VAC Vintage High Power - Manual (Page 3)

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.
As with all high fidelity products, the sound characteristics of the VAC amplifiers change somewhat as they warm up.
Best sound will be achieved after 15 minutes of operation, with subtle changes occurring for up to one hour. However,
we advise against leaving the equipment on at all times because of the attendant acceleration of output tube wear. Life
of the output tubes averages between 2,000 and 5,000 hours depending upon brand fitted and random variations within
the tubes themselves.
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 of operation, preferably
with an audio signal.
Please note that although your VAC amplifiers have been run for 48 hours at the factory, they will continue to "break
in" for approximately 150 hours. The break in is most pronounced in the ultra-linear mode. Note that the optional
triode mode and ultra-linear mode do not require separate break in periods, and that the ultra-linear mode will improve
even if you only operate the amplifiers in the triode mode (this is in most cases the most listenable way to break them
in).
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 approximately every month thereafter. It must also be set whenever an output tube is changed.
Monitoring and adjustment of the output tubes is quite easy, and requires a simple voltmeter. To start, place the black
test lead in the center test point (TP5) and the red lead in TP1 (the test probes will "snap" into the test point jacks with
moderate pressure). Adjust the associated bias control to obtain the DC voltage reading listed on the top of the chassis.
Perform the same operation for TP2, TP3, and TP4. Finally, since the tubes interact slightly, repeat this procedure two
or three times until the readings are unchanged. Lower bias readings will not damage the amplifier or tubes, but may
affect the sound if more than 1 or 2 volts low. It is most important to have all of the tubes read the same within +/- .5
VDC.
When installing new tubes, adjust the bias controls to the centers of their rotations. Check the bias of all tubes as the
amplifier warms up - don't wait ten minutes. Recheck after 10 minutes, one hour, and one week.
Bias levels should be checked monthly to ensure optimum sound quality. Allow the amplifier to warm up for an hour
before checking. It is not unusual for bias current to change with time, particularly when tubes are new. In fact, 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 then later decreased, or vice versa. Also check bias if the sound seems
to be lacking in detail or dynamics.