Vacuum tubes glowing in an amplifier

Filament or Heater Glow, and Red-Plating in Vacuum Tubes

3:21 pm

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.


12ax7 glowFilament 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.

smredglowFinally, 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.

3 Responses

  1. Julio Velazquez
    Julio Velazquez at |

    Hi Jon, will a bad power transformer cause my four 6l6 tube to red plate? Strange all four tube red plate and blew a fuse. Thank you. JR

  2. Arvind
    Arvind at |

    Hi Jon, what would cause the filament glow to be ‘blue’ instead of usual orange in color?
    I notice this in one of the KT88 tube, while others are all orange. This happens at the amp startup and the bias needle goes slightly over the initial set bias as per amp instructions.
    many thanks in advance.


    If all tubes get red-plating, it is an issue with the bias. First of all check the coupling capacitors hooked to the 6L6 (mostly the root cause). Then if they are not voltage leaking check the bias resistors are in good condition (if self bias) or fixed bias voltage.


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