Lightning risetimes are slow enough that back-to-back zener diode limiters can protect the input circuitry for a 1-meter probe. Nothing will stop a direct strike, even a 12ax7 :-). A series resistor on the input helps. Actually, a well-designed active probe antenna, even (shudder) one made by MFJ will be quite adequate for vlf-ulf phenomena. May have a lo-pass filter that needs attention. Measuring the earth/s fair-weather field, 100 v/m or so, or even a field under a thunderstorm, 5000v/m or higher, would require input impedance in excess of 10^15 ohm. Better is to raise a sharp point, say a sewing needle on a nice protected teflon or better kel-f insulator, and measure the small current of around 10 fA. Needs a small breeze to work right. Clean off the spider webs and wasp nests as needed. Better is to build a device called a field mill (q.v.) that essentially chops the field into an AC signal. Getting too far from time nuttery and into atmospheric electric nuttery :-)
Don Brian Lloyd > On Wed, Jun 25, 2014 at 4:42 PM, paul swed <paulsw...@gmail.com> wrote: > >> I am currently using a 12AX7 as a ELF preamp and have for years. >> A note in the coldest part of winter preheat the tube low filament voltage. >> They tend to fracture. >> It sits 200 ft from the house as far away in the woods as possible. >> That said and back to the thread. At these frequencies tubes do work. The >> 12AX7 can be found on vlf.it and numbers of tubes will work. They run 12 V >> on the plate. They also stand up to nearby lightning very well. >> So Diddier now you have no excuse. I can't wait to implement your design on >> one of my stm boards. Not sure how to get this back on time-nuts topics >> Regards >> > > Try a 6DJ8 instead of the 12AX7. It has a higher GM and a LOT more > bandwidth. What kind of risetime are we talking about for a lightning > strike? And why not a loop antenna? That should provide plenty of signal > but not destructive voltages. > > I know you are talking about measuring lightning strikes but if you get the > impedance high enough, you can actually measure the earth's electric field. > (It is about 200V/m if I recall properly.) Interestingly it is affected by > the solar flux and solar wind. > > -- > Brian Lloyd > Lloyd Aviation > 706 Flightline Drive > Spring Branch, TX 78070 > br...@lloyd.com > +1.916.877.5067 > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > -- "The power of accurate observation is commonly called cynicism by those who have not got it." -George Bernard Shaw Dr. Don Latham AJ7LL Six Mile Systems LLC 17850 Six Mile Road Huson, MT, 59846 mail: POBox 404 Frenchtown MT 59834-0404 VOX 406-626-4304 Skype: buffler2 www.lightningforensics.com www.sixmilesystems.com _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.