VAC Phi 300 - Manual (Page 2)

PLEASE READ CAREFULLY
Your Phi Beam Power amplifier is fitted with a unique output tube monitoring circuit. Rather than
directly contacting the audio path, and thus contaminating the pure sound, the temperature of a
small cathode resistance is sensed. The four bi-color LED’s on the front panel indicate the tubes’
conditions.
If the idle current of a KT88 output tube drops below nominal limits, the LED corresponding to that
tube will light green. This indicates that either the bias setting for that tube should be adjusted, or
the tube should be replaced at your convenience, but it is safe to continue operating the amplifier.
The exact idle current of each tube may be tested by means of the meter and the test switches
located on the front panel. The associated bias control should be adjusted so that the needle
points directly at the dot in the center of the meter. When no test switch is selected, the meter
indicates the incoming ac voltage (wall voltage).
Note: These LED’s will also be green when the amplifier is first switched on, and generally will
extinguish when the bias resistors have reached full operating temperature after approximately 30
minutes. The threshold between green and no illumination is not precise; do not be overly
concerned if some positions take longer to extinguish or fail to extinguish. The precise condition
of individual tubes may be checked with the built in meter.
Note: air currents could cause the lights to remain green even in normal operation.
In the event that any output tube draw excessive idle current (a “run away” tube or a tube not
biased correctly), the high voltage supply in the amplifier will shut down automatically and the LED
corresponding to the problematic KT88 tube will indicate red. The red light will stay on until the
power switch is turned off. This tube must be replaced. (Note: The LED’s for the other tubes will
show green after a few minutes...this should be disregarded.)
To minimize difficulties, always source genuine VAC Tested audio tubes for your Phi.