VAC Vintage Mk II User Manual
Page 9
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. We
consider the sound of the KT77 to be superior. 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, and of course the KT77 sounds different as
well. Here is a brief summary of our experiences with currently available tubes from
different sources.
M-O Valve Company/Genalex/GEC KT77 (U.K.):
Accurate and dramatic imaging, great nuance, control of dynamics, sweetness, and
bass quality. Quality is surprisingly variable for a premium tube, and we have had to
reject about 10% of the KT77s we have purchased. Most EL34s are more harsh
sounding by comparison. World wide supply is limited. Highly recommended.
Golden Dragon EL34 (B):
A new version of the EL34 designed in Great Britain and manufactured in China. A
very good tube, much more open, detailed, and dynamic than the East German EL34,
with superb bass performance. Highly recommended.
Gold Aero KT77:
Depending on vintage this can be a relabelled GEC KT77 or an EL34 with the KT77
name rolled onto it. Take care. We recommend staying with tubes that are accurately
labelled.
Russian & German Made KT77:
EL34 variants. Not recommend.
Siemans, EUI, and National EL34 (Germany):
Detailed bass and energetic treble. Good detail. May be too cold or harsh sounding
in some systems. Some recent authors have cast suspicion over this tube due to the
slimness of its design, claiming that it is less durable than versions in a larger bulb.
Do not worry. This style of tube is the original EL34 as made by high quality firms
like Amperex, and occasionally imported by GE. Recommended.