Why do some tubes glow brighter and others darker?

If this occurs with the same type and manufacturer, it is usually due to the position of the heater filament / cathode tube. If the heater filament protrudes far at the top and the tube only slightly, it glows brighter, and vice versa. This is not a defect, but visually it may not be ideal for some users. Where this is recognizable, we naturally take it into account or give tips on how it may possibly be readjusted. If the types are not the same, it is due to the filament temperature. Some tubes operate in the lower temperature range and glow darkly, for example 12AX7WB Sovtek, while others operate in the upper range, for example ECC803S from JJ.