VAC Vintage Mk II - Manual (Page 11)

TIPS & ADVICE
A Word About Low Level Tubes
The small tubes (Voltage Amplifier/Phase Splitter and Driver) in the VAC Amplifiers are
type 12AU7. This tube is essentially the same as types 12AU7A, 5814, 6189, CV4003,
ECC82, and E82CC. The quality of the tube used is by far more important than which of
these (equivalent) types is used.
The preferred 12AU7A is the British/Chinese Golden Dragon. Acceptable alternate versions
include the Brimar CV4003 (ladder plate version), Mullard CV4003 (very midrangy),
generic Chinese 12AU7 (brighter), Sylvania 6189 (darker sound), the GE 12AU7A (slightly
forward and grainy) and the Yugoslavian 12AU7 (cohesive and somewhat coarse).
Many classic tubes worth trying if you have access to them, such as those from Telefunken
and Amperex, although a caution is in order, as we have recently seen East German EL34
relabelled "Telefunken West Germany." Tubes actually manufactured by RCA,
Westinghouse, and Sylvania can be quite good, but the name on the tube doesn't always
indicate who actually made it. Other names to watch for are Valvo, Mazda, Tung-Sol,
Bendix, Mullard, Brimar, and Raytheon.
TIPS & ADVICE
A Word About Bias Levels
The output stage is designed to operate in rich Class AB
1
at an idle current of 65
milliamperes per tube, at approximately 500 VDC on the anodes and 460 VDC across the
tubes. For more technical information about classes of operation see VAC Technical
Monograph 90-8.
Some designers erroneously market a lower voltage and current operating point (as little as
26 mA) as "Class A
1
." Be assured that the VAC and its tubes are strong enough to handle
the more demanding, richer bias level. However, if you prefer the slightly greater tube life
predicted by a lower idle current, you may adjust the bias level down somewhat lower, being
sure to keep all four tubes at the same level. If you do, experiment with the output
impedance connections for best sonic results.