View toward the back on the IEC connector/fuse side. See my big white 5w 220r resistors for fail-safe speaker loads. Also check out my PS/OT/PA ground point at the OT mount bolt - seems to work ok. Update: Ripped out that grounding scheme and ran it all back to the single point at the input jack. Still have some hum so I'm going to have to revisit it - it's probably an grounding issue but I'm also going to try some other heater fixes as well.