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Honestly, the name of this sub-forum is my probelm - Tube Amps

Discussion in 'Shock Brother's DIY Amps' started by Throttleneck, Dec 5, 2012.

  1. Throttleneck

    Throttleneck Tele-Afflicted

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    Ok, in another post I talked about my interesting in building amps. This actually extends to pedals as well so I am getting my feet we messing around with these.

    I am actually having a good time. And I want to continue.

    And I really am fascinated by tube amps. But there is this little problem.

    I don't want to die. :)

    This whole thing about discharging the capacitors, making sure you have one hand behind your back so you don't stop your heart thing messes with my head.

    I am happy to do all kinds of physically dangerous things but I would hate do die just because I wasn't careful enough.

    Which makes me wonder why there isn't some design that would make the capacitors discharge automatically when the power was turned off. Is there some reason that was never done. You would think that some smart person would have figured that out long ago.

    Why isn't there some sort of automatic drain on the capacitors?

    I know this is likely a very ignorant question, but I am curious.

    Thanks!
     
  2. schoolie

    schoolie Tele-Holic

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    There is a way to automatically drain filter caps--just connect a 220K-470K 2-3W resistor across them. They drain fairly quickly, depending on the value of resistor used. I think most modern tube amps have them


    By the way, I was very nervous on first power up (which I think is healthy!), but if you follow safety guidelines the risk is very minimal. I, for one, would never stick my hand in a live amp. That's why alligator clips were invented:lol:
     
  3. celeste

    celeste Friend of Leo's

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    It is not hard to be safe, just be in the moment, think about what you are doing every moment you have a probe in an amp. I am one of the most absent minded people you might ever meet, but I have not killed myself yet. That includes learning on TV's with ~20Kv flyback curcuits, +-5kv polarizing supplies for homebrew electrostatic speakers, ~1.6Kv GM 70 amps. Be honest with yourself about your abilities, strengths and weaknesses. Develop a methodology that keeps you safe from those weaknesses and never deviate.

    I have done some stupid things, placed my palm across the terminals of a fully charged 1800uf 450v cap, but because I had been careful with where my other hand was, the current path was just across my palm, and not across my chest.
     
  4. Wally

    Wally Telefied Ad Free Member

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    But....even with that 'safety' there, it is prudent to drain and check to assure that there is no voltage present, imho. Assuming something when electrical charge is involved can be dangerous. YMMV.....let's hope your heartbeat doesn't!?! (8^O.... (;^)
     
  5. muchxs

    muchxs Doctor of Teleocity

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    "Safe" is a relative term. It's a range.

    There's

    Absolutely Safe (supposedly idiot proof although there's no such thing)

    Relatively Safe

    Relatively Unsafe

    Way I figure it working on amps is relatively safe compared to working on high voltage, working on towers or working with high explosives to name a few. We consider "high voltage" to be around 500v. The power lines out in front of your huse are a lot more than that, 30kv, 50kv and more. There are guy7s out workin' on 'em every day. They're "safe" because they know what to do and what not to do.

    Workin' on a repeater on a cel tower 150' up. There's a way to make it "safe". Don't fall. By itself falling isn't dangerous. The landing is the hard part.

    High explosives can be relatively unsafe if they're not handled properly and carefully. They're not dis- similiar to working with high voltage. Handle them properly and you won't have a problem. It's all about situational awareness.
     
  6. waster

    waster Tele-Holic

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    Check this out for a little help

     
  7. Commodore 64

    Commodore 64 Friend of Leo's

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    Just short the + of the main filter cap to the chassis with a long screwdriver. After the flash, it'll be all good. ;)
     
  8. dsutton24

    dsutton24 Doctor of Teleocity Ad Free Member

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    They did. It's not done in consumer electronics because, generally speaking, once an amp is working and buttoned up, there's little reason for the average Joe to go poking around inside it. 'Course, we're not average. :D

    Many reeeeeally big pieces of electronic gear like motor drives or transmitters do automatically discharge capacitors. It just a matter of hooking up a resistor to the cap, and running it through a contact on the door switch or supply contactor.

    On a guitar amp you could use a dpdt power switch, using one pole as a power switch, and the other as your discharge circuit. That doesn't do you any good, of course, if you yank the power plug without turning the switch off.

    As always, meter the caps to make sure whatever means you're using actually is working.
     
  9. printer2

    printer2 Poster Extraordinaire

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    As anyone with a fair bit of experience and they have stories like these. I got zapped by a car spark plug circuit, ouch, in my early 20's I was messing around with electrostatic speakers and my brother asked whether the resistor in the polarizing supply was hot, touched it with one finger and found myself 10' back on my ass with him laughing.

    Most of the time it ends up being a gentle reminder to watch what you are doing. What the body really does not like is high voltage with enough current that travels across the chest. Always be sure you are not going to be making a complete circuit with both your hands. An accidental brush of your hand in a live circuit will cause you to tuck your tail between your legs but generally you will be around to tell the tale.

    I sometimes have to test for power with standard meter probes in a 600V system. I make sure my hands are holding the probes securely and away from the metallic tips. I make sure I poke my tips close to a right angle of the live metal so that they do not accidentally slip. Quite often inadvertent contact in the wrong part of a circuit is because your probe slipped off the part you were measuring. Sharp tips help reduce that.

    In an amp chassis I will use a test clip and connect the meter to the ground side of the circuit and then I only have to deal with one probe. Don't forget that if you have done this also that if you decide to adjust the volume pot or other control that the control is probably at ground potential and your other hand that is poking around inside could end up being considerably higher if you screw up. Might be best to adjust controls when you are not measuring, at least at the start.

    Also don't go poking around if you are tired or have distractions around.
     
  10. keithb7

    keithb7 Friend of Leo's

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    re Shock Bros

    I too had some apprehension when I first began thinking about building an amp. I really took things slow and studied as much as I could before I began. Safety was my #1 objective. The first amp I plugged in, I too was very nervous. All went well. I too poked around in live amps with a chop stick and my digital multi-meter. I was always very careful and kept one hand in my back pocket. The only time I ever got a shock was when I replaced a broken LCD screen in a digital camera. The flashes work with some wicked capacitance!

    You probably put yourself in more danger when you get behind the wheel of your vehicle and drive everyday, than building an amp. At least you are in control and the amps do drink and drive or swerve in your lane while texting. Take your time, be smart and have a back up plan. Double and triple check. Your work before plugging in for the first time. Maybe plug your project into a GFI plug and have a friend stand by ready to switch a breaker if things really go a-wry.
     
  11. schoolie

    schoolie Tele-Holic

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    I learned about electrical safety from these men

    [​IMG]
     
  12. max_twang

    max_twang Tele-Afflicted

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    Great explanation -- thanks for posting that.

    In the video, Bill M says that simply turning off a warm amp to discharge the caps does not apply to amps with tube rectifiers. Could someone explain this? I'm guessing that it's b/c with a solid state rectifier, the power tubes will conduct even when the power is turned off, but this won't happen with a tube rectifier once it cools off. Could someone more knowledgeable than myself confirm if this is correct?
     
  13. backline

    backline Tele-Afflicted

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    Not sure If I have ever read an account of a guy being killed while repairing a guitar amp. Probably not very likely. But that is "neither here nor there" as they say.
    I think Wally makes a great point. Do not ASSUME anything. Get into a routine of safety. Not unlike driving a car. If you change lanes, look over your shoulder every time. So what if you just looked 5 seconds ago, and there is not a car within 3 miles. THAT is when you have an accident. Just go ahead and get into a routine of being safe, and checking for potential danger(s).
    There is an old saying the world of guns:
    "A loaded gun never killed anybody. It is always the "unloaded" guns that kill people". :)
    As soon as you assume something is safe, unloaded, has no voltage, is a clear lane, a door is locked, etc etc etc. THAT is when you have problems.
    good luck
     
  14. hackworth1

    hackworth1 Friend of Leo's Vendor Member

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    Bill M is correct when he explains in that video that (under normal circumstances) the hot tubes (after you turn off the amp) will use up most of the remaining energy stored in the Filter Capacitors - draining them down to safe levels.

    The problem occurs when someone builds an amp and has not done everything correctly.

    One example:

    You power it up using proper safety precautions and you get no sound. For example, your power tubes and preamp tube dont heat up - your heaters are miswired, perhaps.

    Now let's say that your PT is making good HT (High Voltage) and your Rectifier (Tube or Solid State) is converting that High Voltage AC to High Voltage DC (aka B+). IOW, normal function and working properly.

    It sends that B+ to your First Filter Cap and on to all Filter Caps and Charges them up nice and high. Nothing is Drawing that current. The Filter Caps are holding that energy like a battery.

    So what occurs is that your filter caps have been charged and you've got 400 Volts or so in them. That voltage won't draw down for a long while.

    You need to drain it. You can do it with an insulated jumper wire (stripped on each end) or a long screwdriver (with an insulated handle).

    Put one end to ground and the other end to the positive terminal of any of the big capacitors. You will probably see a spark.

    Check with your meter.

    You can also drain the caps with your meter. Set to high Voltage DC. One terminal on the meter to ground, the other on the positive end of any of the big caps.

    All the big caps will drain by doing this to one of them.
     
  15. printer2

    printer2 Poster Extraordinaire

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    Best to discharge the capacitors with a resistor and not a screwdriver or piece of wire. A dead short puts a lot of stress on the capacitor.
     
  16. tubeswell

    tubeswell Friend of Leo's

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    :eek:
     
  17. Racing

    Racing Tele-Meister

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    +1.
    Regard a tubeamp as would you a loaded 45cal and you´ll be just fine. Feel respect for the voltage. Not fear.
    As the guys have explained above...there is no magic. Just be aware...and TBH having done this for a while by now the dangerous time isn´t when you start out.
    Dangerous era will commence as you get accustomed to what needs to be done...and haven´t got hurt yet.
    I believe i talk for more or less all of us when i say that those that have commented in this thread have all been zapped to a lesser or larger extent.
    So have i,and i´m still here to tell you about it ain´t i? ;)

    Had a "pupil" at the shop recently. 20yrs old and full of himself. Currently becoming an electronic engineer.
    I´ve told him...numerous times..as he stuck his hands into this 30watter of his...
    `N i laughed...n laughed..cause "some men just can´t be reached!:lol:". I think he got struck like 5 or 6 times in that 2hrs period...more cuzz words than you would imagine..:lol:
    On the other hand...well enough.
    He at least took to the golden rule.
    YOU NEVER EVER WORK ON A TUBER WHILE THE CORD IS STILL IN THE WALL!!
    NEVER!
    Measure..yes. With one hand in pocket. Work? No and no and no again.

    He´ll live...and most likely not perform them idiotic stunts ever more..:D
     
  18. andyfromdenver

    andyfromdenver Friend of Leo's

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    Mr. Ohm declares this thread safe for public consumption:
     

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  19. Throttleneck

    Throttleneck Tele-Afflicted

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    Thanks everyone. I know I am being a tad paranoid. I work around horses all day, I am way more likely to get killed there, but for some reason amps are scarier. :)
     
  20. andyfromdenver

    andyfromdenver Friend of Leo's

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    I wouldn't have started had I not had one on one teaching. Try locally to see if someone can show you some safe practices. Building your own amp is really addicting and amazing. If it's been nagging you for a while try to make it happen!
     
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