VAC Vintage High Power - Manual (Page 7)

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 12AX7 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 10 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 the amplifiers can no longer meet specifications or when (if you have access to a tube tester)
the tube's 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 10 years.
VAC will be happy to test tubes for concerned customers, providing transconductance, idle current and/or a
photographic record of the tube's plate family curves.