Ignition coil exploded

[Colin J Brodsky <cbrodsky@xxxxxxxxxx>]

Rather surprising noise in the driveway with my '63 Rambler Classic
today... heard an explosion sound that at first sounded and looked like a
radiator cap that blew, but once I opened up the hood, I found the ignition
coil was blazing hot and apparently ruptured.  I have recently been through
two ignition coils but both were old and I didn't know the history of them.
The car has not been driven a lot; one lasted for quite some time and then
the other died after a few days.  A new replacement exploded today - I had
left the key in the on position without cranking for about 3 hours.

I'm now wondering if a '63 rambler classic needs a ballast resistor added
to it  to prevent this?  I thought the wire to the coil was resistance wire
and it wasn't needed.  The other coil that died quickly also seemed to be
when I was working on the car and had left the key on quite a bit.  Seems
like that is overheating the coil.

Any other possible causes?  The car starts and runs fine other than blowing
up the coils!

-Colin