Switch to a 6SN7, should be much easier to see. Doesn't sound like your problem is related to anything that simple based on what you said below.
i tried listening to the amp earlier today on two separate occasions. both times about 5 minutes into the first song, the left channel doesnt dim and then then die out...it just goes away after a little "bloop" sound. the right channel goes strong as ever but no sound out of the left channel after that "bloop" sound.