rectifier disaster

I made a voltage doubler rectifier and it blew my rectifiers ;/

the components were

2x 4amp at 600V diodes (1N822 I think)

all configured in a voltage doubler circuit

I hooked my variac to the inputs and a DMM on the output to measure the voltage

I started at 0 volts on the variac and when to 70 and everything was ok(the voltage output seemed right).

after about 70V I heard like a small pop or something and my variac started making a loud humming noise so I turned it off... a few seconds later I heard it make a click sound.

I can't imagine why the diodes blew... I tested them using my TV oscilliscope think using the X-Y plot... they conducted both ways and acted like a short... another diode of the same kind looked different(at a loop at one side).

Surely I didn't go past 4 Amps(my variac is only rated at 3 amps and it didn't blow the fuse) and definately not past 600V's.... maybe it had something to do with the caps? and surely I don't have to connect a load? (I suppose though that I did need some resistance in there to limit the current a little atleast? I didn't think about it at the time as I just copied a circuit in a book I had and it didn't have any resistors ;/).

One thing I did was to cut in half one of the broken diodes and then I stuck them back together and it worked! ;) (well, showed the same plot as the working diode).

Thanks, Jon