First solder a 100K resistor in series between the negative Cathode bypass cap lead (the far right one by the preamp section C3) and ground. Now solder a wire to the connection between C3 and the 100K resistor. This wire goes to the center terminal of the soon-to-be "fat" switch. Wire one of the outer terminals of this switch to a nearby ground connection (input jack would be great). When this switch is open, C3 sees 100K between the cap and common ground and therefore C3 does not effect the gain of the circuit. When the switch is closed, C3 sees this ground connection through the switch and the gain of the circuit goes way up. Has anyone here tried this mod? It was recommended to me by Rob Hull from Tube Depot. Any thoughts you may have are appreciated.