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The filament in a tube has the job of heating the tube. It is required to bring the tube up to a temperature where electrons will get excited and start flowing. The filament is also commonly called a heater for this reason. All common audio tubes will have a heater filament although in some tubes they are well hidden behind the plates and other elements making them difficult to see, even in a dark room. Regardless, if the tube is warm and sound is coming from the amplifier, you know the heater is doing its job no matter how bright or dim it appears. An excellent example of this is the reissue Tungsol 12AX7 tube shown in Photo 5. Its heater is extremely well hidden and casts little light. The Chinese 12AX7A also shown casts much more light even though it is operating in exactly the same circuit. On occasion we have people wonder why one tube appears brighter than another in a matched set they have. In almost all cases it’s simply a matter of one heater being more exposed than the other and therefore casting off more light. This is not a sign of a problem. Remember they are all hand made and a variance like this is normal. Photo 6 shows two EL34 tubes where one is brighter than the other due to the filament height. Both tubes test and perform perfectly.
Another thing that needs mention is red-plating or "cherry" plate glow. This will happen when a tube is incorrectly biased, causing the plate to overheat. Generally speaking, tubes do not like this unnecessary stress and will not last long if rebiasing is not done. See Photo 7. In this case the plate itself is actually casting a red glow.