I am trying to convert my old 9V battery operated Wah-wah pedal to a DC converted one, so I can use AC power. I bought a DC plug for it and was told that the center pin is for ground. However there are TWO additional pins. I only expected one since there's just a positive and negative wire. Which pin on the DC plug is for the positive wire? And is it true that the center pin is for the negative wire?
The pin with the tab on the bottom left is the center pin. The tab at the very top is the sleeve. The middle pin is a break contact on the sleeve connection.
Most old guitar pedals use center (-) - but not all. Check for continuity between the center pin of the connector and the battery connector.
You can either ignore the break connection or, if you still want to have the option of a battery inside the pedal, connect whatever lead corresponds to the sleeve to the break connection. This will disconnect the battery when external power is plugged into the pedal.
For most barrel jack/plugs, the center pin is usually positive terminal, but there are exceptions, which cause severe confusion and a lot of burned gadgets who use this reversed polarity.
The plug polarity is usually marked on a body of AC-DC adapter, by approximately this sign,
When a plug goes in, it breaks the contact, so the wire connected to #3 gets disconnected from ground. This signal usually used in devices to switch between external and internal power sources. SO you need to check carefully the arrangement in your device before changing the jack.
The sleeve or outer contact often includes a switch which will open when the mating plug is inserted. This switch may be used to disconnect an internal battery when external power is used.
I think (but don't quote me!) that the center pin is usually positive, but both ploarities can be found. If in doubt, measure with a DVM or other voltmeter.
An ohmmeter or continuity checker can be used to determine which switch contact is which.