VAC PA90 - Manual (Page 13)

13
TIPS & ADVICE
A Word About Output Tubes
Your VAC Amplifier can use 2 different output tube types: the KT77 Beam Power Tetrode and
the EL34/6CA7 Power Pentode. From the engineering perspective, beam power tubes have
sharper "knees" to their plate characteristics, more second harmonic distortion, less third
harmonic distortion, and are somewhat less tolerant of load impedance mismatches.
As with interconnects and speaker cables, each tube manufacturer's EL34 tends to have a
distinct sound when used in VAC amplifiers, and of course the KT77 sounds different as well.
Feel free to experiment with different brands and types to customize the sound to your tastes.
A Word About Low Level Tubes
The Voltage Amplifier/Phase Splitter tube in the VAC Amplifiers is a type 12AX7. This tube is
essentially the same as types 12AX7A, 12AX7WA, 7025, CV4004, ECC83, and E83CC. The
driver tube type 12AU7 is also known as types 12AU7A, 6189, CV4003, E82CC, and ECC82.
The quality of the tube used is by far more important than which of these (equivalent) types
is used.
The preferred 12AX7A is the British/Chinese Golden Dragon. Acceptable alternate versions of
the 12AX7 include the Chinese 12AX7, the old USA Sylvania 12AX7 stock sometimes available
under the ECG/Phillips label, the Brimar CV4004, the Hungarian 7025, and the GE 12AX7. The
Yugoslavian 12AX7 yields a somewhat coarse and forward sound.
The best tubes for the 12AU7 spot are the Golden Dragon 12AU7 and the Brimar CV4003
(ribbed plate style,
not the smooth plate version). A darker sound is obtained with the Sylvania
6189, and the Mullard CV4003 is a more lean and hollow sounding cousin. The Yugoslavian
made 12AU7 is adequate but not preferred.
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.
A Word About Bias Levels
The PA90 is designed to operate in Class A at an idle current of 54 (mA) per tube, at
approximately 480 VDC. This represents Class A
1
operation with no output tube reaching cut-
off or drawing grid current (see VAC Technical Monograph 90-8).
You may have noticed that many competing amplifiers consider a much less demanding 26 mA
as "enriched" Class AB
1
, representing operation without output tube cut-off ("Class A") only up
to approximately 16 watts. Some even erroneously refer to this as Class A
1.
Be assured that
the PA90 is strong enough to handle the more demanding 54 mA level. However, if you prefer
the slightly greater tube life predicted by a lower idle current, you may adjust the bias level
down to as little as 40 mA. This will change the amplifier's class of operation to AB
1
. If you do,
experiment with the output impedance connections for best sonic results.