Westinghouse h251-Arcing 5u4

[fsjonsey]

I brought home a good condition Westinghouse h251 from a junk store today. I paid a whole $20 for it. I tested every tube and looked for any burned resistors or capacitors. One 5u4 was bad, so i replaced it. I hooked the set up to an isolation transformer and brought it up. The 5u4 i replaced started arcing internally. I quickly turned off the isolation transformer, and looked at the tube socket. There was no evidence of arching. One electrolytic can was extremely warm. Could my problem lie in a shorted filter cap? To be clear, I am going recap the entire set. I just want to make sure it isn't something like a shorted power transformer winding before i start putting effort into it.