VAC PA160 Mk I - Manual (Page 4)

4
Operation
Continuous operation is not recommended. If the amplifiers will not be auditioned for a few hours it is best
to turn them off. This avoids undue stress to the KT88 output tubes, whose lives average between 3,000
and 8,000 hours depending upon brand fitted and random variations within the tubes themselves.
Do not operate the tube type switch while the unit is turned on.
As with all high fidelity products, the sound characteristics of the VAC amplifiers change somewhat as they
warm up. Good sound will be achieved within 15 minutes of operation, with subtle changes occurring for
up to one hour.
Please note that the initial break-in time of high resolution audio equipment is infuriatingly long. The PA160
will continue to season for at least 200 hours. Typically, the early sound of the amplifier will be less
extended, dynamic, and coherent. Then the sound will improve noticeable, followed by a period of darker
sound, finally giving way to the desired musicality. Patience, as they say, is a virtue.
Any time that the VAC Power Amplifier has not been used for a few weeks the sound may be different. This
is also normal for high resolution audio equipment. Optimum sound should return after a few hours. Also
be aware that many components display the need for a new break in period after being transported in
unheated cargo aircraft.
Bias Adjustment
Your VAC Power Amplifier has been shipped with output tube bias preset. This should be checked when
you install your amplifier, and periodically thereafter. It must also be set whenever an output tube is changed
or when the output operating mode is changed.
Adjustment of the output tubes is quite easy, and requires only a small screwdriver. On the front top of the
amplifier are four green LEDs, each with an adjacent adjustment control. Adjust the LEDs to be off, just
shy of green, when the amplifier is not playing music. To set, turn the control clockwise until the LED just
begins to glow green, and then back the control off slightly until the LED goes out.
As music plays, the LEDs will flicker off and on. Check the bias if you ever see an LED stay green while
music is not playing, or if an LED does not illuminate when music is playing.
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
green when music is not playing.
When installing new tubes, set the bias controls counter-clockwise. Check the bias of both channels 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.