10
VAC iQ Continuous Automatic Bias System
In engineering, the abbreviation “Iq” denotes the quiescent current of a vacuum tube, also known as the
idle current or bias point. For best performance, the Iq must be set and maintained precisely, even as the
amplifier warms up, the power line varies, the tubes drift, and regardless of whether you're playing the
music softly or loudly.
No system has been able to do this. Until now.
The result of 17 years of research, the VAC iQ Bias Control System is the first and only system able to
monitor continuously the true bias point of each output tube and hold it precisely at the desired value,
thus ensuring optimal performance at all times. It actually outperforms manual systems because it can
correct for the heating effects that occur when reproducing loud passages of music. The VAC circuit both
MONITORS and CONTROLS, continuously, in real time.
The resulting performance difference is one that you can easily hear. And not only does it sound better, it
radically reduces the rate of tube failures, and is self-diagnosing.
Operation
In operation, the iQ circuit monitors and maintains the correct bias adjustment for the KT88 output tubes
at all times, regardless of the music playing.
The iQ system contains two other components that inform you of tube condition and protect the amplifier;
the operation of these two circuits is indicated by the bicolor LEDs.
If an LED illuminates GREEN, this is an indication that the associated tube is becoming weak. In a
conventional amplifier, you would have advanced the bias control well clockwise for this tube. No
immediate action is required; simply replace this KT88 with a fresh one when time permits, and the
amplifier will make the best of the weak tube until then.
An LED will illuminate RED if an output tube draws excessive current, i.e., if a ‘run away’ occurs. Because of
the stabilizing action of the auto bias circuit, this is most likely to occur if something inside the tube has
physically broken. The iQ system detects the fault within a fraction of a second and shuts down the main
power supply, thus preventing any damage; all of this happens long before the fuse can react. Switch
power off, and replace and discard the indicated tube after it cools down.
See the chart on the next page.
Summary of Contents for Musicbloc Master 300 iQ
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