help with fairfield County Signature (John Mayer Two Rock Signature)

[matoni]

Hello,
I am pretty new to amp building, hopefully one of you could help me out. I am building a Two Rock John Mayer signature clone using an amplified nation chassis and boards from the tone geek (Ryan Colgan). I just finished the build and the amp sounds really good, but there are a couple problems.
I drilled a hole for a half power switch on the bottom of the chassis (because 100 watts is loud) and the amp works fine in half power mode, but in regular power mode there is a loud feedback and I have no idea what the issue is. If any of you have any experience with this problem, some advice would be great
There is also hum coming from what i assume is a ground loop that is constantly present in the amp's sound. what is the best way to systematically go through the amp to try and isolate the cause of this problem. I do own an oscilloscope which should make this problem a little easier, but I'm still not sure how to go through and find this hum, or even what it would look like on the oscilloscope. Here are some pictures of the inside of the amp. I apologize for the messiness, please let me know if you need better pictures to help diagnose the feedback problem.

[xtian]

I drilled a hole for a half power switch on the bottom of the chassis (because 100 watts is loud) and the amp works fine in half power mode, but in regular power mode there is a loud feedback and I have no idea what the issue is. If any of you have any experience with this problem, some advice would be great

Sounds like positive feedback. You may have to swap the OT primaries. You can confirm by simply disconnection the NFB wire--if the howling stops, that's the issue.

There is also hum coming from what i assume is a ground loop that is constantly present in the amp's sound. what is the best way to systematically go through the amp to try and isolate the cause of this problem.

Divide and conquer. Pull out the PI tube and evaluate the hum. Replace the PI, but pull out the previous preamp tubes and evaluate the hum. Or, attack it from the opposite end--pull out preamp tubes beginning with V1 and see when the hum vanishes.

[matoni]

If positive feedback is the issue, why would it sound completely fine when the half-power switch is engaged? You are probably right, but I just want to make sure before I switch these wires because I don't want to end up cutting them too short. And thank you for the method for debugging I will start with that tomorrow. Thanks so much for your reply!!

[matoni]

NEVERMIND!! I Just switched the output transformer primaries and it works now. Thank you so much for your help. Thanks for helping a 15 year old get that much closer to finishing his second amp build ever. Now to bust the hum using your strategy.

[Tweezy]

Were you able to figure out the hum?