On Wed, 29 Mar 2017, Jeff Liebermann wrote:


The "classic RF probe" is a marginal and crude device that was long
ago replaced by far better amplified probes. The "classic RF probe"
quits at about 100MHz, has a rather lumpy frequency response, and will
load down high impedance circuits. If you have volts of signal to
work with in a tube device, it will work as expected. If you're
working on a modern low voltage device, you'll have problems. What's
better is a FET input RF probe, something like these, which can be
built:
http://cjh.polyplex.org/electronics/RFCascodeProbe/
http://www.instructables.com/id/DIY-1GHz-Active-Probe-for-Under-20/?ALLSTEPS
If you must use tubes, I could probably design a cascode Nuvistor RF
probe for the occasion.

I always found the AC probe on the HP 410B to be great. But that uses a
tube, a rectifier, right in the probe.

But you're probably right. A buffer would help, right where it's needed.
And probably wasn't common in the old days because of cost, or getting a
small tube in the probe. Or hobbyists never gave it much thought. SInce
a lot of this is about relative voltage, or tuning for peak, having a flat
buffer isn't even necessary.

Michael