Gain and Volume Pots

[MullisMan]

Are there certain rules that you need to follow when deciding where to place the pot that will be your gain control?  For example, I'm using both sections of a 4558 chip for the main clipping stages in a circuit, and then I'll be adding a JFET section on either the input or the output.  As of right now, the gain control is in the first clipping stage of the opamp.  The only problem is, when I add in the JFET on the input or output, I'm loving the gain i'm getting, but when I turn the gain pot down low, it gets to a certain point and then the signal disappears completely.  It's not smooth.  I'm using a linear pot, and I wasn't sure how to decide what resistance the pot needs to be rated at.

[GibsonGM]

Well, there actually ARE rules, but they're not set in concrete - they have more to do with what you expect the circuit to do.  When you get into AC signal analysis, you move from just worrying about how DC levels change; it becomes about impedance, and how changes to it will affect your design.

A volume pot before an active stage will cause attenuation (of course) and lower the input signal. But it will also drop your highs (like a guitar turned down without a 'bright capacitor') - you are losing highs faster than lows due to the capacitance of the device input working with circuit resistance as a filter(Miller Effect), and also are changing the input of the device's impedance, which is a little bit complicated.  Not to mention, the signal-to-noise ratio gets worse, so as you turn down, the the hiss goes up....

In your case, it might just be a simple tapering issue.  A 'regular' linear pot doesn't have the smoothness an audio taper pot would.   1Megohm is a good value to try....I keep a 1M, 500K and 100K around to experiment with just like you're doing. 
If your pot is right after the JFET input stage (a buffer?), then it's probably in the right (or a good) place.   Devices like a high input impedance and low output impedance, and the JFET section will offer that if you're using a 'known good' input stage.

If you don't want the signal to die completely, use a resistor (experiment with the value, maybe 10K?) between one leg of the pot and ground to assure that you never completely shunt the signal to ground...without it, it will also be a volume control!