VAC PA35/35 - Manual (Page 8)

TIPS & ADVICE
A Word About Output Tubes
Your VAC Amplifier uses the EL34 in PENTODE VERSION ONLY. Some 6CA7/EL34 have been made as
"beam power" tubes, and will damage the PA35/35, as will any output tube that internally ties pins 1 and 8
(the cathode and supressor grid).
As long as the above requirement is met, feel free to experiment with different brands and types to customize
the sound to your tastes.
As with interconnects and speaker cables, each tube manufacturer's EL34 tends to have a distinct sound when
used in VAC amplifiers. The PA35/35 was developed primarily using the Golden Dragon EL34.
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
. For more technical information about classes of
operation see VAC Technical Monograph 90-8.
Some designers erroneously market a lower 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.