Safety sticky thoughts?

Discussion in 'Shock Brother's DIY Amps' started by Ben Harmless, Feb 17, 2009.

  1. Ben Harmless

    Ben Harmless Friend of Leo's

    Mar 10, 2003
    Salem, Mass
    I posted over in the forum issues forum regarding a possible safety sticky here in the Shock Bros forum. I figure it would be a good move, and we could all just link or refer to it when it's warranted, instead of typing out countless warnings about the dangers of sticking your tongue on a charged up filter capacitor.

    Paul's on board, but someone needs to write something. I thought maybe a team effort might be in order.

    I figure we need a paragraph on the overall danger, and then one or more suggestions on how to safely discharge filter caps. We could compile them here, and then someone can repost the result for a cleaner sticky.

    Make sense? I can't tell. It's too early in the morning for me - I've got a road trip for work, and I'm waiting for my comrades to show up.

    How's about this for a start:

    Vacuum tube (valve) amplifiers contain potentially lethal electrical charges, even when powered down and unplugged. Do not open an amplifier if you are not completely versed in the necessary safety measures. This is a hobby for some, and a profession for others, but like many others it involves a certain amount of risk that can be minimized by following a set of best practices.

    First and foremost, never perform any repairs or modifications to the circuit of any amplifier when it is plugged in, whether it is powered on or not.

    Secondly, the filter capacitors in tube amplifiers are of significant concern. They are typically the largest components inside the amplifier, and in some cases are located outside of the main chassis. Capacitors by their nature maintain a charge after current has stopped flowing to them. This charge can be extremely hazardous. To work inside of a tube amplifier safely, the capacitors must be discharged."

    ...and then I figured we could follow up with alternate cap discharge methods and other safety tips. I'm loathe to type out the "screwdriver method" myself, just because I find it somewhat clumsy, and generally don't think it's a good idea.

    I personally use a 150 ohm 10w power resistor in the middle of a length of wire with an alligator clip on one end, and a probe on the other. All wire connections are covered by heatshrink. A few seconds on each cap and they read clear. One should note that the filter caps in some amps may not be visible - the cap for the preamp supply on some Marshalls is under the board, and of course the caps in some Fenders are mounted to the underside of the chassis. In this case, I touch both ends of the power supply resistors on the board until they measure safe as well.

    Keeping one hand in your pocket during this procedure can be a good idea too I suppose, but I'm typically holding my multimeter probe with my free hand, ready to measure things just in case my resistor shorted and I didn't know it. Either way, using both hands on a live or charged amp is a good way to pass a nasty shock right through your heart.

    Anyone care to add anything?

    ...By the way, there used to be a great little module (2 minute) show on the public radio station that I work for that covered the etymology of words that people probably think they're using correctly. Did you know that the word "electrocuted" necessarily indicates that the victim died from the experience? Otherwise it's just being shocked. The show was funded by Merriam-Webster, who just had to pull its support because of financial difficulties. And so it goes.

    Okay, yes, I'm bored.
  2. ThermionicScott

    ThermionicScott Poster Extraordinaire

    Dec 3, 2005
    I remember there were several attempts at a Tube Amp Safety sticky thread in the Amp forum, but at the time, they got denied sticky status.

    I gotta run to work, but I'm sure they're findable here... some good stuff in them.

    - Scott
  3. Natstrat79

    Natstrat79 Tele-Meister

    Jul 28, 2007
    I think its a good idea to have the warning but I wouldn't put the part about how to discharge capacitors. Seems like it could cause a liability problem.
  4. dangelico603

    dangelico603 Tele-Meister

    Jan 19, 2007
    Oklahoma City
    Maybe just a bunch of links to articles on electronics/high voltage safety?
  5. wnorcott

    wnorcott Tele-Holic

    Sep 30, 2008
    New Hampshire, USA
    "Wear a lineman's glove and use an insulated screwdriver to short across capacitors before working on the amplifier"
    Last edited: Feb 17, 2009
  6. shadowfan

    shadowfan Tele-Meister

    Dec 13, 2008
    Atlanta, GA US

    I have read that you could discharge an amp's power supply by leaving the amp on (both power and standby switches ON) and then unplugging. That seems logical as the caps remain across a load through which they can discharge while the tubes are still hot; can anyone comfirm or, on the contrary, refute this? (I'm thinking in terms of an AB763 circuit here).
  7. mofugly13

    mofugly13 Tele-Meister

    May 4, 2007
    San Francisco

    I doubt that it would be a liability problem, but if there was a liability issue, don't you think it would stem from talking about working on amps, and not having clear concise instructions on discharging caps??

    When I built my 5e3, I had a hell of a time on the net trying to find out how to safely discharge caps. I started by just putting my meter across them and watching it count down, but I think a 5e3 drains the caps by itself after it's shut down. Not sure if this is righ, or why it happens.

    FWIW, Ben, I think a sticky is a great idea!!
  8. EdMax

    EdMax Tele-Meister

    Mar 18, 2008
    Northern Illinois
    I belong to several great forums, and I have always felt that safety sticky's were extremely important.

    One of my favorites is over at the DIYaudio forum. Yeah Yeah, audiophiles can be a bit eccentric, but the thread over there covers a lot of important issues.

    Most likely you will need to sign up (free) to view it, but it is indeed well done by tube masters.
  9. mofugly13

    mofugly13 Tele-Meister

    May 4, 2007
    San Francisco
    Originally posted by Tele Dave:

    Disclaimer: Amplifiers contain DANGEROUS AND DEADLY VOLTAGES. If You do not understand proper safety procedures, DO NOTN ATTEMPT WORKING ON THE EQUIPMENT.

    That said, unplug the equipment.

    First thing, go to radio shack and get a set of test leads with alligator clips on one end. Dont use speaker wire or lamp zip cord or ordinary hookup wire. Test lead is rated for at least 750V. Plus the alligator clips ends are insulated also to at least same voltage. Dont skimp out here. YOU DO NEED THE PROPER INSULATED WIRES AND CLIPS.
    Purchase a 1meg ohm resistor of at least 1/2 watt rated.
    Anything less than about 700k ohm will throw a sizeable spark on hookup
    Get some shrink wrap of a couple of sizes. (Diameters)
    Shorten leads on resistor to about 1/2 inch.
    Shorten test leads to about 18-24 inches to make more convenient lenght to work with.
    Solder resistor onto wire end of a test lead.
    Slip at least 3 different sizes of shrink wrap over resistor and slide down to end. Shrink wrap should be sized to fit inside of each other and be at least 3 inches long
    Take other test lead and solder to other end of resistor
    Slide shrink wrap up over resistor I piece at a time and center resistor inside length of wrap and heat to shrink. Do all 3 pieces the same.
    Now you have a draining tool.

    Clip black end to chassis. (Doesn't make any difference as to which color goes where).
    Be very careful here. Use ONE HAND RULE. Only reach into amp with one hand and do not touch anything with other hand. Put it in your pocket if you have to. You to not want to have a complete circuit so voltage will go in one hand and out the other. almost sure to kill you.
    Clip other (red) end to positive (+) lead of the biggest cap you see.
    I meg resistor provides enough resistance so that there will be no or a very small spark upon connection to cap.
    After about 30 seconds start checking voltage on + side of cap using one hand rule. Might take a minute or two to drain.

    When voltage is down to about 2-3 volts, check other caps voltage also.
    If everything is down below 2-3 volts, it is safe to start work, but leave drain hooked up for a short time because caps self charge to several volts after a fiew minutes and this way you wont be surprised by a little tickle after a short time.

    Then, enjoy your work.

    P.S. I have a friend who used to use a screwdriver to ground cap directly to chassis, untill he welded screwdriver to chassis, blew a chunk of metal off and almost blinded himself. DC voltages this high are not to be taken lightly. It will at least burn you very badly. AC at these voltages, (300V) you would most likely survive unless you were standing in a puddle of water or holding onto a water pipe. Please be very careful
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