Just picked up a new tube amp with a terminal case of tube rattle in the power tubes. They're 6L6b Groove Tubes and I can't find them for sale. I find a bunch of 6L6gc for sale however. I read that all modern 6L6s are gc's but mine clearly are modern and clearly display 6L6b. What am I missing? I LOVE the tone so I want to know so I can order new tubes without changing tone and headroom too much. Thanks guys!