Biasing? "lethal", "danger", "NOOO!"

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Cambian Dawn, Sep 29, 2008.

  1. Cambian Dawn

    Cambian Dawn Member

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    Hey all, I thought I'd post on here about a not-so-recent issue I've had with my guitar amp - be warned, this is mainly out of frustration so as much as I'll try to be coherent if I start typing gibberish you'll know why!

    I have a valve/tube amp. One or more of the four output tubes died. I get a new set. I need to bias. BUT! I can't - I don't have ready access to a tech. Now, I'd like to do this myself and have found a tool that will allow for minimal pokage on the innerds of the amp. I've tried asking a number of people what to be wary of when adjusting the bias trim pots, but all anyone seems to want to say is "Don't do it", "lethal voltages", "get a tech!".

    I'm not an idiot (which is why I haven't adjusted it yet), but I'd like to be able to do this myself without getting a damn electronics degree! Can anyone explain whether I am likely to kill myself by adjusting the bias trim pots inside my amp, or whether there is/are specific points inside a tube amp that can give you this kind of shock? Is it all of it? If I touched a capacitor would it throw me across the room?! If I brushed against a wire would it fry me like a piece of bacon?

    If anyone would care to shed some light I'd be most appreciative. I'm not advocating going off and poking around amps, but my situation dictates that I can't get anyone else on the case. It used to be, but now it isn't.

    Cheers!
    C
     
  2. scottl

    scottl Member

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    It would help to know what amp.....

    But, biasing is easy. As long as you learn how to safely work, it is no issue.
     
  3. Cambian Dawn

    Cambian Dawn Member

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    Hey scottl, the amp in question is a Matamp LX (PTP wired on turret boards), but I may be down-sizing to a smaller amp soon.

    On the subject of safety, do you have any pointers on how not to kill myself? I know "how" to bias an amp, but I've always been wary due to all of the warnings I've had. As I said before, I'm not an idiot - I wouldn't go randomly poking at the circuits with a screwdriver - but any info would be great.

    Thanks again,
    C
     
  4. Adwex

    Adwex Member

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    Usually the danger is from the large capacitors that hold a significant amount of charge. You can discharge them by grounding them through a resistor so they discharge slowly. I don't know what a good R value would be though. Once the big caps are discharged, I believe you're safe, but just to be sure it is highly recommended that whenever poking around in a tube amp you NEVER use both hands...they say you should always keep one hand in your pocket. This is to prevent the current from going through your heart if you happen to touch something "hot" with one hand, while you're other hand is touching the chassis or some other ground. You don't want give the current a path to ground that runs through your chest. One hand only in the amp, and wear sneakers or other insulating shoes.
     
  5. scottl

    scottl Member

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    Just turn the bias pot/trims with a nice insulated screwdriver. Don't touch the chassis with both hands. If using a multimeter, clipping it on will help. If you have rear test points, you are golden.

    If you are concerned about brushing up against stuff, you could measure the bias. Then shut off amp. Tweak trims a millimeter. Then turn on and remeasure. This way you can set the bias with the amp off and still get where you want to be. Not the best way, but one that might appeal to you as you are nervous.
     
  6. Cambian Dawn

    Cambian Dawn Member

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    Thanks for all the info guys - breathing a sigh of relief and hoping to try this out sometime soon. I will post an update of success... he says tentatively :D
     
  7. PRNDL

    PRNDL Member

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    Perhaps a better question may be "How did you shock yourself working on a tube amp?"
    The answers will be much more revealing.

    I'll start with my latest ...

    I flipped the chassis over on the bench with it plugged in but turned off. In the process, I touched a wire on the fuse/switch ... 120 volts AC ... ouch!

    Anyone else?
     
  8. GearHeadFred

    GearHeadFred Member

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    Discharging the caps is great advise but it won't help you bias the amp. The amp needs to be running to measure and adjust the bias.

    The most safe way is if the amp has bias measuring jacks.. just plug in a multimeter and dial the desired number.

    If the amp does not have external measurement points, use a bias probe (plugged in between the amp and one of the power tubes). Again, the high voltage is kept safely away from you in the chassis..

    When you open up the chassis is when things get dangerous..
     
  9. WaltC

    WaltC Member

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  10. Hacksaw

    Hacksaw Time Warped Gold Supporting Member

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    [​IMG]

    Sorry, I couldnt resist! :D

    I built a amp stand. made out of wood that cradles the amp almost (angled) upside down.. you can use the amps headshell sometimes to cradle the amp upside down while you are setting the bias. This way you can have one hand do the work to turn a bias adjuster. You dont need to be doing the balancing act as well as setting bias.. Just be diligent in your moves inside the amp with a insulated screw driver like mentioned. I covered a screw driver with electrical tape.

    a shocking story, Working on an amp, and flipped it on, I received a zzzzzap from the metal ON switch mounted (grounded) to the chassis... This felt like I was riding a jack hammer. found out what was wrong really quick!
     
  11. ked

    ked Supporting Member

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    Unless you have a screwdriver that is certified as being insulated, I would not use it inside an amp which has voltages in excess of 400 volts. If the handle has the tiniest unseen crack in it, the voltage will find you. I just get a small diameter wooden dowel and grind the end down to the shape of a flat blade screw driver tip. Works fine and cheap.

    Ken
     
  12. stratman_el84

    stratman_el84 Member

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    As far as the proper tool to adjust bias trim-pots, Radio Shack to the rescue for only $5.97 (nerdy pocket-protector extra):

    http://www.radioshack.com/product/i...+tool&origkw=alignment+tool&parentPage=search

    Here are some tips to working safely with amps:

    When working in a live chassis, especially if inexperienced, the most important thing is to not work alone. Make sure someone is there to kill power/knock you away if needed. Set a wooden-handled broom close, and instruct the other person to use that to push/knock you away if needed, and to NOT grab you bare-handed, as they'll get shocked too. Make sure there's a power-kill switch that kills power to the whole bench if possible, close and handy.

    Make sure the amp is well-supported and won't fall over. Keep the area clear of anything that might get in the way of your arms and body, or fall/get knocked into the chassis around where you're working. Clear away any wires/cables on the floor around where you'll stand/work.

    Plan each step of what you'll be doing and how you'll do it before starting. If possible, use clip-leads to attach meter leads to test points before applying power. Always keep one hand behind your back or in your back pocket so as not to provide a ground path through your chest and heart.

    Wear safety glasses. I know many will pooh-pooh this, but all it takes to realize the value of this is one time of having a cap explode or a violent short to happen when you've got your face down there.

    Triple-check your meter settings for correct measurement type and range before applying power, and also check meter leads are properly and tightly seated.

    If planning on working on the amp powered-down (replacing parts, attaching meter leads, etc), you can make a simple discharging tool of two lengths of wire, a 100K 1 watt resistor and two insulated alligator clips (available at Radio Shack). Solder a wire to each end of the 100K resistor, then an insulated alligator clip to the remaining ends. Put a piece of heat-shrink tubing over the resistor and its' leads (remember to slide the heat-shrink tubing on *before* you attach the second insulated alligator clip!).

    Attach one clip to the chassis FIRST! Clip the other end to the metal shaft of an insulated-handle screwdriver or you can use a piece of wood with a nail in the end, and attach the clip to the nail. Touch the screwdriver/nail tip against the hot side of each filter cap for about 30 seconds, starting at the lowest-voltage point in the series of caps and working towards the cap closest to the rectifier diodes or tube. Then attach the clip to the hot lead of the filter cap closest to the rectifier diodes/tube and leave it there while you work, for safety. DON'T FORGET TO REMOVE IT WHEN DONE!

    Here are some additional general rules to follow whenever working on or around any equipment containing dangerous voltages and current:

    NEVER work tired, frustrated, sick, hungry, impaired, distracted, or upset. Wait for another better time.

    NEVER rush or hurry! Always think two or more moves ahead. Be situationally-aware.

    Keep distractions to a minimum, and don't try to carry on a conversation/talk on the phone. Don't have a crowd around.

    Wear shirt-sleeves to avoid cuffs snagging. Remove necklaces/rings/watches/bracelets, etc.

    Keep tools out of the way, but handy. Don't stretch to reach things.

    Most importantly, try to relax. Be methodical. Make and follow a checklist if this is new to you. Don't fear electricity, but DO respect it.

    I've worked on equipment that contains extremely lethal voltages/current for most of my life and haven't been hurt because I developed safe work habits as described above, just as one must develop safe driving habits. Always think things through before you do them. Safety around electricity is a mindset, not a magic list.

    Cheers!

    Strat
     
    Last edited: Sep 30, 2008
  13. 5er Driver

    5er Driver Member

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    I also apply this train of thought when filing my tax returns.....I'm so behind.........
     
  14. skipm45

    skipm45 Member

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  15. gtrnstuff

    gtrnstuff Member

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    This ought to be a sticky. Good summary.
     
  16. rockon1

    rockon1 Member

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    I realise this doesnt always apply however all my current(unintentional pun)amps will drain the caps if switched off without using the stand by switch. Bob
     

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