VAC PA160 Mk I - Manual (Page 5)

5
Fine Tuning Your System
The VAC PA160 offers unparalleled opportunities for fine tuning your system for best subjective
reproduction. At first glance this may seem a bit intimidating, but it's actually quite easy to do. The possible
adjustments are output stage operating mode, adjustable negative feedback, output impedance matching,
and output tube type.
Before critical tuning, be sure that the amplifier is fully broken in, having operated for at least 150 hours.
Play several different recordings that you are familiar with to get used to the sound of the system.
Start with the factory settings. Listen to a bit more feedback, then a bit less. If your speakers' impedance
falls between two output taps, try them both. Selecting the combination that best matches your system is not
a question of right or wrong in the engineering sense. It is a subjective exercise, much like adding seasonings
to food. Experiment at your leisure, and select the settings which make the overall system sound most like
music.
Output Stage Operating Mode
The PA160 output stage may be operated in pentode/beam power, partial-triode (ultra-linear) or full triode
strapped modes, at your discretion via a single switch. Although we often change the mode with the amplifier
in operation, we strongly suggest that you turn the amplifier off before operating the mode switch. When
changing the mode, be careful not to burn yourself on the output tubes. Turn the switch to the position
indicated, then power the unit up and listen.
The greatest power output is available in the pentode and ultra-linear modes, while power in the triode mode
is least. Damping factor is greatest in the triode and ultra-linear modes. Triode operation offers somewhat
more distortion at low power levels, somewhat less distortion at moderate power levels, and somewhat less
critical load impedance matching. Speaker impedance matching is the most critical in the pentode mode.
Experience reveals that the triode mode will generally have the leanest and tightest sound, with somewhat
more image depth. Ultra-linear typically shows more drive and "jump" with a wider soundstage. Pentode
is richer still, but might lose air and openness on some speaker systems. In many ways this is an issue of
system matching, and universal recommendations do not exist.
To get an idea of the sonic differences, start with the ultra-linear mode with feedback set to "D." Then try
pentode with feedback at "D" or "E." Compare this to triode with feedback at "A."
Any level of feedback may be used with any operating mode.
Note that the bias current must be reset for each mode. Follow the bias procedure to restore the idle current
to proper value.
Note also that the sensitivity of the amplifier is different in the three modes. Adjust the volume controls on
your source component accordingly.