filter caps

[Anonymous]

hi i need to replace my filter cap in a a coliseum p.a. and was wondering if any one had any advice on how not to electricute my self, i have a 1k 10watt bleed resistor, which should do the job.

[JoeArthur]

Let's see... about 500volts of B+ across a 1K resistor would be about 1/2 amp of current.  That would probably electrocute you and roast you to serving temperature all at the same time.

Maybe you should really take it to a professional.

[Anonymous]

thanks for "the maby you should take it to a professional" comment, it would seem Tom Mitchell  auther of How To Service Your Own Tube Amp got it wrong and should be informed of this.
other than comments that are defeating in nature could you offer any practicle advice on discharging filter caps and other electrolytic type caps that have the potential to electrocute.

[Leftee]

I personally like a much higher resistor. 100 times higher at least. He is right, that is a quite a bit of current. Leave the resistor clamped across the cap, too. They can reform a charge sometimes.

[Anonymous]

ok something like a 1m ohm 5watt across the cap should work good then shoul?

[Bill K.]

Those books are good, as a reference. Read the disclaimer. Here you will get some feedback from people who use, and some who actually built Sunn amps. The info I have garnered from this site has been invaluable.

[JoeArthur]

I don't take chances with the internals of amps.  I usually wait an hour at least after switching the amp off and unplugging it.  Sometimes I wait over night.  

Either way, I use a 100K 1/2 watt that I connect across the cap terminals using a pair of insulated alligator clips.  I leave it on each cap for five minutes, and then I check it with a volt meter.

Only when everything safely reads zero volts do I even think about working on the amp.

If you think my comments were trying to defeat you instead of possibly saving your life, then feel free to follow the advice of your Tom Mitchell guru.  Sounds to me he either doesn't understand Ohms Law or prefers mysticism to sell his product.

[Anonymous]

no, staying alive is by far the most important part. ive recived quite a bit of advice on draining caps, from resisters of various sizes,across the cap, to a screw driver from the caps positive terminal to ground, to take it to a professional, and was just seeking a little more being that i could get a deadly shock, doing this particular job. just as bill k. put it books do have good info but real life feed back is invalueable, im glad we have this site. thank you to joe arthur for keeping me from dieing, if would have followed the advice of the book i may not have been typing this right now. i apologize for the deffeting comment, i was a little fed up with the take it to a profesional advice, as it is i will need to do just that to have the new filtercaps charged up properly on a variac, (unless there is another way that will do), so i hope there are no hard feelings. so a 100k 1/2 watt will do, thank you. would there happen to be any other spots under the chassis to look out for in reguards to serious injury? if so any advice. thanks again

[Greg Z]

I use a voltmeter ( picked up on e-bay cheap,
0-600v) with a resistor across it (forget the  value) as a dedicated cap drainer,
it stays on the circuit till I'm done.
Don't forget some caps exhibit a rebound effect,after you drain them and remove the load they can build up quite a charge.

[Anonymous]

yeah a coliseum p.a. i own has some after market  filter caps in stalled with the permenant bleed resistor, good idea. i to have reciently picked up a dmm, quite usefull.

[Anonymous]

As an electronic tech most of my life, 1966 to present, I was a bit surprised to read something about needing a variac to "charge up" filter caps after installation. I have never heard of such a requirement. John  jhumphre@netidea.com

[JoeArthur]

In some places... a comment like that might be considered a "troll"!!

I agree with you, never did it, never saw a reason for it, will never do it.  IMHO... It probably developed as a way for some tech to charge extra money.

[Tom Loizeaux]

  Before I work on replacing filter caps, I always leave the amp standby in "play" position and leave a speaker plugged in. I usually wait for a few hours. I've used the resistor/ground wire a few times, but never had any sense that I was actually draining any voltage. I've never been zapped either.
  The idea of bringing up caps after replacing them in an amp is not needed if you use fairly fresh caps. If they have been sitting around ( a year or more) it is considered good to bring them up on a variac to slowly reform the electrolyte. If they are weak, and you "zap" then with full 120 AC, you could be shortening their life expectancy.

Tom

[Leftee]

Yeah, I never use the variac to bring an amp up after a recap. If I'm bringing up an amp that has sat around for awhile, I'll use the variac. Sometimes it's just the thing for old caps.