VAC PA160 Mk I - Manual (Page 8)

8
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.
Different types of tubes will also tend to sound different (ex: 6L6GC vs. EL34 vs. KT88).
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.
It is normal to see a slight violet glow in a power tube such as a KT88 or EL34. However, a vivid violet
indicates excess current flow through the tube and should be investigated.
VAC can test tubes for concerned customers.
Low Level Tubes
The Voltage Amplifier/Phase Splitter and Driver tubes in the PA160 are the 6SN7 medium mu octal twin
triode. Your amplifier is fitted with the current production British/Chinese Golden Dragon, which we find
superior to the NOS types we have tried. There are dozens of versions of this tube available in new old stock
(NOS) from a variety of sources. It would be impossible to characterize them all. The usually are not terribly
expensive, and may be worth experimenting with. VAC welcomes your comments.
One NOS variant worth mentioning is the RCA "Special Red" industrial version 5692. These represent the
pinnacle of RCA's tube knowledge.
Other equivalent type numbers are 5692, 13D2, B65, ECC32, QA2408, QB65, and CV1988.