VAC Vintage Mk II - Manual (Page 13)

TIPS & ADVICE
A Word About Output Stage Operating Mode (triode internal wiring option)
Triode amplification is the oldest form known, while partial triode ("ultra-linear") operation
of pentodes and beam power tubes dates to 1937. 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.
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.
VAC can provide directions allowing a skilled technician to convert the output stage to
triode operation.