VAC Vintage Mk II - Manual (Page 8)

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.