Adjusting cap value in the Classic 30 Feedback loop

[tubeswell]

I installed a 500kA pot in place of the stock 220k R56, as a resonance control. It provides a little bit of variation in the sound, but not much. I was wondering whether changing the value of the parallel cap (c37 - stock value .0047uF), to something a little smaller (say .001 uF or even down to 680pF) would increase the dynamic range of the resonance pot by much. At the moment with the 500kA pot on zero, the signal bypasses the cap and you get a less-bassy sound. When the pot is turned up the bassiness increases a bit, but not by much. Any suggestions on what value cap to biff in there for more bass?

Also since putting in the resonnace control, I am getting a bit of signal interference that goes away when I touch the amp chassis or my guitar strings. I wonder whether the way I have the cable shield for the 500kA pot grounded (to the amp chassis) is the cuplprit, and whether I shoudl be better off grounding it to the same ground trace as the 'other end' of R70?

Helpful suggestions appreciated.

Schematic with comments and arrows attached FYI

[Enzo]

The modification should not affect the guitar grounding issue. But you added shielded wire? That is not a high gain point in the circuit, so I don't hink it will pick up much, but your should COULD upset the grounding. Try disconnecting the shield altogether and see if it helps. Is you shield now only connected at one end? That would be the proper way, but I'd like to hear it gone.

But don't overlook the possibility the grounding thing is due to a loss of grounding elsewhere. Find R61,62 and measure across each. You should get about zero ohms. If you get 47 ohms or something, then there is an open ground path.