[35Z5]

The 5X4 is basically a 5U4 with different basing. It has approx double current rating of 5Y4(which has same ratings as 5Y3). The 5Y4 is not a good sub but I'd think it's at least power up radio. If the arc occurs with other tubes removed, there is a short in P/S(my bet).

You can measure resistance across 1st filter cap, if it isn't several hundred ohms, there is a short in P/S.