VAC PA90 - Manual (Page 12)

12
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.