Here's what Truetone say about the Voltage Doubler:
The 1 SPOT Voltage Doubler (TVD) is an intelligent cable which is mainly for converting 9Vdc to 18Vdc, but it can also convert 12Vdc to 24Vdc automatically, without a switch! Able to handle up to 100mA of current, it can help you power just about any 18V or 24V pedal*.
All you do is take the output of your 9Vdc or 12Vdc power supply and connect it to the Voltage Doubler and plug the TVD into your pedal. Simple. Noiseless. Inexpensive.
*The only 18V pedal we know of that the Voltage Doubler can’t power is the Pigtronix Infinity Looper, which requires it’s own power supply.