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Discussion in 'Amp Central Station' started by rich602, Aug 28, 2010.
Discharge filter caps. What's that mean?
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.
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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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.
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
I do know, but thank you for protecting my fingers from getting burn!
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.
What about solid state amps. I have a Vox Pathfinder. Thanks
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.
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.
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.