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Tips: A Word About Tubes in General
It is true that each brand of tube sounds different in a particular high resolution circuit. This is because no two
manufacturers make a tube type in quite the same way, and the central tendencies of the performance
parameters will differ slightly with each maker. To emphasize the point, examine the plate structure of any
two 6SN7 from different manufacturers will probably find that they may not even the same shape and size.
(Be careful here, as often a tube is made by a firm other than indicated on its label. In the heyday of tubes it
was common to crossbrand between major labels, such as GE and RCA. Today many labels do not
manufacture their tubes at all, including Gold Aero and RAM.)
This sonic variability may at first seem a liability, but further thought will reveal that it is an advantage, just like
the ability to adjust VTA on a tone arm. The owner of a tube amplifier can select those tubes which sound
like the real thing in his/her specific system. Of course, if the manufacturer you prefer is rare you may want to
purchase a few spare tubes for the future.
How long should tubes last? It has long been known in professional circles (and probably now forgotten)
that a tube such as the 12AX7 will display better performance characteristics after two years of continual
operation than when it was new. In normal use it is not unusual for a low level tube to last 5 years or longer.
Output tubes are another story, as they are continually providing significant amounts of current. Here the
sound is your best guide. Certainly a tube should be replaced when its emission is significantly down or its
transconductance is substantially out of specification. In normal use, output tubes will last at least 2 years and
perhaps more than 5 years.
It is normal to see a slight violet glow in a power tube such as a KT88. However, a vivid violet indicates
excess current flow through the tube and should be investigated.
VAC can test tubes for concerned customers.
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