20 percent NaCl with calcium carbonate seems to show the undesired affect
over a few days so maybe the Marshall contaminated boards have 1 or 2
percent contamination.
Pitty no Chinese? E number on their boards so we could tell if they turn up
with other makers products, but perhaps not with valve voltages.

So after a few days the dry test cell showed about 500M and over a few hours
dropped to 80K or so ,maybe the Megger high V across the probes accelerating
this effect , and light heating with hot air , drops to 40K or so, returning
to 80K or so soon after. Much the same today as yesterday