VAC Phi 300 - Manual (Page 13)

11
CHECKING OUTPUT TUBE CONDITION
As described at the beginning of this manual, the Phi is equipped with the KT88 Sentry circuit,
which continuously monitors each output tube to ensure that it is operating within normal limits.
Each tube has a corresponding indicator light on the front panel.
A green indicator light will be seen as the amplifier warms up, or when a tube’s idle current is less
than normal. It is safe to use your amplifier under these conditions, but the tube should be replaced
at your convenience. The threshold between green and no illumination is not precise; do not be
overly concerned if some positions take longer to extinguish. The condition of individual tubes may
be checked with the built in meter.
By operating the test switches beside the light, the meter will display the exact idle current in the
tube - this is used in setting the bias. Be certain to stop the music before making the reading.
Typically, the meter will indicate near the dot at the center of the scale. A lower indication will also
be observed if your power voltage is low.
If necessary, adjust the level of each tube with the screwdriver adjustment adjacent to each switch.
If a strong air current is blowing on the amplifier, it is possible that a tube within the normal current
range will still display the green indicator light.
If a tube draws excessive current, the indicator light will turn red and the high voltage supply will
shut down. This tube should be replaced before using the amplifier.
Please note that operating the amplifier without adequate ventilation will cause the red indicator
lights to come on and the amplifier to shut off. Operating the amplifier on a thick carpet will cause
this, and is not safe.
Output Tube Bias Adjustment:
Bias settings it should be checked when you install your amplifier and approximately once every
month thereafter. It must also be adjusted each time a tube is replaced.
Allow the amplifier to reach normal operating temperature (about ten minutes of warmup should
be sufficient). Mute the preamplifier and utilize the eight toggle test switches on the front panel.
Operate the first toggle switch and observe the bias meter. Using a small flat bladed screwdriver,
rotate the control directly beside the switch until the needle is centered on the small black dot near
the top of the meter. Then continue to the other seven switches and adjust the adjacent controls
as necessary. Since the adjustments can interact slightly, recheck all eight tubes, starting once again
with first toggle.