DRAIN those Caps!!!!

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by timeforheroes, Jun 24, 2008.

  1. timeforheroes

    timeforheroes Member

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    Hey guys,
    I am facinated by the DIY possibilities concerning guitar gear. I am getting into amplifiers quite a bit. I would like to try taking a look into some of my amps...and who knows maybe get a mod for some of em too!

    But, before i do.....how do i drain the caps! i dont want to get the shock. OUCH !!! KILLER!!!!


    so, do i need a tool? can i just use my mulitleter? what is the basic process of draining the stored voltage?


    any tips appreciated!
    Thanks
     
  2. 5E3

    5E3 Member

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    I made up a wire with an alligator clip at both ends, and a resistor in the middle. I clip one side to ground and the other to the caps.
     
  3. Jason Carter

    Jason Carter Member

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    Always keep one hand behing your back or in your pocket when draining caps. I did the same thing as 5E3, 1000 ohm 3 watt resistor between two alligator clips with some heat shrink covering the resistor.
     
  4. 12guitdown

    12guitdown Member

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    Also note, to connect the ground first.

    Question. Are caps the only thing that store voltages in amps. Do transformers store any?
     
  5. billstets

    billstets Member

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    If you do a search, you'll find a number of options for how to discharge them. You should have a multimeter to see how much is in the caps to begin with, and if any is left after you've drained them. Here are some options for draining caps:

    1. Do as 5E3 suggests, though I believe this can spark in some cases. At least it often does when people do it with a big-assed screwdriver (cap to chassis).
    2. Clip one end of the alligator to the chassis and the other to one of the pins for the plate wires on one of the preamp tubes.
    3. In a tube rectified amp, you can pull the rectifier and then turn on the amp (including standby) without the rectifier tube and run it for a few minutes.
    4. While the amp is on, pull the plug and strum your guitar and while the sound is fading away, most of the electricity is discharged.
     
  6. timeforheroes

    timeforheroes Member

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    1. I havent heard of sparking alligator clips.....but, I have heard that some high voltage discharging was enough to weld the screwdriver? WOW
    2. self explanatory
    3. is this true?
    4. Is this true? you say MOST of he electricity. should i follow step 4 with step 1?
     
  7. timeforheroes

    timeforheroes Member

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    for all of you alligator clip users.....is there just a little thing that i can buy? or do i have to make one?
     
  8. billstets

    billstets Member

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    Well I always use #2, and it works. You can actually see the voltage drop with the MM. I've heard builders say that after about 5 minutes an amp is safe even if you don't drain the caps with a jumper, especially if you unplug it while it's on. But I would check with an MM to be sure!! As for the rectifier trick, again you'd want to check with a MM to be sure, but one of the gurus on this site (was it John Phillips?) posted about this method.
     
  9. billstets

    billstets Member

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    I don't know, but I don't put a resistor in my jumper that I use for draining throught the plates. So it's pennies to make it your self - a wire with two alligators at either end. I insulate the end I attach to the preamp tube pin. I put the other end on the chassis first.
     
  10. timeforheroes

    timeforheroes Member

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    you know what...now that i think about it.... how do i get some of these alligator clips? you guys say that you built em?

    also.....once i have some. I ground one end. and I clip the other end to what?
     
  11. billstets

    billstets Member

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    Two alligator clips that you can buy from Radio Shack. Then you solder them to either end of a wire (I think I used 18 gauge). You can insulate each end with shrink tubing and/or electrical tape, or you can buy insulated alligators to begin with. I think if you attach to the filter cap, you also put a resistor in the middle of the wire. The method I use is to clip one end on the chassis and then clip one end to one of the plate wire pins on a preamp tube. I have my multi-meter attached to one of the large filter caps and the other end to the chassis, and I watch the current drain out on the MM. Leave the jumper attached as you do your work, because I've read the caps can somehow re-acqiure a charge. As mentioned above, always keep one hand behind your back as you clip one end of the MM to the cap or the alligator to pin, until they're discharged. Then of course you'll need two hands to do your work!
     
  12. timeforheroes

    timeforheroes Member

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    WOW thanks. I guess ill be going to radioshack later today!
     
  13. Nolatone Ampworks

    Nolatone Ampworks Silver Supporting Member

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    Ditto. In a lot of amps it's also safer to clip to pin one of the first preamp tube too.
     
  14. billstets

    billstets Member

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    On my JTM45 clone, I just clip it to pin one of the preamp tube. Even before draining, I've never measured more than 40volts on either of the large can filter caps, and if I turn the amp off leaving the standby in the on position, the voltage is much lower.
     
  15. timeforheroes

    timeforheroes Member

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    By the way. here is a gut sot of the amp to be worked on.

    [​IMG]
     
  16. jay42

    jay42 Member

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    Matchless (clone)? ...k, some of us are having a hard time believing that you don't know what a roach...er...alligator clip is.

    http://en.wikipedia.org/wiki/Alligator_clip

    There should be a resistor across one of the blue caps - maybe the grey one in between the black and blue ones - that has a value between 200k and 500k. It should be there to bleed the caps down after the amp is turned off. With functioning bleeders, you can unplug the amp from the wall, turn it ON (standby too) and pound away on a guitar. Keep the speaker connected. The fizzling is the sound of the caps draining. Just remember to turn the switches back to OFF.

    If there isn't a bleed resistor, you should add one. A 2 or 3W 470K will do fine.
     
  17. The Smith

    The Smith Supporting Member

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    I never understood how you can attach a clip to a tube pin if the pin is inserted into the tube socket. Isn't it inaccessible? I'm sure I'm just misunderstanding a really simple concept. Explanations welcome!
     
  18. jay42

    jay42 Member

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    Just shorthand. Connect to the socket lug corresponding to the appropriate pin.
     
  19. billstets

    billstets Member

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    Sorry, I mean the terminal for the pin inside the chassis.
     
  20. timeforheroes

    timeforheroes Member

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    the chassis pictured is a hayseed
     

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