Bb;
AMP Indicator can come full ON for two very different reasons (see:
http://www.sw-em.com/AMP_Indicator_ON.htm ) but since it is going OFF without intervention at Fuse 1, it is definitely not due to Condition 3.
AMP indicator should go OFF almost immediately after Starting, as soon as RPMs rise off idle and CH Sys starts putting out, so your action is actually not normal (just normal and typical for your vehicle)...if it takes longer than that, or higher RPMs, it indicates cut-out relay in Regulator is not closing as soon as it should. Immediate cause is low Ch Sys output.
These symptoms are most often a result of worn Generator Brushes (resulting in low contact pressure to commutator) , or poor contact between Brushes and Commutator (resulting from that dark surface coating on copper [ it has a specific name, which I used to know back in my DC Motor days] which Brushes are immediately cutting through due to low pressure - see above).
I recommend you first procure some replacement Brushes, to have them on hand...but you might get some temporary relief by simply removing Brush access band (Gen in-situ, Battery disco) and cleaning Commutator to bright shiny copper.
Cheers from down(up) the Road