VAC Avatar SE - Manual (Page 7)

Page 7
OBTAINING THE BEST PERFORMANCE FROM AVATAR
Although surprisingly simple to use, Avatar 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 Avatar performs optimally in
your system.
Output Tube Bias Adjustment:
Your Avatar 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 amplifier 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 EL34 output tubes (V8-V11,
respectively). Press the button for V8 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 V9, V10 and V11 and then, because the adjustments can
interact slightly, recheck all four tubes, starting once again with V8.
Output Tube Sentry Circuit
A unique circuit has been fitted individually to each of the output EL34 tubes. In the event
that an EL34 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 30
milliamperes (about 60% or 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 EL34.