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 PA100/100. 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.
VAC Tested KT88-T:
The best of the versions from China, and our current first choice. Good combination of punch, air,
sweetness, and detail. Reliability is very good.
Golden Dragon KT88-M:
No longer available from production, but an excellent tube. A bit more matter-of-fact than the KT88-T,
having more of an NOS US tone, but still preserving the airiness of the KT88.
Golden Dragon KT88, KT88 Classic, KT88 Super:
Generally fairly good for sound; reliability somewhat inconsistent.
National NL-KT88-USA:
A version produced by the US arm of the Richardson's organization. Visually very similar to
the original and sonically satisfying, slightly lightweight sound, but sweet. Very expensive.
Svetlana KT88:
Several versions have been made, so it is difficult to give a blanket comment. The newest version (as
of summer 2000) shows promise, with a sound somewhat similar to the GE 6550A.
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):
Somewhat soft and lean, with a slightly buzzy character, but fairly sweet.
Phillips/ECG/Sylvania 6550A:
Similar to the GE tube; later production samples may have a somewhat hard upper midrange.
Sovtek 6550:
Samples tested do not bias correctly, having insufficient current flow. Not recommended.
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