the bottom chassis—two on each end—and remove the lid. Remove the old tubes
from their sockets and insert new tubes. Replace the lid and the four screws that
hold it in place when finished.
The preamp tubes should be replaced only with tubes of the same type: 6SL7. At
the time of this writing, the only current production tube of this type available is
the TungSol brand. These are good, reliable tubes, and less prone to
microphonics than vintage tubes.
You may also find Sovtek 6SL7s available for purchase. These are actually new
old stock Sovietera tubes, and our experience with them is that they sound good
until they start getting microphonic or noisy—which tends to be early and often.
For this reason, Sovteks are not recommended. Preamp tubes, in general, don't
wear out in the same way as power tubes, so unless a tube is noticeably
problematic, there's probably not much reason to replace it.
As far as replacing the power tubes, see the above section on
Power Tube
Rolling.
There are a bewildering variety of manufacturers and tube types that will work as
the power tubes in this circuit. The good news is that they almost all sound pretty
good, so don't sweat it too much. We use TungSol 6L6GC “STR” tubes and
recommend these without reservation. The performance of power tubes can
degrade over time, so sometimes replacing the power tubes can create an
impression of breathing new life into an amplifier. How long this takes to happen
varies and, of course, depends upon how often and how hard the amp is played.
In “typical” usage, power tubes should last several years before replacement
yields a noticeable improvement.
When replacing power tubes you should purchase and install matched pairs,
replacing both power tubes at the same time. There is no such matching
requirement for the preamp tubes.
Replacing the Fuse
You might also have a fuse fail, in which case it should be replaced with a fuse of
the same type and rating. Make sure any replacement fuse is a slowblow type.
2A fuses are used in countries with 120V and 100V power. 1A fuses are used in
countries with 230V. The fuse holder is located on the rear of the amplifier. If the
fuse fails more than once in a short period of time this indicates that there is a
problem with the amplifier, in which case it should be referred to Dusky
Electronics or a qualified technician for repair.