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 70 milliamperes per tube, at
approximately 535 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 @ 550
VDC) as "Class A
1
." Be assured that the VAC and its tubes are strong enough to handle the more demanding,
richer bias level.
