Farts on 8, but doesn't on 12. Why?

[dadydg]

I've got a Gibson GA-6 that I converted to a 5E3. The only topography difference is that it uses a 6sl7 for V2 instead of a 12ax7. I am using .047uf caps for all four coupling caps. Speaker is a vintage Jensen P12N.

Here's the thing... on the "bright" channel, when I get the volume to about 8 and pluck hard the low E on my Tele, the amp farts (not too unusually). The fartting is there as I crank up the volume until it get to about 11, at which point, the fartting suddenly drops away and the bass get tight.

I've check the volume pot on my meter. The resistance changes smoothly and consistantly with it's audio taper. Also, it's a relatively new pot.

What could be going on? The fact that it farts out some above 8 isn't too surprising, but the fact that it suddenly tightens up is. If it's possible, I'd like to keep it tight through the entire sweep.

Dan

[Matt T.]

Isn't the AC impedance seen by the following grid (grid just after the volume pot) at it's highest when the pot is somewhere near 1/2 (which occurs near 7 or so on an audio taper pot)? Remember that the B+ rail is ground as far as AC is concerned (so from that grid there are two paths to AC ground...one to regular ground and one to the B+ rail). Look at the schematic...see what I mean? I can't give you an answer why...but whenever I see behavior that occurs right near '7' on an audio pot...this is along the lines of what I'm thinking.