[Radioroslyn]

I'm no expert but what could cause this problem is one of two things.
If you had a bad connection at the filter choke or open filter choke it
wouldn't load the power supply and #78 would just keep charging up more
and more till it passes the maximum voltage rating and it blows it's lid!!!
Or
If 78 was installed backwards however the set would not have played
at all. Shortly after snapping in on 78 would look like a dead short and
would heat up and blow it's lid. A good chance that it would take the
80 tube with it. If left on for a length of time the shorted 80 tube could
take the power transformer too.

This is the same sort of scenario you can have when you try out a set that
have the original filter caps in it. Although the original aren't in backwards they
can over a long time develop a low resistance.

Terry