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Old August 28th, 2010, 11:32 PM   #1 (permalink)
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Discharge filter caps????

Discharge filter caps. What's that mean?

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Old August 28th, 2010, 11:40 PM   #2 (permalink)
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The caps in the power supply off your amp retain a high voltage, even after the amp is turned off. Prior to working on an amp, it's important to discharge this voltage so that you're not putting yourself in danger.
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Old August 29th, 2010, 01:58 AM   #3 (permalink)
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The filter capacitors can store a charge for a long time after the power is turned off,
so it's a good idea to discharge them by shorting across them after the power is off and before sticking your fingers in there.
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Old August 29th, 2010, 03:13 AM   #4 (permalink)
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It's not a good idea to short across the caps, use a 100 or 150 ohm, 10 watt resistor and clip the + side to ground. Another way to discharge 70% or 80% of the charge is to keep playing your guitar, chords, while turning the power switch off, leaving the standby on.
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Old August 29th, 2010, 05:00 AM   #5 (permalink)
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I always connect an alligator clip to pin 1 of the first preamp tube and the other end to the chassis, with the amp disconnected from the wall, the turn the amp on and leave it like that for a few minutes. Don't know if it is the best way, but I'm still alive :S
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Old August 29th, 2010, 08:41 AM   #6 (permalink)
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Quote:
Originally Posted by polymolly View Post
I always connect an alligator clip to pin 1 of the first preamp tube and the other end to the chassis, with the amp disconnected from the wall, the turn the amp on and leave it like that for a few minutes. Don't know if it is the best way, but I'm still alive :S
That is fine if you know that the first preamp tube has a plate connected to pin 1. That would not work for say a Vox with a EF86
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Old August 29th, 2010, 10:40 AM   #7 (permalink)
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Quote:
Originally Posted by celeste View Post
That is fine if you know that the first preamp tube has a plate connected to pin 1. That would not work for say a Vox with a EF86
I do know, but thank you for protecting my fingers from getting burn!
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Old August 29th, 2010, 11:45 AM   #8 (permalink)
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If your amp has a standby switch, leave it in "play" position when you turn the amp off. If the amp is fully warmed, enough to play through, it will self-discharge when you turn it off.

And as Ronnie Reagan said, "Trust, but verify." Check from ground to the + side of the filter caps with your meter. You will usually see a 10-15 volt residual charge on the caps. This is harmless; it's the chemistry in the electrolytics. You can discharge it with a jumper wire if you want or just ignore it.
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Old August 29th, 2010, 11:50 AM   #9 (permalink)
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What about solid state amps. I have a Vox Pathfinder. Thanks
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Old August 29th, 2010, 12:12 PM   #10 (permalink)
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Solid state amps generally discharge themselves pretty well. Generally.

Always just take a meter and check. Just check and discharge as needed.

A light bulb with clip leads attached, like the one you clip in place of a fuse for testing also works well.
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Old August 29th, 2010, 09:28 PM   #11 (permalink)
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I use the method advocated by Doug Hoffman; with amp on and strumming guitar, pull the plug while continuing to strum. You can actually hear the volume die as the caps drain and it gives you a pretty good idea how strong your caps are once you get the hang off it.

With weak caps the sound goes almost instantly, while strong caps give you noise for a while.

I always check with a meter the first time I do this with a amp.

I find this the quickest and easiest method for draining the caps on an assembled amp.
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Old August 29th, 2010, 09:35 PM   #12 (permalink)
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There's no difference between pulling the plug and turning the amp off. Strumming the guitar is also not a necessary part of the process. The amp will discharge in 30 seconds or less either way.

Of course, you should unplug it before you take it apart.
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