Seeking Grounding Help with the Lyman Amp (Improved Little Gem MkII)

[schrectacular]

Also, sorry for the basic clarification but.. well this is just where I'm at what can I say?

Is this capacitor of which you speak a "coupling capacitor" that is meant to act as a short to those really high frequency AC signals that might be coming in through my (admittedly long, old, and probably well trampled) guitar cable? And that's why we choose a low value - because the guitar signal itself is AC, so we don't want that to be shorted to ground. So the signals in our ~20 to 20k audible range we want through, but those AM medium wave (which start at ~500kH) in the US can go straight to ground through this 330pf capacitor I'm about to solder in.

I'm sure I have something wrong but is this the idea?

EDIT: Soldering the 330pf capacitor across signal and ground of the guitar input jack did indeed make a world of difference. I still hear some faint radio, but the gain is now totally usable and it doesn't react nearly so strongly to my fingers coming near the knobs and dials. I will probably open the case and twist the speaker cable up better, that could be done much better in places.

It's so exciting to play from something you made, kind of like eating a tomato you grew.

[anotherjim]

That was exactly what I meant -  although it is a coupling cap in the sense that it couples point A to point B, it's more of a "bypass" capacitor, which is also the name for those caps fitted from the power supply to 0v. And you've made the important discovery that sizing the cap can select the frequencies that are passed.
More specifically, the input bypass cap forms a low pass filter with the source impedance of whatever is feeding into it. Although a guitar has a complex source impedance because pickups are inductors and there's other stuff, for thinking we consider it as a source resistor of about 10k.
So if you type in 10k resistance and 330p (or any other cap value) capacitance into this calculator...
http://sim.okawa-denshi.jp/en/CRtool.php
...you have an idea of the frequencies the filter can reject.

[schrectacular]

This is extremely helpful. That calculator page is... information heavy. I assume I should look at the "Bode Diagram" which will appears to show how much attenuation I will get across frequency spectrum on a log graph. So I'm getting like 3db attenuation at 10k and about 20db at 100k? So that's why I still hear the signal, just much more faintly.

Oh and out of curiosity I looked up the station that I pick up, it broadcasts at 1110 kHz, 50k watts. The towers are 2000m from my house. I guess I'm going to have to be very careful about this for anything I build. Ugh.

[anotherjim]

I only bother to look at the text cut-off frequency result (the "Fc=" part) in the calculator. Keep it simple!
If I was to do the calc longhand, it's 1/(2pi(RC)) which is all that calculator does to show Fc.