VAC PA90 - Manual (Page 8)

8
AC BALANCE ADJUSTMENT
The AC balance adjustment, located on the top of the PA90 between the low level tubes,
allows the amplifier to be adjusted for maximum symmetry of the push-pull signal. It has been
set at the factory for lowest distortion performance with the supplied tubes.
Ordinarily, you need not change the setting of this control, even when the low signal tubes are
changed, for two reasons. Firstly, most low level tubes will call for approximately the same
setting, such that very little imbalance is to be expected from a tube change. Secondly,
although a change in the AC Balance setting can have an impact upon measured distortion,
it can be very difficult to hear.
Proper adjustment of the AC Balance control requires access to a low distortion signal
generator and a harmonic distortion meter. Connect the signal generator to the amplifier input,
and the output to an appropriate load resistor. (NOTE: Do not connect a grounded test lead to
ANY output tap except "-2 ohms".) Adjust the signal generator to 1000 Hertz. Set the level such
that the amplifier is producing approximately 10 watts. Adjust the AC Balance control to
minimize distortion measured across the load resistor.
The precise setting for minimum measured distortion is slightly different for the triode and
ultra-linear output modes. Given that the AC balance is adjusted for one mode, the measured
full power distortion in the other mode will be approximately .2% THD higher than when that
other mode is optimized. The PA90 is supplied optimized for triode operation.
If for some reason you need to make an adjustment without access to test equipment, the
following rule of thumb will result in a good setting. Turn the AC balance control full counter-
clockwise. Think of the screwdriver slot as if it is pointing to the numbers on a clock's face. For
ultra-linear operation, turn the control less than 1/4 turn, until the screwdriver slot is
approximately sideways (as in 9 and 3 on a clock's face). For triode operation, turn the control
slightly farther to approximately 9:30 and 3:30.