In conjunction with British weather/ garage storage.
Well I can't believe I'm unlucky enough to have 2 such repairs.
Earlier one Marshall 1962, year 2003 bluish-white flash from the HT area of
the mains Tx secondaries.
Small arc over just 1mm or so , moving the wires apart and cleaning and
telling the owner to store the amp properly, cured that (or at least did not
bounce back).

This time Marshall JTM Tremolo 50W , 1998. The giveaway is that the HT fuse
failed at switch off to standby, mains sw on. Replacing fuse and running up
on a variac amp would work fine up to 95 percent mains and switching back to
standby but 100 percent and switch to standby and Tx saturation noise and
dramatic long bluish-white arc from near an HT(ac) terminal. As fuse is in
the ac line and the o/c voltage goes up on removing the load on the valve
rectifier at sw off.
Between the purple "o" s on this pic,
http://www.diverse.4mg.com/Marshall_jtm_tx.jpg
vertical black smudge presumably a carbon track. The wire removed from the
red tag by me, 2 wires to the black ground tag. The black mark and the 2
bare copper patches lie along the edge of the nylon , the o-o line , not
distinguishable in the pic. As the wire is .27mm diameter you can see the
"carbon arc" was 2.5mm or so long and colour white, not green or blue.

No obvious organic mold marks elsewhere on the amp or even abnormal rusting
of the steel bits.
Any opinions on the derivation of this problem. I assume marginal invisible
mold growth that carbonise over amp use, and chafing? and 500V or so pk-pk
then carbonising of the nylon or Tx lacquer.
Hopefully recoverable by cutting away/cleaning nylon and silicone sleeving
over the wire and exrending to rejoin the tag, to move it away from the
edge. Would there be some Marshall treatment/coating that may promote the
initiation of this problem? yes I know its due more to owners than
manufacturer