Mazda VAC PA100/100 Betriebsanleitung PDF herunterladen (Seite 9)

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

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

." Be assured that the VAC and its tubes are strong enough to handle the more demanding,

richer bias level.

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Mazda VAC PA100/100 Betriebsanleitung Seite 9