1) Tube not plugged in firmly 2) Tube pins dirty 3) Tube socket holes are too large for smaller NOS tube pins 4) Bad pin soldier joint (usually on Russian tubes) 5) Tube is ready to s**t the bed. 6) Momentary household voltage change caused the grid bias to change and let the plate voltage "run away". Clean the pins/sockets with Caig DeOxit, then use G-X5 (high temp.) preservative. Caig also has some great cleaning swabs and brushes to fit the sockets. Next, look at the socket holes, and see if any look bigger than the others. You can use a very fine jewelers screwdriver to GENTLY close the metal socket contact. UNPLUG YOUR AMP FOR 24 HOURS BEFORE THESE STEPS, AND DO NOT GROUND YOURSELF TO ANY OTHER METAL ITEM (CHASSIS, STAND, ETC.) Definitely keep an eye on that tube! Also, is the GE 6550A power tube a proper tube for your amp (check with the manufacturer)?
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