$250! What a score!, sounds like that amp is a CBS era Fender, as far as I know Fender has never put out a re-issue with a silver-face, and you mentioned the 12ax7's still have the metal covers on, so it sounds like it is in very original condition, those metal covers are usally gone, so thats good, as Lordathestrings said the 6L6's are a push-pull pair, and the amp is designed to operate in Class AB, setting the bias makes the tubes operate efficently within that class. Your amp will have either have an adjustable bias circuit, ( Very Early Silverface ) or the typical CBS era Hum adjusting circuit which was a stupid idea but not a big problem since it is very easy to convert to the adjustable bias, you will need two resistors, it takes longer to get the amp out of the cabinent than it does to convert it to adjustable bias. Your amp should have the following tubes in this order, looking in the back of the amp from right to left, 2-12ax7, 1-12at7, 2-12ax7, 1-12at7, 2-6l6, 1-gz34, that GZ34 is a rectifier tube, I'm going to check a website to see if I can find you a schematic and a board layout.............