VAC Phi 300.1a - Manual (Page 15)

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Tips: Output Tubes
Your VAC Amplifier uses the KT88 kinkless tetrode. It is strongly recommended that replacement
tubes be purchased only from VAC. If, however, you want to customize the sound to your tastes,
be aware that as with interconnects and speaker cables, each tube manufacturer's KT88 tends to
have a distinct sound, as well as its own reliability profile.
Tips: Low Level Tubes
The Voltage Amplifier/Phase Splitter and driver tubes in the Phi is the 6SN7 medium mu octal twin
triode. Your amplifier is fitted with the current production VAC Tested 6SN7 from China, which we
find superior to the NOS types we have tried. There are dozens of versions of this tube available
in new old stock (NOS) from a variety of sources. It would be impossible to characterize them all.
Tubes V1 and V2 should be transconductance matched, and tubes V3 and V4 should be
transconductance matched; this minimizes distortion.
We strongly recommend obtaining tubes as sets from VAC.