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Potentiometer Fire, Smoke and Sparks

Discussion in 'Pickups & Electronics [BG]' started by Rebop, Mar 12, 2013.

  1. Rebop


    Jul 9, 2008
    La Honda, CA
    So one of my customers purchases a 250k mini Alpha pot from me and I get an email a week later saying he wants to return it as a flame came out of the Potentiometer. He said he also saw smoke and sparks as well.

    How in the heck!? :eek:

    How is this possible unless he was trying to use it as a dimmer for his dining room light fixture? He said it was a Lyon Washburn guitar (not that it matters but it does let me know it was likely just a run of the mill passive instrument).

    I can't figure this one out. :confused:

    Anyone have an idea how this could happen? I've been working on guitars professionally since the mid-late 90's and have never even heard of this. He's lying... right?
  2. hdracer


    Feb 15, 2009
    Elk River, MN.
    Could it be his amp?
  3. Maybe the smoke, sparks and flame was from the bong.
  4. Rebop


    Jul 9, 2008
    La Honda, CA
    I don't think so.
    I find this all very funny though.
  5. awamori


    Dec 10, 2009
    It happens when they wired in reverse IIRC.
  6. bassbenj


    Aug 11, 2009
    Well, he could be lying, or his life could be in danger! Quite frankly the only thing I know that can get smoke, heat and melted parts out of a 250K pot would be some kind of grounding problem on his amp. Improper grounds or the large RF shunting caps they used to put from power to chassis on old vintage amps are about the only thing that can generate that kind of voltage and current. And if that was the problem you tell him to get that amp and house wiring checked for proper grounds PRONTO! This is one way rock stars die.
  7. The Power in a passive electric bass will always be well below the rating of any pot. In an active bass, the Power will be a maximum of the battery voltage times the maximum current of a battery under a load approaching zero Ohms. I just grabbed a 9V Alkaline battery on my desk and measured it quickly. I got 8.890V and 3.3221A, directly across an ammeter. This is 23.53W, which exceeds the standard 1W or 2W rating of a pot. This means pot failure is possible if DC power is applied directly to the pot. This is possible if a coupling capacitor in a preamp leaks. It is certainly not likely, however.
  8. bumperbass

    bumperbass Supporting Member

    Jun 19, 2012
    In a real world situation, you're not going to get much more than 600ma out of a typical alkaline 9v battery. IMO, if the guy's using a tube amp, there could be a problem with the coupling circuit in it around the grid of the input tube, but I think it's more likely there's a grounding issue or possibly some terribly dangerous AC wiring contributing to this. As bassbenj said, this guy's life is in danger! I once had a customer come to me with his guitar strings melted. He said it looked like a toaster when the strings got red. Problem? Improper wiring in some makeshift outlets in his basement practicing area. It certainly is NOT your problem with your pot.
  9. walterw

    walterw Supportive Fender Gold Supporting Member Commercial User

    Feb 20, 2009
    i heard about a guy who plugged his bass into the speaker jack of his bass amp, whereupon the pickup blew up with a loud pop.

    remember, the difference between genius and stupidity is that genius has its limits.
  10. Not true. The impedance of the path from battery to pot can approach zero rather easily, leaving the pot free to vary current flow. The pot will also approach zero Ohms at one end of its rotation, regardless of rated/actual range. A bad pot may present a few Ohms of resistance, however. In any case, 600mA at even a very low voltage, like say 8V, would still exceed the pot's capacity for thermal dissipation. (Those two numbers come out to ~5W.) That's not enough for fire and sparks, though. Just lots of smoke.
  11. Hi.

    He's probably not lying about what happened, but he sure is lying (or clueless) why exactly it happened.

    Really, why lie about a few $ part since he's going to need to send it back at his cost for a replacement anyway?

    A 9V battery has definitely enough juice to fry a regular 1/4-1W potentiometer, and depending on the circuit and the freshness of the battery, it may be quite spectacular looking sight as well.

    Why on earth would he use a 250K potentiometer in an active circuit is anyones guess though.

    A wall voltage leakage through his amp is also a possibility, but in that case he's scheduled for electrocution in no-time.
    Just wait him to prove Darvins theory to be correct :).

    Whatever happened to the potentiometer was an user error though, but if You're selling a lot of parts, a buyers positive feeling about one transaction will IME be worth more than clinging onto some change.

    Granted, these kind of whiners pi$$ me of as much as any other dorks, but either suck it up or stop selling.
    I pretty much have chosen the latter.
    Just not worth it to me anymore :(.


  12. Seems more likely to me that the customer tried to use it in a cab as an L-pad. That would do it.
  13. What's wrong with using a 250k pot in an active circuit? It all depends on signal impedance, and most basses that are "active" have standard high impedance pickups and passive controls before the preamp input.

    That is almost impossible. AC power is rectified and preferably filtered, so you would have to have both an unlikely failure of a power supply, and a leak from whatever input stage component to the input. And also not damage anything else but a pot. On the other hand, if you have a vacuum tube-based input stage on an amp or pedal or whatever, and there is a leak from the plate to the control grid, it can place the B+ voltage across the signal path. This B+ is generally a few hundred Volts, DC.
  14. fisticuffs


    May 3, 2011
    Madison, WI
    Your potentiometer has just chosen a new Pope. Nothing to be worried about.
  15. I'm curious, though, if the pot was in an instrument, how did he see the flames and sparks?
  16. Scott in Dallas

    Scott in Dallas Commercial User

    Aug 16, 2005
    Dallas, north Texas
    Builder and Owner: DJ Ash Guitars
    I L'd my AO at that.
  17. Cadfael


    Jan 4, 2013
    Germany, EU
    The biggest fault sits in front of the PC (or soldering iron) ...

    Sorrry; but I can not imagine fire / smoke +++ from a pot if the user of the pot has even "sub-basic" skills ...

    Even after 24 hours bass powerplay every pot feels cold.
    There is NO heating, NO reason for fire or smoke.

    I have seen human beings doing unbelievable things (see Darwin Award) ...

    "English > German > English" (surely differnt in the English original by Brian Addams?): "If you think you constructed something fool-secure, you have underrated the imagination of fools ..."

    But (2):
    As a dealer/trader it is forbidden to say so. You can only think this - and be annoyed ...
  18. khutch

    khutch Praise Harp

    Aug 20, 2011
    suburban Chicago
    He may not be lying, it may actually have happened, but he is not telling the whole truth either. I have no idea what he did but the failure is due to what he did, or to a problem with whatever it was he was trying to fix. Do whatever you feel like doing to placate him and then never, ever deal with him again is my advice. He clearly does not know what he is doing and should not be allowed to play with a soldering iron.

    Are you saying that the battery terminal voltage was 7.08V while the battery was delivering 3.32A? That is quite a 9V battery! That is an internal resistance of only about 0.54 Ohms.

    The customer could have a serious wiring problem with his 9V battery if there is one in the bass. Whether there is a battery in the bass or not there could be a serious problem with his amp or pedal board or whatever he is plugging the bass into and when I say serious I mean potentially lethal. I would advise him to seek professional help with his equipment.

  19. JdoubleH

    JdoubleH Cold, Daring. No flies on me. Supporting Member

    Jul 10, 2008
    Ellerslie, Georgia
    THAT is so totally Rock N Roll!!!! Almost as cool as spontaneously combusting drummers! Seriously though, I once had a Fender Bassman with a wicked ground problem which was responsible for some ridiculous and painful shocks if the user touched his strings and say a mic plugged in elsewhere. No glowing strings or melted flesh, but still quite exhilarating.

    On this one, have we ruled out any kind of flammable contact cleaner as an accelerant? I once set a cheapo paper shredder on fire that way. Or perhaps the plaintiff was channeling Jimi and it was lighter fluid...

    As for 9v batteries, yes they can set stuff on fire quite easily:
  20. I call BS. Not possible.

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