After getting the 4.5V sweet spot, I demo'ed my Fetzer Valve with the drain V from 2.5 to 7ish just to hear what's happening. (an exciting Friday night for anyone, for sure...)

Down in the 2.5V neighborhood, full bore it's clear and very tubey, just on the verge of growing some hair... With a 250k log pot volume control after the circuit I notice there seems to be some audible distortion when I back off the output.

I'll go back and check it with the magic 4.5V, but I'm curious why this happens. I'ld think the distortion would only appear when the circuit's maxed rather than when the output volume is rolled off some. Is this because of the lower voltage at the drain, or is the 250k pot doing it?
Maybe my amp speakers are blown? :roll:

the pot shouldnt change anything, regardless of were it is set, the Jfet should see the same load (me thinks). However, when you bias the drain down to 2.5v, one half of the wave gets 7.5v of swing, and the other only has 2.5v to swing, so one side will go into clipping much earlier than the other, so id say youre probably getting asymettrical clipping...might be worth getting out the ocilioscope...

Thanks for the straight up reply Alpha579! I want learn the "why", so far I've only been tinkering with the "how".

Looks like I'm headed for some reading on AC and transistor theory...how transistors create gain, signal swing, clipping, everything I should know already before jumping in here...

And I'm embarassed to say I don't own a scope, let alone know how to use one.

[cue "Kung Fu" voice over]
Patience, grasshopper.

That being said, the Fetzer Valve is a great sounding circuit! When V at the drain is correct, that is :oops

edit: Does the drain pin need to always see 1/2 Vs? In that case, what about voltage divider biasing, rather than setting the drain resistor's value to get 4.5V assuming a fresh battery? Maybe I'm not thinking of this correctly.