VAC Phi 300.1 - Manual (Page 11)

10
CHECKING OUTPUT TUBE CONDITION
The condition of each individual KT88 output tube should 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.
Please note that operating the amplifier without adequate ventilation may not be safe, may shorten
the amplifier’s life, and may result in tube failures. Operating the amplifier on a thick carpet will also
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 (or the Phi 300.1) and
utilize the eight toggle test switches on the front panel. Activate and hold the
first toggle switch and observe the bias meter. Using the supplied tool (or 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. Release
the toggle switch when the adjustment is complete, and 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 with the first
toggle.