WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
Sometimes, a little acetone or MEK will reactivate the old cement. If that does not work, try a small amount of fast setting, liquid, cyanoacrylate, such as "Crazy Glue".