VAC Phi 70 - Manual (Page 12)

10
CHECKING OUTPUT TUBE CONDITION
As described at the beginning of this manual, the Phi 70 is equipped with the 300B 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 until the amplifier warms up, and 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 below the light, the meter will display the exact idle current in the
tube. 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 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.