I just started playing again after about a 20 year break. I bought a new Orange 35RT amp and plugged my old epiphone SG into it, and the amp makes a very loud buzzing/humming sound when I turn up the gain past 20%. I tried new cables as well without any change. It stops if I put my finger on the string or another metal part of the guitar. Thinking it was the old guitar, I went out and bought a new one, and the exact same thing happens.

Unfortunately I don't have any other amp to test on at home, but seems like the issue is with the amp. I'm thinking of returning it and exchanging for another one, but wondering if this has happened to someone else before. Thanks!