As I said I couldn’t remember the kind of amplitude you get so I decided to breadboard the circuit as Sam shows it, with a 2N9018 transistor and a 9 V supply. One thing I’d forgotten about is if you take the signal from the collector, as in the Kassutronics circuit, the amplitude is smaller than if you take it from the emitter, as in Sam’s:
It’s almost 400 mV peak to peak from the emitter vs only about 100 mV from the collector. On the emitter side the DC offset is about 7 V, on the collector side it’s about 700 mV.
That makes it (on the emitter side) somewhat hotter than a typical guitar signal (though guitar signals vary a lot), so using a guitar amplifier circuit you’d probably want to attenuate it first… and attenuating then amplifying, as opposed to just buffering, doesn’t make a lot of sense to me.