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There are a few things to be aware of. First your 6v system is positive ground so make sure that you hook positive of the jumper to positive of the jumped car (and negative to negative). Most people will naturally want to put the negative cable from the jumper car to ground of the A. DO NOT DO THAT! Do not let the vehicles touch each other either while the cables are hooked up.
Make sure all the lights are off, putting 12v to 6v lights can burn the bulbs out.
I like the idea of letting your battery charge and then unhooking the cables before trying to start the car. I don't think leaving them hooked up would be a problem on a stock A but if you have upgraded to electronic ignition it could be damaged by the 12v.
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