[EricS]

Rectifier tube is my suspect as well. Caps have a maximum rating for ripple (AC across the cap). If this is too high, the caps overheat and blow. My guess is that your rectifier tube is not really rectifying all that well and is passing a high component of AC along to the cap. Put your meter on the output from your rectifier tube and set it to AC and see what you get. Most run of the mill electrolytic caps can only handle ripple in the low tenths-of-a-volt range, any more and you will be severely stressing the cap unless you have specifically purchased a high-ripple capacity cap.

Eric