make sure the cable you are using to connect the head to your cabinet is a good one. make sure the cable you are using to your guitar is a good one. The pre gain is how much dirt for the amp, the post gain is the master volume. make sure all the tubes are seated correctly in their sockets. do this by just looking at them and making sure they are snug.
do not touch anything inside of the amp or you will die. i'm not joking.