The push-pull output stage of this Ampeg® SVT-VR
bass amp wouldn't balance at any setting of the Balance trimpot.
The two Bias trimmers could be adjusted to light the green LEDs but when the amp was driven to an output of 25VAC at 40Hz the Balance
trimmer wouldn't light the yellow LED.
In addition, the amp sounded weak when turned up. And sometimes it would
go into Standby mode by itself, or else not come out of Standby.
Since all the power tubes tested good, we decided to remove the power
amp chassis and look for other problems in the output stage.
We ended up replacing four open resistors on the SVT-VR Tube Board.