Hey UG,

I have a wah pedal that needs a DC9V 100mA~ adaptor.

I have one (from an old phone we used around that house) that's DC9V and 300mA.

Is it safe to use ?
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No, you can't use just any old adapter. Like gibson jack said, you'll fry it. Check online for adapters from the company that produced your wah, or find a daisy chain for your pedalboard (unless wah is the only one you're using) that'll accept it.
Nah, 9VDC is 9VDC any way you slice it, and the adapter has enough juice to run the wah, so why not?

The risk of frying your wah comes from mixing up the POLARITY of the power supply, so make SURE that's correct before plugging it in.

As long as you match polarity and are sure it says DC not AC output, you're good.
What the hell are you on about, 'it'll fry it'?

The mA doesn't matter in the slightest unless you're powering multiple pedals and are worried you won't have enough juice. The adaptor will only provide as much current as the pedal(s) it's powering uses, and i doubt there's an adaptor on the market without the juice to power a single pedal.

However, you must ensure the polarity is the right way round (most guitar pedals are center negative), or you could damage the pedal and/or the plug.

Your adaptor (and hopefully the wah) should have, somewhere on it, a little symbol like this:

If it's the wrong way round, it's not particularly difficult to snip the wire and solder it back together the other way round, or else look for a new adaptor.
Yeah I'm not sure why people think its going to explode... ^^ Kyles post is the only thing i'd check
Quote by mespinos
Yeah I'm not sure why people think its going to explode... ^^ Kyles post is the only thing i'd check

Aw, no love? jus' playin', Kyle's diagram is perfect.

As stated, the current rating of the power adapter (300ma) is the maximum, and the pedal(s) will only take what they need.
I go by the 3/4 rule:
If your pedal(s) need 300ma, make sure your power supply's rated to at least 400ma, just to give yourself some breathing room.

In this case, you've got plenty of headroom.
Good to go as long as polarity matches!
I'm going to slightly dissent and say an adapter of the specified rating would be adequate. For manufacturer liability reasons a spec should always be conservative. That means an adapter should easily provide what it specifies, and a device should never exceed what it specifies.

IOW, it never hurts to have more but just having enough should be plenty. I've actually been running a multifx that specs 2.1 amps with an adapter that supplies 1.3 amps for quite a while with no problems or excessive heating of the adapter.
Nobody mentions if it's regulated or unregulated?

Big difference, many modern pedals would not take unregulated adaptor without shortening the life of components.