One other thing to do is to check that there aren't any cracked solder-joints on pins 4,5, and 9 of the two 9-pin sockets for the 12AX7 tubes, pins 4,5, and 9 are used for the heater connections, with pin 9 being the common connection for each triode's heater, the 12AX7 is what's called a high-gain twin-triode tube (theoretically capable of a gain of 100) and is a very common tube in guitar amps, each of the 12AX7 tubes in your amp can have it's heaters wired to operate on 12.6V AC (only pins 4 and 5 used with no connection to pin 9, both triode heaters in series), or 6.3V AC (pins 4 and 5 tied together, with pin 9 being the common, both triode heaters in parallel).
Also check pins 1,2,3 and 6,7,8, on both sockets for cracked solder-joints as well, you can do this by examining the solder-joints under a strong magnifying-glass, if you see any joints that look cracked or suspect, just heat the joint up with a soldering iron and add a little extra fresh solder to it, that should fix any cracked solder-joints, but make sure the amp is fully powered-off and disconnected from the mains, and that no dangerous voltages are present first before you do anything else.
Sometimes those amps with board-mounted 9-pin sockets can suffer from cracked solder-joints on the socket pins, usually due to heat and vibration when the amps are operating, or when transported on the road in a car, if it turns out to be a faulty 12AX7 tube with a burnt-out heater, then it's just a simple matter of replacing the faulty 12AX7 with a good one.
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