Blowing voltage regulators

I'm having trouble with my electro mechanical voltage regulator. In the past 4 years I have bought 3 and today I blew another one. I don't seem to get them to last more than a couple of thousand miles at the most.

I had to change the alternator about at the same time the first one blew but can't remember which one went first. Could this be the alternator causing this or something else? Where should I start looking for faults? Could this be solved with an electrical regulator that fits the single field alternator?

When the system has been working it has charged fine, no overcharging or anything it just gives up after a while.
What part(s) 'blew'?