VAC Phi Alpha 160 Musicbloc - Manual (Page 6)

Page 6
Output Tube Bias Adjustment:
Your amplifier has been shipped with the output tube bias properly set. Nonetheless, 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.
Adjustment of the output tubes is quite easy, and requires only a small screwdriver. On the front
panel are four test switches, each with an adjacent adjustment control. With no music playing, lift
and hold the first test switch while viewing the meter: the meter’s needle should point straight up,
directly to the central black dot (note: slightly below the dot is acceptable; above the dot is never
acceptable). Using a small screwdriver, adjust the control for that tube if necessary to attain this
indication. Now, release the first test switch; move to the next test switch and repeat the process,
and so on, until all four tubes have been tested.
Any tube that can not be adjusted to the correct bias point should be replaced. Any tube which
requires constant readjustment should be replaced immediately. Never operate the amplifier with
a tube that is indicating above the meter’s central dot.
When installing new tubes, set the bias controls counter-clockwise. Check the bias of all tubes as
the amplifier warms up - don't wait ten minutes.
Bias levels should be checked monthly to ensure optimum sound quality. It is not unusual for the
bias to change with time, particularly when tubes are new. The greatest amount of drift occurs
during the first 200 hours of a tube's life. The drift may change direction periodically, such that the
bias control must be increased and later decreased, or vice versa. Check bias if the sound seems
lacking in detail or dynamics.
Note: When not testing bias, the meter is indicating power line voltage.