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The TriAmp
®
MK II ships with EL34 and 12AX7 tubes.
Once they’ve been burned in (initial continuous
operation under a load), these undergo an
indepth selection process where their electrical
values and mechanical status (microphonics)
are checked. Beyond that, they’re installed in
a finished amp and their sound is auditioned
in a listening benchtest.
One of the most important steps is power tube
matching, a process whereby tubes with the same
characteristics are selected and combined to
make matched sets.
When is it a good idea
to replace tubes?
The tubes in the TriAmp
®
MK II are exemplary in
terms of quality, workmanship and long service
life. If you nevertheless come across a problem,
please be sure to run down the following checklist
before you swap out your old tubes for new ones:
Was the fault or failure of the power tube caused
by the tube itself or a flawed peripheral device or
component, perhaps a defective speaker cable?
(If you don’t get to the bottom of the problem
and remedy it, it may crop up again even after
you replace the tubes.)
Did the mains voltage fluctuate or spike while
the amp was on? In all-tube amps, over-voltage
surges in the mains net can certainly blow fuses
or even the amps. Over-voltages are often
caused by generators or improperly installed
high-current power circuits.
Did a fuse blow even though none of the tubes is
actually defective? An old fuse, tube de-ionization
or mains voltage power surges could conceivably
have caused it to blow. In this case, replacing
tubes of course is a waste of money and time.
Tubes definitely show signs of wear when their
service life is coming to an end, such as increased
microphonics, noise, muddier tone through loss of
high-end frequencies, degraded performance,
etc. You should take these indications seriously
and replace old tubes because not only do these
side effects take their toll on sound quality, they
also indicate the affected tube is about to fail.
NOTE:
Although it may be tempting to find out what
an amp sounds like with different tubes,
we urgently recommend that you refrain from
replacing tubes for experimentation purposes.
Handled improperly, this will damage the amp
and lead to incredibly steep repair costs.
What should you keep in mind
when you’re replacing tubes?
The golden rule: Replacing tubes is a job best left
to qualified professionals.
Accordingly, these guidelines are addressed to
reputable technicians who earn their living with
a screwdriver:
Pull the mains plug of the TriAmp
®
MK II and
allow for a discharge time of at least two minutes
before removing the sheet metal panel on the
rear of the amp. Once the panel is off, you can
cautiously ease the tubes out of their sockets.
A single power tube may only be replaced if the
replacement tube is precisely matched to the ori-
ginal, in other words, it has the same characteristics.
As a rule, if you are replacing power tubes,
use matched sets only.
If you’re installing a new set of power tubes with
characteristics identical to the old set, it is not
absolutely necessary to re-bias the amp.
TRIAMP MK II - MANUAL
12
12
5.0 REPLACING TUBES, SERVICE
AND PREVENTIVE MAINTENANCE