Its official, my 15GP22 is bad because of gas
I left it unused for a couple of months, and when I turned on
last night, it arced badly with full HV, maybe twice a second for the
three seconds I left it on. With the socket removed there was no arcing.
With the HV removed there was a tiny ticking sound. Trying it a third time
with both connected I saw an arc near the base, inside it, and immediately
shut it off.
I will now wait the final results of Yurkon's work, unless a working one,
or even another set, shows up for sale.