Bias Windows / Tube Life - Decware ZMA Operation Manual

OPERATION MANUAL
WARNING
This amplifier is designed to operate with a 50ma bias on each output tube.
Running higher bias currents for any length of time will overheat the power supply
transformers and could cause pre-mature failure. We recommend no more than
60ma per tube at any point. Running less is not an issue for the power supply but
doing so will reduce the power output of the tubes and change how they sound.
Some will use this technique as an esoteric voicing tool.
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NOTE - Different output tubes will bias at different levels so you will have to re-
adjust the bias level control when changing tubes.
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INSIGHT - The two small 9 pin tubes (6N23P) that make up the input stage and
phase inverter are used to drive the output stage. These tubes have an effect on
the overall matching of a given pair of output tubes. That means you can have a
perfectly matched pair and an odd or misbehaving 6N23P could cause the tubes to
be off sightly. Of course with the Mystery Amp you can simply compensate with the
ingenious balance control.
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BIAS WINDOWS and TUBE LIFE
The biasing capabilities of this amplifier operate on two pre-determined windows.
Window one is how high and how low you can adjust the bias level, for example,
between 20ma and 80ma. This rather wide window is needed to accommodate
such a wide range of output tubes. It is nevertheless narrow enough to keep you
from getting into trouble and prevents the possibility of the popular term "run away
bias" when an output tube completely melts down, shorts out and ruins your
amplifier.
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How to tell when your output tubes no longer match well enough to use.
The second is the balance window that has also been set to a predefined width.
This is very important because it is what tells you when it's time to replace your
output tubes. Quite simply, when you have the bias balance control all the way to
the right or left and still can't quite get the meters to match, one of the output
tubes has drifted so far away from the other, that they no longer can be used
together in the amplifier. If both tubes were installed at the same time we typically
will just replace the entire quad so that both channels are new and match.
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MODEL : ZMA - ZEN MYSTERY AMP REV 1 MAR 2014
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