Really interesting series of videos and a lot of work I will go back over them when I get around to the higher bandwidth option.
Did you completely stop using the Super Corona Dope and if so any reason? I see it is made up in part with xylene and a few other ene's but how does it stand up to repeated cleaning with IPA? I have found a localish source for not to many peso's for it so I can give it a test for adhesion on PLA.
I'm glad you are enjoying them. The dope will come off with IPA and other solvents. It can be easily damaged with a finger nail. I clean the parts, then apply the dope. I'm not aware of a better product for this sort of application and will continue to keep it in my bag of tricks.
One of the reasons to roll this probe was even a simple Calibration point of a 20MHz Scope last week was out of the range of gear I own. Having some bandwidth over a simple resistive divider would see it getting more use.
The response on my 40KV probe isn't very good. It took some effort to tame it as well as I did. Having the GDTs built in and using the two 40KV resistors (to handle a single point failure), I have no concern about using it with any of my scopes.
Of less interest, I did demonstrate a few simple low end, low voltage, low energy differential probes. I don't think these were part of my HV probe playlist. The last one I made on some perf board from scrap parts I had. It's pretty funny to look at. It does have some decent BW for being such a mess.
https://youtu.be/0thOfk4I3qsIf you can keep your single ended probe within a dB from DC to 20MHz, I would say you have done a VERY good job!
Beyond the obvious advice, like keeping parts CLEAN and keeping your source to a safe level a few things to consider. While experimenting, I suggest making some sort of protection circuits for when things go wrong. No point in blowing a scope channel or your spectrum analyzer's front end. Even some Citel GDTs and attenuators may save you a big repair bill. Keep in mind what you want to measure in the future and minimize the loading effects. While toilet paper rolls, aluminum foil and other things laying around the house can look stupid, they can provide a lot of insight into how things will behave with little time invested.
I followed this guy's channel until he stopped posting. He did a series on AC high voltage probes as well that you may find of interest. He was after a differential setup. Here you can see them in operation. The probes are the soup cans:
https://youtu.be/cogG29Z7wrc?list=PLmj4qMKo6w6etRqmdffwfg9JqIeAR0tzLDave's friend also walked through an AC probe. I attempted something similar to this but my skills were too lacking to get it to work as good as my toilet (shitter) paper roll and some aluminum foil.