From Support@tekkeon.com:
Hi Sean,
To make the adapter tip center negative, all you have to do is rotate the adapter before connecting it to the cable 180 degrees. That will reverse the polarity for you.
Chris
The adaptaplugs from radio shack DO fit the tekkeon cable. In fact, the adaptaplug fits MUCH better than reversing the adapter from tekkeon, but it is POSSIBLE to reverse the adapter even though it seems like it only goes one way.
The cable you currently have, acidjack, would not work with the tekkeon since the tekkeon is completely different but, as I said earlier, the adaptaplug tip WOULD work.
I hope this can clear things up.
A word of warning when using the tekkeon: plug the cable in and select the voltage and then plug the device in. Once the device is powered by tekkeon, don't be tempted to adjust the voltage.