I'm finding tons of information and equations on calculating this for solid state devices, but nothing for good old fashion particle accelerator based amplifiers :P
Currently I'm revamping the K270 guitar preamp to use components that are readily available. See the original design here: http://s13.postimg.org/patw5krxj/k270.png
This is my current progress: https://123d.circuits.io/circuits/852190-jan-6418-vaccum-tube-valve-preamp
Here is a snapshot at the time of writing this:
The problem I think I might run into is that my guitar's pizeo pickup requires a ridiculously high input impedance on the other side of the cable. I've been reading about jfet based piezo preamps and it's not uncommon to see people have 5 - 10 mohm input impedances on their piezo preamps.
1) How do I go about figuring out what the input impedance of my current circuit is?
2) How do I raise it if necessary?