I have an old 68 pro reverb. I got a good quality 3 prong chord. I cut out the wires for the extension plug and the ground switch. I attached the black power cord wire to the power switch, the white wire to the end of the fuse holder, and the green to one of the ground commons. I"m getting this staticy harsh scratching sound when I hit a chord hard at high volumes. Very strange. My only guess is that I should check the ground connection, maybe put it under the nut that holds one corner of the power tranny. What do you think? Am I on the right track for troubleshooting? Thanks.