15
Installing Tubes
When installing tubes, you may find it helpful to lay the amp on its side on a table, and
shine a light directly into the back of the amp, to aid in locating the position of each
tube. Tubes must be installed in the correct orientation. Both 12AX7s and EL84s have
the sample base, with a blank space in the ring of pins which must line up with the
blank space in the ring of tube pins in the tube socket.
Replacing Output Tubes
When changing output tubes, I recommend using a matched pair, but if a matched pair
is not available, an unmatched pair will work fine.
Any time you change output tubes, you must re-set the bias current, by adjusting the
bias control on the back panel, while observing the bias current with a multimeter. This
process is described in detail in the Calibration section of this manual.
Tube Quality
Currently manufactured tubes are built in China, Russia, or the Czech Republic, and
are not built to the same quality standards as tubes manufactured in the “glory days” of
tube manufacturing.
We musicians choose to use to vacuum tubes because of their tone, but we need to
accept that the tubes available to us these days can be imperfect devices, and are most
certainly the least reliable component in the entire amplifier.
It would seem that the obvious solution would be to use new-old-stock tubes that were
made back in the “glory days”, when tubes were very well made. However, I’ve lately
been finding that much of the available stocks of such “new old stock” tubes are either
gassy, noisy, or unreliable – I do believe that we’ve reached the bottom of the barrel of
NOS tubes.
Tube problems generally reveal themselves as a crackling noise which can occur
continuously, sporadically, in response to mechanical vibration, or in response to your
playing (e.g. a crackling or other type of noise which occurs only when you hit a note).
We subject all tubes to a thorough burn-in and test procedure to ensure that they are
fully up to spec and operating perfectly. However, you must be aware that the majority
of tube failures occur early in their life, and may come about as a result of the jostling
and jarring that an amp can receive in shipping. In spite of the exhaustive testing we
perform at the shop, early-life tube problems cannot always be found in such testing.
The first two months or so are the most tenuous period for any set of tubes. Most
manufacturing defects will be revealed in the first two months of their life, but many are
not detectable in initial testing, even after a burn-in period.
If you notice any noises, cracking, or any other odd behavior of your amplifier in this
period, note that it is most likely to be the result of a defective tube, and should be
debugged as such.
