11
TIPS & ADVICE SECTION
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.
In the event of unusual noise in one channel, or a loss of sound in one channel, a bad tube may
be located easily. If the problem is audible in the line stage, exchange the two 8416 tubes; if the
problem changes channels then you have found the bad tube. If the problem is only audible on
phono, swap the rear pair, then the middle pair, then the front pair, each time listening to see if
the symptom follows the tube.
VAC can test tubes for concerned customers.
