my impression now is that casually you are on something: do you remember the stories about TV set reported by SM (?)...just about funny things happening with flybacks...we are on muddy terrain as, at least in your case, situation appears to be unrepeatable due to kind of internal secondary shorting you are experiencing...MAY BE THAT THE PARTIAL INTERNAL FUSING IS IN A WAY ENHANCING RESONATION, who knows?
Please do not forget that original basic #2 design provided for an 'internal resonation' putting a cap in parallel to a part of the HV secondary....and T-1000 clearly says that's particular & important. Unfortunately it is very difficult to replicate as the Russian unit the STAAAR group used is vintage TBC.
I agree that there could be OU source.
It could be interesting for me to have a rough idea on light intensity you see (for example: what kind of lamp, filament red, white, low shining....). In my case before Ventex neon power supply died I obtained white filament (low shining) on 240V/60W lamp.
Finally I reccomend to use solution easy replicable for every aspect as contrary impossible to have any confirmation.
"...Remember I mentioned to you to try and put a second coil over your Katcher coil, like @woopy did, but @woopy did not send the HV negative through that second coil and was still getting some great effects.
So look at the TK coil in your last posted diagram. There is not that much difference from that coil (L27 to L31) and your Katcher coil if it had a second coil wound over it to pass the HV-...."
I did not notice and neither did any action for putting additional coil on kacher and routing there also the Hv...but I can try it soon. Tnx for remembering.
I have also your same impression about last STAAAR group implementation. It is rather easy to understand that there are let's say 2 kachers-like coils: one connected to control grid and another one connected to tube's plate...all with proper phasing connections.
The design does provide also the secondary TC coils resonating at lambda and lambda/4 wavelength's...all mixed with 50Hz modulation. In output we should have 3 freqs f1 from TC#1, f2 from TC#2 in harmonic relationship and f3=50Hz). Coils internals air Ionization promoted by the huge electrical field by TC1 and TC2 big magnifier coils could really be the key factor.
I've almost finished the coil set (sse pic) and I'm now working on circuit while waiting for ordered components .