Good morning, Looking for a little help. I have a 1986 Marshall JCM 800 2205, the 50 watt head. I haven't played it in a few years and looking to sell it. But The gain/boost channel is not boosting or gaining at all. I've checked the pre amp tubes, and the power tubes and they look fine. I've also swapped them around to try different tube positions. But still the boost channel is producing nothing. Any clues as to what might be going on?