[baz]

i have found this to be a normal thing for any car with a carb
the only cars that don't do it are the ones with fuel injection they have an idle control valve to deal with the rev drop as the load on the engine increases as more power is pulled from the alternator(even though it just spins in a magnetic field it still pulls power from the engine)
as jim says you can fit an idle solenoid to up the revs or just turn up the idle 50-75rpm it is more stable (more revs make more power so it looses less revs as it has more power)

 

[baz]

The cure I use is to set the idle to about 1050rpms, so with accessories on I get an idle-speed of 850-900, to keep my voltage up, but during daytime driving such a high idle is a bit iritating.

another thought is to put an over speed pully set on (the pully ratios are changed so the alternator runs faster at idle) this will give you more power at lower revs in the altenator but may give you problems at constant high running but will keep the output up at idle so you can be at normal revs