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The Nature Of Tubes: Why (And When) To Replace Them:
Tubes are made up of a number of fragile mechanical components that are vacuum-sealed in a glass envelope
or bubble. The tube's longevity is based on a number of factors which include how hard and often the ampli-
fier is played, vibration from the speakers, road travel, repeated set up and tear down, etc.
Any time you notice a change in your amplifier's performance, check the tubes first.
If it's been a while since the tubes were replaced and the sound from your amplifier lacks punch, fades
in and out, loses highs or lows or produces unusual sounds, the power tubes probably need to be replaced.
If your amplifier squeals, makes noise, loses gain, starts to hum, lacks "sensitivity", or feels as if it
is working against you, the preamplifier tubes may need to be replaced.
The power tubes are subjected to considerably more stress than the preamplifier tubes. Consequently, they
almost always fail/degrade first. If deteriorating power tubes aren't replaced they will ultimately fail.
Depending on the failure mode, they may even cause severe damage to the audio output transformer and/or
other components in the amplifier. Replacing the tubes before they fail completely has the potential to
save you time, money and unwanted trouble. Since power tubes work together in an amplifier, it is crucial
that they (if there is more than one) be replaced by a matched set. If you're on the road a lot, we rec-
ommend that you carry a spare matched set of replacement power tubes and their associated driver tubes.
After turning off the power and disconnecting the amplifier from the power source, carefully check the
tubes (in bright light) for cracks or white spots inside the glass or any other apparent damage. Then, with
the power on, view the tubes in a dark room. Look for preamplifier tubes that do not glow at all or power
tubes that glow excessively red.
Whenever you replace the power tube(s):
• Always have the amplifier's bias voltage checked by a qualified service center. Improper bias voltage
will cause degradation in performance and possibly damage the tubes and/or the amplifier. (See the section
below entitled, "The Importance of Proper Biasing", for more information on this subject).
• We highly recommend that you replace the driver tube(s) as well. The driver tube determines the shape
and amplitude of the signal applied to the power tube(s) and has to work almost as hard as the power
tube(s).
You can check your preamplifier tubes for microphonics by turning the amplifier on, turning up the gain
and tapping lightly on each tube with the end of a pencil or a chop stick (my favorite). You will be able
to hear the tapping through your speakers, which is normal. It is not normal for a tube to ring like a
bell after it’s tapped. If it does ring then it’s microphonic and should be replaced. Remember to use only
high quality, low microphonic tubes in the preamplifier section.
Even though power tubes are rarely microphonic, you should check them anyway. The power tubes can be checked
for microphonics just like pre-amp tubes.
In the case of very high gain amps, you may be able to reduce the amount of noise generated by simply swap-
ping the preamp tubes around.
The Importance Of Proper Biasing:
For the best performance and longest tube life, proper biasing is imperative. Bias is the negative volt-
age which is applied to the power tube’s control grid to set the level of idle current. We cannot over
emphasize the difference in warmth of tone and dynamic response that come with proper biasing. If the bias
is set too high (overbiased), the sound from the amp will be distorted at all levels. If the bias is set
too low, (under biased) the power tubes will run hot (the plates inside the tubes may glow red due to
excessive heat) and the sound from the amplifier will lack power and punch. The excessive heat greatly
reduces tube life – from a few days to as little as a few hours in extreme cases. Setting the bias on your
amp is like setting the idle on your car. If it’s too high or hot it’s running away with you and if it’s
too low or cold it will choke when you step on it.
The bias is adjusted at the factory in accordance with the type of power tube(s) installed in your ampli-
fier. It is important to point out that tubes of the same type and specification typically exhibit dif-
ferent performance characteristics. Consequently, whenever power tubes are replaced, the bias voltage must
be checked (unless the amplifier is equipped with "self-biasing circuitry) and readjusted to accommodate
the operating parameters of the replacement tubes.
Depending on the model and amplifier type, there may be hum balance controls, trim pots, or bias adjust-
ment controls on its rear panel. However, the bias adjustment should be performed only by qualified serv-
ice personnel with the proper, calibrated test equipment.
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