I apologize if someone has already posted something like this, but I couldn't find it... I recently bought a Spider Valve 212 mk1 from a friend. It was in good shape except the tubes, and so he bought some for me to use so it would be in perfect working order (as he didn't want to sell me something that didn't work.) I replaced the power tubes with Sovtek 6l6's, which work perfectly fine, and sound pretty good. They work well with the stock China preamp tubes. He also gave me a matched quartet of Sovtek el84's. I tried both matched pairs in succession, and neither are working. I get a ton of sound coming from the XLR line-out, but I can't coax anything out of the speakers. Both the 6l6's and the el84's are lit, but still no speaker sound. If I put the chinas back in, the speakers start working again. Is there a compatibility issue with el84's on this amp? I work with electronics quite a bit, but I honestly know next to nothing about tubes.