I don't think you'll find one that's "efficient".
Best luck will be stepping it up from 12V to 120 or so, then driving a common CRT FBT, which will do 20-30kV depending on size (up to 60kV for the most powerful projection sets, I believe?).
If you're lucky, you can use a portable TV FBT ... but I doubt these were large enough to need 10-20kV. You could use a CW multiplier, but this is only possible if the output is AC, which it usually isn't. You may get lucky and find one that's used with a tripler already, so the transformer itself is AC and can be multiplied up as needed.
FBTs are also optimized for a harmonic of 15kHz. 70 exactly might not work quite well enough. (Why so particular, anyway?)
You might also have some luck with unconventional approaches, like a very small Van de Graff, which need not require any transformer (or 70kHz) at all.
Tim
--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
Website: http://seventransistorlabs.com
"Kevin Foster" wrote in message
news:qn13sahv99emeub4o0kq84t52a8dfg145s@4ax.com...
I am looking for an off-the-shelf transformer that will step up a
12Vpp 70KHz square wave to around 10-20KVAC.
It is for an electrostatic experiment, so very little output current
is needed.
I have tried ignition transformers, auto coils, audio coupling, and TV
flybacks, but none were efficient at that high frequency.
I can not afford to have one custom wound.
Does anyone have any suggestions? I am happy to put a few in series if
that is what it takes.
I am thinking maybe a tranny from an inverter or SMPS, but not sure
exactly where to look.
Thanks,
Kevin Foster