VAC Standard 220 - Manual (Page 6)

Page 6
OBTAINING THE BEST PERFORMANCE FROM Standard 220
Although surprisingly simple to use, Standard 220 is also remarkably versatile, and has been
outfitted with an assortment of useful features that enable it to excel in a broad range of
applications. Familiarity with these facilities will assure that Standard 220 performs optimally in
your system.
Output Tube Bias Adjustment:
Your Standard 220 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.
Allow the amplifier to reach normal operating temperature (about ten minutes of warmup should
be sufficient). Mute the preamplifier and remove the small, gold-handled access panel (located
below the bias meter) from the front of the unit. You will see a four pairs of buttons and knobs,
each pair of which corresponds to one of the four KT88 output. Press the first button and observe
the bias meter. Rotate the knob directly above and to the right of the button until the needle is
centered on the small black dot near the top of the meter. Repeat for the other three tubes, and then,
because the adjustments can interact slightly, recheck all four tubes, starting once again with first
push button.
Output Tube Sentry Circuit
A unique circuit has been fitted individually to each of the output KT88 tubes. In the event that a
KT88 enters a ‘run away’ condition and begins to draw excessive current, the Sentry circuit will
engage and limit the current that the tube can draw to approximately 40 milliamperes (about 50%
of normal).
If the Sentry circuit has engaged, you will note low bias meter readings for that tube, which will not
increase even if you advance the bias control fully clockwise.
The Sentry circuit can trigger either due to a tube that needs to be replaced or due to misadjustment
of the bias. To test this, turn the bias control fully counter-clockwise and turn the amplifier off.
Wait 15 minutes, then restart the amplifier. Follow the normal bias adjustment procedure. If the
tube can be biased normally and does not re-trigger the Sentry circuit, then all is fine. If the bias can
not be adjusted to the normal value, or if the Sentry circuit is triggered again, then replace that
KT88.