VAC PA90 - Manual (Page 15)

15
TIPS & ADVICE
A Word About Output Stage Operating Mode
Triode amplification is the oldest form of amplification known, while partial triode ("ultra-linear")
operation of pentodes and beam power tubes dates back to 1937. Both modes of operation are
available with your VAC Power Amplifier.
From the engineering perspective, the triode mode differs from the ultra-linear mode in the
following ways:
1) Somewhat more distortion at low power levels
2) Somewhat less distortion at moderate power levels
3) Reduced maximum power output
4) Output impedance matching is somewhat less critical
5) Increased damping factor.
The ultra-linear mode applies negative voltage feedback from the primary of the output
transformer to the screens of the output tubes. The ultra-linear circuit displays different
characteristics (efficiency, distortion components and levels, etc.) depending upon how much
of the primary winding is common to both the screen and plate of the output tubes. One
commonly sees 20% to 45% winding in common. The VAC implementation of the ultra-linear
circuit has been selected to achieve performance quality as close to pure triode as possible
while still increasing power output and efficiency.
Subjectively, the triode mode often produces a more natural sound provided that sufficient
power is available for your application. Listen to both, and select the mode most pleasing in
your system.