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... with a single '9v' effects pedal, using just an adaptor plug for the wall socket? Or will it die immediately, or just not turn on...

(I'll be visiting, and pretty pleased I'm thinking of this stuff in advance!)

only if the power supply has the ability to switch between UK 220V and USA 110V. they sometimes have a switch on the outside/back panel which allows you to select the voltage, of the sort you may have seen previously on guitar amps or consumer electronics.

Where are you visiting? I'm pretty sure what you are thinking will work with a wall wart. Devices that have direct plug ins may or may not though. I think the uk one spots are exactly the same as the ones here just with different prongs. Im fairly certain they convert voltages automatically.

I'm visiting NAMM in LA! Taking my stuff over! Gonna show it to anyone who'll stand still for a moment. Might be a bit heavy on the batteries...

I think they sell batteries and One Spots in L.A...

A 1spot wouldn't be much more than one of those adapters; I'd just pick one up when you're over here to be safe.

What's a one spot?
And... they have batteries there??

Tis a daisy chain.
http://www.musiciansfriend.com/accessor ... dc-adapter

another alternative is ofc a stepup transformer. thwy shouls be easily avail in LA at fry's or whatever but it might be a pain if you're operating out of a hotel. I mention it anyway because in my exprience oneapots can be slightly noisy (not that anykne would hear it on the Namm floor). Good luck! knock em dead