Example: I63
Δ 7L2
I6 = ECC83/12AX7
I63 = ECC83/12AX7 #3 = manufacture series
Valvo Holland and Hamburg used color print and design for the special features as follows:
Military tubes: can consist of ALL the below attributes, including special alloys too expensive for commercial use. Also, tubes with special Military markings were pre-tested for tight tolerances and matched sections, and made with the more expensive earth metals, and better glass bottles that can withstand temperature variations in combat, etc.
Red Label: Industrial tubes designed for long heater hours, durability, high tolerance, shock and vibration resistance, special cathode construction
Yellow label: designed for long heater hours, durability, high tolerance
Green label: computers, designed for long heater hours, durability, high tolerance curve points, special cathode construction
Blue label: Aviation/aerospace. Special switching filaments. Used in missiles as an on-off switch, Industrial tubes designed for long heater hours, durability, high tolerance, shock and vibration resistance, special cathode construction
Nonetheless, all tube specifications change over time or don't change from the original factory testing, whether for Military, Industrial, or commercial, and they have to be re-tested now and today! That's why proper tube testing is necessary for any tube, regardless of what a color-code, or class of tube designates. Heater-to-Cathode leaks in a tube render it useless and too many of these undesirables are in circulation on the internet and sold by other sellers unbenownst to them or perhaps they know and don't care. We care, Tube Museum tubes takes the guesswork out of the process of audio tube selection, providing the best possible audio tubes available.
