User guide

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 83 milliamperes per tube, at
approximately 550 VDC on the anodes and 500 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.