I recently got my hands on a Mesa F-30 in extremely good condition and when i received it I proceeded to give it a test run. After about an hour of testing it when I was using the channel two + contour the channel suddenly turned clean.

It still is running clean/ semi-distorted(but not nearly as much as when i first tested it)and when I attempt to turn up the gain past about 4 o'clock it gets very quiet. If anyone has any idea how to fix this or what the problem is I would appreciate any input. Thanks