I don't get this volume control

[ayayay!]

I was just glancing at this schem and realized I don't understand what's happening w/ the volume control on VR2.  The wiper goes to ground.  Also the junction from the input of the pot, right after C10, goes to pin 7 on the Input jack?  There's a master volume at the end, and everything else makes sense, I just don't get this volume VR2 control. 

What's going on here? 

http://farm4.static.flickr.com/3495/3798718778_46f7734fc2_b.jpg

[Nasse]

I think it shows in the pic that there is a switch in the input jack so that when nothing plugged the switch connects that point to ground.

My amp does not have any switch for clipping diodes and does have line out

I have the manual somewhere that describes how volume gain and master controls work vs distortion and "tone" but can not find it just now...

[earthtonesaudio]

Picture the wiper of that volume control all the way at the top.  Signal comes from IC1A through a voltage divider formed by R8 and VR2 and into the + input of IC1B.  The gain of IC1B is determined by the R3/R10 divider.  
Now what happens if you start to turn the volume down?  First, the signal from IC1A is attenuated before it gets to IC1B, because VR1 becomes effectively smaller compared to R8.  Meanwhile, the gain of IC1B is reduced because R3 (effectively) becomes larger with respect to R10.  Keep turning the volume down, and the right side of R8 goes through C10 straight to ground, and no signal appears at the + input of IC1B.
So what you have is a combination of attenuator and volume control in a single potentiometer.

See also:
Graphic EQs
Tube Screamer tone control

[ayayay!]

I think it shows in the pic that there is a switch in the input jack so that when nothing plugged the switch connects that point to ground. 

Ok I'll buy that.  Thx!

Keep turning the volume down, and the right side of R8 goes through C10 straight to ground, and no signal appears at the + input of IC1B.

And there's the money!  Got it.  Thx! 

[earthtonesaudio]

Incidentally, the bass, mid, "tre," and presence controls all use a similar design, just with different frequencies rather than straight-to-ground.

[teemuk]

At low volume settings the control also introduces a high-frequency boost because at higher frequencies C12 bypasses VR2 (which is also part of the gain equation along with R3 and R10).

When volume control is turned up the input signal gets attenuated more but likewise the portion of VR2 (in the feedback loop) also decreases in resistance, which then increases the gain. At the same time C12 has an increasingly lower parallel resistance to ground and therefore its effect as high-frequency bypass decreases. Thus at higher volumes the gain is more constant throughout the bandwidth.