Can i use a 9v 500mA adapter with a pedal that should use a 9v 300mA ? no?
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I dunno. Try the manufacturer's website.

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i wouldn't. if the ma were the other way around, 300ma pedal and 500 ma pedal i'd say try it but it prolly wouldn't work, but that sounds like you're asking to fry something inside.
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Quote by the_perdestrian
listen to revelation, for he is wise in the way of bass-fu
Nothing will fry as long as:
- The mA output rating on the adapter is higher than the required mA rating of the pedal (doesn't matter what the difference is!)
- The polarity is correct
- The voltage is the same

The only problems you might encounter would be noise - the cheaper adapters do not have power regulation (or power conditioning, filtering, whatever you want to call it), so you might get some buzz by using it. But if you're not doing anything too hectic, this noise level could be acceptable, so worth trying out.

If you're looking to reduce noise, try the Visual Sound 1Spot, works well and its affordable.

Oh, and I should mention - those cheap adapters tend to produce a higher voltage than they specify. Generally, about 1 V or so shouldn't cause a problem, but any more might cause damage. Buy or borrow a multimeter and measure the output of the adapter if you really want to be safe.
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it should be fine. most things only draw as much current as they need, so supplying more isnt going to really be a problem. stuff like that is a voltage source, so as long as it is around 9V, your pedal will only draw the 300mA of current it needs to run.
totally fine, you could even run a 2nd pedal of that supply at the same time of the 2nd pedal was needing 200ma or less.

99% of pedal can also take 12v, so power supplies like the the T-Res Fuel tank exsist. Overdrives & Distortions oftern sound better and have more headroom with a few more volts.