Old Muntz TV for sale Locally to me, Should I pick it up?

[Electronic M]

Possible but highly unlikely.

To troubleshoot this (and I have had to troubleshoot similar issues before) disconnect the B+ rail and filter cap connection to your the 5U4 (remove all load from it). If it stops arcing you have confirmed the B+ rail has a short or severe overload. If it continues to arc with no load then the socket may have a short or the transformer may be supplying too high an AC voltage.

Assuming the arcing stopped reconnect the first filter cap then reconnect small parts of the B+ 1 at a time (not all at once) the first one to cause arcing is your short, and you'll have to pick it over with a fine tooth comb to find the root cause.

I've fought a few B+ shorts. Sometimes they are obvious if you look everywhere, sometimes they are hidden... Case in point: a 40s zenith chassis with only a couple rubber wires in seemingly good shape...the single rubber wire feeding the eye tube had one dry patch of insulation that had disintegrated right at the eyelet where it passed through the chassis, and was invisibly shorting to chassis there... just because you can't see a problem, and have replaced the likely suspects doesn't mean the problem isn't there.