VAC PA90 User Manual
Page 9
9
SELECTION OF OUTPUT STAGE OPERATING MODE
All VAC amplifier output stages may be operated in partial-triode (ultralinear) or full triode
modes, at your discretion via a single switch. Although the mode may be changed with the
amplifier in operation, we strongly suggest that you turn the OPERATE switch to off before
operating the mode switch. To change the mode, insert a flat blade screwdriver into the switch
opening between the output tubes (be careful not to burn yourself on the output tubes). Turn
the switch to the position indicated on the brass plate. Then power the unit up and listen.
Less power is available in the full triode mode. Experience reveals that the triode mode will
sound somewhat lean on some systems and better defined on others. In many ways this is an
issue of system matching, and universal recommendations do not exist.
Note that the bias current will typically read 2 milliamperes lower in the triode mode than in
the ultralinear mode. Simply follow the BIAS procedure to restore the idle current to 54
milliamperes per tube.
For further information, refer to Tips & Advice: A Word About Output Stage Operating Mode.