VAC Renaissance 30/70 - Manual (Page 7)

5
TIPS & ADVICE
A Word About Tubes in General
It is a truth 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 tubes 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 300B or EL34. However, a vivid violet indicates excess
current flow through the tube and should be investigated.
VAC can test tubes for concerned customers.