AC15 project...trebley distortion...

[Stanz]

Just finished KOC's version of an AC15, Volume, Treble, Bass & Cut.There is a weird, grating, distortion (only higher frequencies have this distortion) that sounds like small metal pieces shaking against each other, almost like a tambourine being shaken very softly. If I roll the cut so the tone is on the bass end, the amp plays fine. Nice clean or dirty tones depending on how much volume is applied. I tried pulling the cut pot & cap out of the circuit and that didn't fix it.

If I position the cut pot so it is wide open, the treble pot comes into play. If it is in the middle position, the grating distortion is at its strongest. If I roll the treble all the way down, no problem, OR, if I roll it all the way up, it goes away, but only at the 0 or 10 positions.

Any ideas on a good place to start looking?

[Stanz]

Some further details:

With the cut wide open (full treble), the treble control has these characteristics. In the middle position, it also loses power, as well as having a fizzy distortion on the higher frequencies. I have also noticed that with the volume all the way down, there is still some guitar coming through.

With the amp cranked all the way up, the treble has no effect. I do not know if this is normal operation or not.