TIPS & ADVICE
A Word About Output Tubes
Your VAC Amplifier can use 2 different output tube types: the KT88 Beam Power Kinkless Tetrode and the
6550A Beam Power Tetrode. We consider the sound of a good KT88 to be superior. Properly manufactured
6550/6550A will work, but may have a slightly higher incidence of failure in the PA80/80. 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 KT88 tends to have a distinct sound when
used in VAC amplifiers, and of course the 6550As sound different as well. Here is a brief summary of our
experiences with currently available tubes from different sources.
M-O Valve Company/Genalex/GEC KT88 (U.K.) PRE-1970:
An accurate and sweet tube with good bass quality. Quality is surprisingly variable for a premium tube.
World wide supply is limited.
Highly recommended.
M-O Valve Company/Genalex/GEC KT88 (U.K.) POST-1980:
Hard, glaring sound, only fair detail. Not recommended.
Golden Dragon KT88 (July 1994 and after):
A new variant of the Chinese KT88 developed in Great Britain. An very good tube, close to the riginal,
great speed, openness, imaging, and balance.
Highly recommended.
Generic Chinese KT88 (including ARS, Gold Aero, Ram, Penta, Jolida, etc.):
Actually a fairly good sounding tube. Bass quality somewhat phasey, some mechanical sound due to a
subjective dip in the upper-mid-range. Some samples make rather alarming noises on warm up and cool
down, often heard through the speakers.
National NL-KT88-USA:
A new version on the KT88 produced by the US arm of the Richardson's organization. Visually
very similar to the original and sonically satisfying, close to both the original and the Golden
Dragon. Slightly lightweight sound, but sweet. Very expensive.
Golden Dragon 6550A:
Excellent tube with more subterranean bass and mid-range zing than a good KT88. Loses in terms of
ease and dimensionality. May help wake up a sleepy system, but overall less natural. Good sound.
US made RCA and Tung-Sol 6550:
Early 6550 type in the "coke glass" bottle. Pretty good sound.
GE 6550A (applies to those manufactured in USA by MPD):
Overall hard and lacking in air. Not recommended.
Phillips/ECG/Sylvania 6550A:
Similar to the GE tube, perhaps a bit better.
Sovtek 6550:
Samples tested do not bias correctly, having insufficient current flow. Not recommended.
