coming out of the amp.. Never ever replace a fuse of specific rating with one of a higher rating... Do so and instantly your picture appears next to the word "stupid" in the dictionary..:p

    Rock on, safely..
    Last edited: Sep 17, 2020
    ronzhd likes this.
  5. tubegeek

    tubegeek Friend of Leo's

    Jan 31, 2020
    Brooklyn, NY
  6. jtcnj

    jtcnj Tele-Holic

    Feb 2, 2015
    N.J. USA
    I dont know how you all run things, my gear is only plugged in during my play session.
    I plug whatever amp I am playing into a surge power strip, which also powers the power supplies for pedal power.

    I leave the amp on if I leave to fill a water glass or go potty (#1 only) .
    Anything more than that the power strip gets unplugged from the wall receptacle.

    They are all scratch builds except for one store - bought.

    In this scenario, is it "wrong" to wrap that with electrical tape or an acceptable method?
    I probably would take notice of the outer insulation pulled back and not do anything at all.

    If you do end up with odd colored cord wires, or just to be sure in any case, meter the raw end to the plug prong for continuity to verify which conductor is which.
    The wide prong is neutral.
  7. Alamo

    Alamo Doctor of Teleocity

    Nov 15, 2006
    Different countries - different standards ;)
    just like metric vs. imperial, left side driving and much more

    it's a pita for sure but better to get one's head around.
    especially when dealing with electrical current.

  8. Blrfl

    Blrfl Tele-Afflicted

    May 3, 2018
    Northern Virginia
    Right. Specifically, where I had a problem was with "18 AWG should be fine since its still rated at 7x the amps fuse rating." That leaves the impression that the right wire gauge for a power cord is a function of the fuse in the load it feeds. It isn't, and, having seen people go off half-cocked and install unsafe accessory wiring in motorcycles, I felt the need to point it out. Wasn't trying to be difficult, but I do try to make safety-related stuff crystal-clear.

    I stand by that. By the power cord, I mean the wire that plugs into the wall outlet, not anything internal and downstream of the fuse that carries line power. The internal stuff is what the fuse protects. I think we're in violent agreement on that.

    Yes, but I should have been clearer in my comment: The cord has to be rated to carry the full 20 amps for the the time it would take for whatever circuit protection is upstream to bring the situation to a halt. More on that below.

    Here's a non-far-fetched scenario: A cord rated for 7A continuous is plugged into a residential outlet with a 15A breaker in the panel develops a partial short that allows current to flow through the wire at 15A.

    Looking at the trip curve for a UL-listed breaker like you'd find in a house (see page 2), 15A is a 1x multiple of the breaker's rating and the trip time is ten minutes. Double the current to 30A and the breaker has to trip in 25 seconds. Triple it and you're down to five. You can bet that any UL-listed wire will have been tested to not have an insulation failure under continuous current up to 20A and higher currents for whatever time a UL-listed breaker will take to trip.

    18's fine for the load and for the line. 20 would be, too, but 18's more common because it's physically more durable. Nobody sells line cords smaller than 20 AWG because it will fail before any breaker likely to be upstream (15A or 20A) trips. UL won't list it and they'd be liable for damage.
    Nickfl likes this.
  9. Dreadnut

    Dreadnut Friend of Leo's

    Nov 4, 2019
    Grand Rapids, Michigan
    Replace it, they are cheap
    trancedental and agent_zed like this.
  10. agent_zed

    agent_zed TDPRI Member

    Aug 21, 2019
    In the UK (and a few other countries) we have PAT (Portable appliance testing) in places like schools and offices where any plug in device is tested for electrical safety. This would be an instant fail as it is physically damaged.

    Whilst it might continue to work there is the chance that the earth/ground cable could be damaged to the point of providing no protection. You might never know if its broken inside. You might be ok for the life of the amp or someone might spill a beer and electrocute you. The entire point of the outer covering is to provide additional physical protection and prevent over flexing, along with holding the wires together.

    Cut it off and add a new socket.

    Or cut it off and add a female socket and then you can use an extension lead with the correct male end to have a longer cable. I did this on my amp so i don't need extension leads in most situations.
    trancedental likes this.
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