Last time we talked about blue glow in vacuum tubes and what causes it. Here we’ll look at filament/heater glow as well as red plate glow in power tubes.
Filament or heater glow is also something you’ll typically see when viewing a vacuum tube in use. The filament in a tube has the job of heating the tube’s cathode up to a temperature where electrons are excited and start flowing toward the positively charged plate/anode. All common audio tubes will have a heater filament. Some modern preamp tubes have filaments that are well hidden behind the plates making them difficult to see, even in a dark room. Regardless, if the tube is warm and sound is coming from the tube amplifier, the heater filament is functioning no matter how bright or dim it appears.
Finally, another type of tube glow that needs mention is red-plating or “cherry” plate glow. This happens when a tube is incorrectly biased, causing the plate to overheat and literally get red-hot. Tubes will not last long if they are operated under this condition and rebiasing or amp repair should be done quickly.
We hope this article will help you become a more informed musician or audiophile by learning about internal tube glow and how it affects quality. You can find some more in-depth info on this topic at our website.