Hi everyone, I have been having various issues with my early 70s Ampeg SVT (pops crackles etc) so decided to change all the valves. When I came to do the job there were all 12AX7s instead of 12DW7s. I've put all the correct new valves in their correct places but now there is a fair bit of hum so I want to correctly callibrate the amp. The directions are to put a short across pins 2 and 3 of connector P3, and a 4ohm dummy load across pins 1 and 4. I've studied the schematic for the amp and taken the power amp out of the chassis but I can't find P3. Anybody help me identify P3 and also any other tips would be gratefully accepted. I'm well aware that I need to be very careful whilst carying out this operation! Jase.