> (Actually about 4/7 of the heat...)
> Appears to me to be a waste of a 2 watt resistor, and probably a
> real safe bet for a half watt of ANY composition.

Well, maybe so. Depends what you have in the junk box I guess.
My resistor drawer is fair groaning with ex-PSU resistors, and
some of them are quite compact for the rating

> In my experience, wall warts are VERY close to their voltage ratings 
> when fully loaded. I suppose if a wall wart had a 35% loss ratio...

The OP reckoned just 71mA. I've quite a few "12V" wallwarts of
various descriptions and the unregulated ones are all over the place.
A 500mA I use a lot of, because it's cheap, is 22V unloaded !!!
My 45mA application drops it to 19V, and 70mA takes it down to
17.5V. That's still 10V over what's needed for 7805 regulation. To
be fair it does have a separate label warning about low loads

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