DeepCut
I used no circuit at all; just the cable with a 820Kohm resistor on one probe.Then the mic level set to 1 of 100

@DeepCut

It looks like the red trace is influencing the green one through induction. The problem probably is in unshielded multimeter leads. I might be wrong though...
You might want to try different ratio of resistance and capacitance (smaller cap and bigger resistor). If your potentiometer is wirewound then it has got some inductance and with a capacitor it acts as LC tank causing those oscillations... Just thinking aloud...

Thanks Marius and keyho. ION at overunityresearch.com has given me a much better circuit for HV use with clipping diodes.

I will look into shielding.


Cheers,

Gary.

@ Mari:
   Was looking at your build and realized you have air core choke coils coming out of your main coils. This cant be much of a hindrance , but then due to an AC signal, it may be  a lot. Then it may also be helpful. Just a thought.

thay

My new setup:
http://www.youtube.com/watch?v=DJ5SkvBBYC8&hd=1