Glowing Plate - Common Occurrences

Common Occurrences

In modern-day repair and maintenance of early tube-based consumer electronics devices, a glowing plate will be rarely encountered. As a visible symptom of a destructive failure, checking for an overheated tube is essential to ensure the safe and reliable operation of the equipment undergoing maintenance.

Most frequently, a glowing plate overload will be found in rectifiers and output tubes. In particular:

• Rectifier tubes (5U4, 80, etc.), usually due to shorted capacitors or other substantial short circuits within the device. The power supply's output may be easily disconnected from the rest of the circuit in order to determine whether the overload is occurring in the supply or elsewhere.
• Horizontal and vertical output tubes in television sets (6CD6, 21LU8, etc.), usually due to a shorted coupling capacitor upsetting the bias or the driving oscillator operating well out of its design range. Deflection yokes are sometimes faulty.
  • Horizontal output tubes may also be overloaded by a flyback transformer with shorted turns, a defective damper (i.e. 6W4), high voltage rectifier (i.e. 1B3) or high voltage regulator (i.e. 6BK4) tube.
  • Early color television receivers also used a complex dynamic convergence system, which was driven by both the horizontal and vertical output stages. Failure of part of this system could overload either tube, though this author has never seen such a failure in person.
• Audio output tubes in radios, stereos, public address systems and guitar amplifiers (50C5, 6L6, etc.), usually due to a bad coupling capacitor upsetting the bias or a shorted load.

If any tube presents a glowing plate, the equipment should be shut down immediately to avoid further damage.