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5
Replacing Tubes, Maintenance & Care
The Duotone® ships with EL34 and 12AX7 tubes by RUBY®. Once
they've been burned in (initial continuous operation under a load),
these undergo an in-depth 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 Duotone® 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 Duotone® 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 original, in other words, it has the same characteristics. As a rule, if
you are replacing Power tubes, use matched sets only.
Duotone Head
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. If, on the
other hand, you install a new set with characteristics that deviate from
the old set, you definitely should re-bias the amp. AGain, biasing an amp
is not a DIY task. It takes experience and a extensive working knowledge
in measuring techniques, which is why this job should be carried out
exclusively by qualified technicians with tube amp tuning experience.
How can you extend the service life of tubes?
Never operate the Duotone® without connecting a load (loudspeaker)!
With this in mind, you should always use high-quality, heavy-duty speaker
cables that won't crimp or snap.
Use the STAND BY switch! Warming tubes up unnecessarily wears them
out that much quicker. Make a habit of shutting down the anode voltage
via the Standby function and you'll enjoy many more hours of operation.
Avoid exposing the amp to vibrations, especially when it's Powered up.
Switch the amp off a good while before moving the amp so that the tubes
can cool off completely first.
Proper biasing and trimming extend service life. Whenever you feel that
tube wear is on the rise, be sure to have a technician check the amp's bias
and trim settings.
Here are a few more basic rules that will extend the service life of
your amp enormously if you follow them:
•
Make sure all peripheral devices and connecting cables are in a state
of good repair! - Ensure plenty of air can circulate around the amp's
ventilation slots at all times - your Duotone®'s life depends on it.
•
Definitely avoid exposure to mechanical shocks and extreme heat, dust
and particularly moisture.
•
Be very careful about the kind of peripheral devices you connect to your
amp and always check out their specs before you plug them in. Never
connect speaker cabinets with an impedance (ohm) rating lower than
the Duotone® is designed to handle. Under no circumstances should you
connect devices with high output signal levels (e.g. Power amps) to your
amp's Input.
•
Be sure the AC Power source delivers the current that your amp is
designed to handle before you plug it in. When in doubt about the local
rating, ask the venue's sound technician or a stage hand.
•
Refrain from DIY repairs! To be on the safe side, you should also have a
qualified technician replace internal fuses.
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Troubleshooting
The Duotone® won't Power up when you switch it on:
•
It's not getting AC Power. Check the mains cable to see if it is connected
properly!
•
The mains fuse is defective. Replace the fuse with another fuse of the
same type. If this fuse also blows, be sure to talk to your local Hughes &
Kettner® dealer.
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