Low volume Runt

[mgwhit]

Well, the diodes are the only other thing in your signal path between the negative lead of C7 and lug 3 of your Cut pot.  Process of elimination!  I'm not saying I'm 100% sure that's where your problem is, but it's really the last place we can effectively test components.

Diodes only pass current in one direction, but even when they pass it in that one direction a little bit of voltage is lost.  This voltage that doesn't pass through is called the forward voltage.  The maximum forward voltage rating of a 1n914 or 1n4148 is 1V.  Your normal guitar signal is probably ~10-50mV, so to achieve clipping your op amp amplifies the signal to greater than one volt and then your clipping diodes let all the signal greater than 1V (or thereabouts -- the forward voltage is current-dependent) gets shunted to ground.  This leaves you with a signal smaller than 1V with the tops "clipped" off the sine wave of your signal: distortion.  Then you use the Volume pot as a voltage divider to lower the signal back down to something reasonable.  Google "diode clipping" -- the Internet can do a better, more accurate job of describing the whole process than I can.

If your clipping diodes weren't conducting at all, you would get no diode clipping at all.  You would essentially get that signal that you're probing on your Gain pot.  Any distortion there is op-amp clipping, which is supposed to be tasty in the LM308.  If your clipping diodes had too high of a forward voltage the signal would be louder than normal and it might be harder to get any diode clipping at all.  That's obviously not your problem.  If your diodes had too low of a forward voltage, your signal would be quieter, but since more of the wave form would be chopped off you would get a synthy, buzzy "square wave" type of distortion.

You see where this leaves us?  None of these scenarios really sounds like your problem -- very quiet, but normal sounding distortion.  That's why I'm not 100% sure it's the diodes.  However, you can easily test this by removing them from the board.  If you remove the clipping diodes (D1, D2) and you get essentially the same signal you were getting at the Gain pot, then we know that the diodes had something to do with it.  Good luck!

[NightOwl]

I pulled one side of D1, and the output volume jumped like crazy.  It sounds essentially the same as before, only with adequate volume.  So can I infer from this that specifically D1 is bad, or just that one of the diodes isn't working correctly?

[mgwhit]

I think that as long as you have one leg off the board, you should be able to do a valid continuity test on the diode.  Try it in both directions, and if it conducts both ways you've got a bad diode.

[madbean]

The 1n914s will clamp the volume for sure. One thing you could do to increase volume output is put two 1n914's in series on each side, or use a couple of LEDs instead.

[jkokura]

It's also possible you had one of your diodes oriented the wrong way.

Jacob

[NightOwl]

It is finished and working beautifully now!  D1 was a faulty part, it conducts both ways and I assume it was shunting a lot of the signal to ground.  I've replaced it with a spare and the circuit is working great, and I've just finished boxing it.

Thank you all for your help, especially mgwhit: you sir are a gentleman and a scholar.  I sure have learned a lot through this troubleshooting, hopefully someday I will be experienced enough to help out newbies myself.

I'm putting up my first build report now, whoohoo!

[mgwhit]

Congratulations!  You did a fantastic job of debugging it: voltages up front, good photos and great work with the audio probe.  I know it can feel like like it's taking years off your life while you're working on it, but nothing teaches you more about these circuits than successfully debugging with a multimeter and an audio probe.  Pat yourself on the back, crank that mother up and rock.