VAC Phi 70 - Manual (Page 2)

PLEASE READ CAREFULLY
Your Phi Seventy is fitted with a thermal sensor circuit. It works in conjunction with the individual
indicator lights located on the front panel. It serves two purposes:
1) If the idle current of a 300B output tube should drop below nominal limit, the light
corresponding to that tube will light green. This indicates that 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. Typical readings are near 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 lights will also be green when the amplifier is first switched on, and will
extinguish when the bias resistors have reached normal operating temperature after several
minutes. 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.
Note: a strong air current could cause the lights to remain green even in normal operation.
2) Should any output tube draw excessive idle current (a “run away” tube or a tube
with an internal short):
A) The high voltage supply in the amplifier will shut down automatically.
B) The light corresponding to the bad 300B tube will glow red. The red light will stay
on until the power switch is turned off. This tube must be replaced.
The lights for the other tubes will show green after a few minutes...this should be
disregarded.
This protection feature normally will operate with much greater speed than the AC line fuse,
and provides a high degree of protection against elevated temperature or fire due to a
major failure in an output tube. However, completely failsafe methods are not possible. As
with any high power electrical device, do not leave the amplifier running unattended. Turn
the amplifier off immediately if you smell any strong odor, or if you see any parts glowing
orange through the ventillation holes between the tubes.
To minimize difficulties, always source the highest quality tubes, avoiding inexpensive or generic
300B tubes. We recommend obtaining replacement tubes from VAC, stating that they are for use
in the Phi Seventy.