VAC Standard Musicbloc 160 - 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 bi-color LEDs, each with an adjacent adjustment control. View the LEDs from
directly from above. Adjust the LEDs to be green when the amplifier is not playing music. To set,
turn the control clockwise until the LED just begins to change to an amber/red color, and then back
the control off just slightly to restore the pure green color.
As music plays, the LEDs will vary from green to red. Check the bias if you ever see an LED stay
red while music is not playing, or if an LED does not tend toward amber or red when music is
playing. If the flashing of the LEDs is bothersome while listening, the switch on the front labeled
"Bias Test" can be set to "0" (OFF), disabling the LEDs. The switch must then be set to "1" to
check the bias of the output tubes.
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
an LED that shows red when music is not playing.
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.