VAC Vintage Mk II User Manual
Page 4
Next, place the black test lead in TP3. The meter may indicate a positive or negative reading.
Adjust the balance control to read a near to 0 millivolts as possible.
Since the bias and balance controls interact slightly, repeat this procedure two or three times
until the readings are unchanged.
When installing new tubes, adjust the bias and balance controls to the center of their
rotations. 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. 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.
Tube Types
The input/splitter tube V1 and the driver tube V2 may be type 12AU7A/ECC82/E82CC or
type 12BH7 (typically richer sound). The output tubes V3 and V4 may be type EL34/6CA7
or type KT77. Golden Dragon tubes are generally recommended and have been used
extensively during the development of this amplifier. Please feel free to contact VAC if you
require additional information or advice.
Installing New Output Tubes
Replacement tubes are available from VAC and other sources. Output tubes may be EL34
pentodes, 6CA7 beam power tetrodes, or KT77 beam power Kinkless Tetrodes. It is not
necessary that they be matched pairs, although an improvement in measured performance
may be achieved in this way. Make certain that each tube fits firmly in its socket. A tube that
fits loosely may not make correct contact on all pins and might "run away" (read on). DO
NOT MIX TUBE TYPES. DO NOT MIX BRANDS OF TUBES.
ALL POWER MUST BE OFF. Remove the old tubes after they have cooled down (TUBES
BECOME HOT ENOUGH TO CAUSE SERIOUS BURNS WHEN IN OPERATION AND
MAY TAKE SEVERAL MINUTES TO COOL DOWN). Install the new tubes firmly and
fully in the sockets, taking care to observe the direction of the locating ridge on the plastic
center pin of each tube.