Sorry that this caused such a controversy, everyone :-(
I guess I failed to note on my diagram that the speaker is 8 ohm.
Last night I stuck an IRF510 into the circuit instead of the 2n7000
(observing different pinout, of course). Now the speaker at least
crackles a little bit when I touch parts of the circuit and/or plug in
the guitar, so it's a little more alive. I still didn't get any guitar
sound out of it, but the voltage measurements were much closer to what
has been discussed here. It seems that turning the pot changes the
drain and gate voltages and their relationship to each other. I moved
it all around and got no sound out of it, still.
I did find that the cap was leaking, so I changed it. I measured the
resistance of the speaker to test for continuity (7.5 ohms was what i
read). I poked around the circuit with an audio probe* and got guitar
signal on all 3 pins, with the drain being the loudest. In fact, i got
it everywhere, including where the speaker connections are, just no
sound out of the speaker. Ran out of time before I could change
speakers, but if the speaker had continuity and was roughly in the
expected range, why wouldn't it do [i]something[/i]? :-P
I'll have to piddle with it s'more. Thanks for all the suggestions
* Audio Probe: essentially a piece of wire plugged into a known,
working amplifier, isolated with a 2.2uf capacitor in series.