Question about draining caps

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by JPenn, Nov 26, 2005.


  1. JPenn

    JPenn Member

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    How long does it take for a caps to naturally drain? I need to change out the speaker in a Hot Rod Deluxe and don't want to fry myself!! I have a gig Saturday night and don't have another till Thursday night, could I safely switch out the speaker on Wednesday? Will leaving the stand-by switch on when powering down help? Thanks
     
  2. Geetarpicker

    Geetarpicker Member

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    Years sometimes.

    Though you can bleed off some of the charge by keeping the standby on, cutting the power while you crank the amp and keep playing as it fades.

    That said you need to learn how to discharge them. I use a little test lead with alligator clips on both ends. Got them at Radio Shack. I connect it to ground (chassis) then to the tube side of one of the 100k resistors on the 1st preamp tube. Let it sit a couple minutes which gets most of it, then I jump the lead over to the main power supply filters and leave it connected there. You need to get a schematic so that you can find these points.

    Be VERY carefull, you can stop your heart here if you are not carefull or at the least get pretty shook up.

    I just noticed you are just changing speakers. You shouldn't have to worry about the charge of the caps if you are just pulling off the speaker wires. There will be no residual charge on the speaker leads. IF you have to pull the chassis you would still be cool if you are VERY carefull and just don't touch anything which is sometimes hard with an awkward chassis.
     
  3. samwheat

    samwheat Member

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    All my builds have a 180K/3W resistor on the B+ feeding the last preamp tube which goes to ground to auto drain the amp

    IMHO, this should be on all amps

    BTW, I'm an old-timer from ax84 and that scheme can be found at their schematics
     
  4. Wakarusa

    Wakarusa Member

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    Alligator clips (even insulated ones) can get you into trouble if something slips while you're trying to clip on. A better scheme is clip on the grounded end, meter probe on the other. Even better is to install a small resistor inline in the probe (there was a thread on this a while back).

    It is also usually unnecessary to leave it sit for "minutes". Even with a sizeable inline resistor, the discharge time is usually on the order of milliseconds (let me know if you want to see the math behind this).

    Finally, even for amps that have auto-discharge circuitry installed, or where you've done the standby switch thing, it's just good practice to always manually discharge caps. You'll be forced to learn where they are in the amp, it takes mere seconds, and once the habit is formed you won't have those nasty surprises when you encounter an amp that doesn't have discharge circuitry ;)
     
  5. axpro

    axpro Member

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    I usually use my tongue... Just keep the closest part of your tongue on the chassis, and attempt to lick the first cap after the rectifier, unless someone puts a wierd draining cap into the amp, it'll at very least give you the afternoon off.

    :)

    Dave McCulloch.

    But, as has been said earlier, changing a speaker is fine, just keep your mits out of the chassis.
     
  6. JPenn

    JPenn Member

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    Come on down, I'll let you swap it out.
     
  7. donnyjaguar

    donnyjaguar Member

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    I concur with the other post-er. Changing a speaker will not subject you to any high voltages and is very safe to do with the amplifier turned off.

    As for how long it takes to discharge an amp, I had a look at your schematic and believe it or not there is no provision for discharging the capacitors in this amplifier. When turning the amplifier back to standby it is going to discharge them a lot anyway. The problem I see is when the amplifier is powered up with no tubes in it. In this case the capactiros are going to hold their charge.

    Normally you can dertimine how quickly an ampliifer will discharge its capacitors by taking the capacitance and multiplying by the bleeder resistor value. So if you had say (collectively) 100uF of capacitance with a 470k resistor tied across them: 0.0001 x 470000 = 47 seconds

    I'd consider having someone install one on this ampliifer. That is unless my eyes are playing tricks on me and there is one somewhere on the factory schematic. :)

    There are a lot of semiconductors in this "tube" amplifier. ;)

    DJ
     

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