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Hello folks,
I am wondering if anyone has had issues with the bias adjustment on this 1x12 Ampeg combo.
It appears it is like the HiWatts where they were originally designed for tubes that naturally have lower idle bias current, but with the newer tubes no longer produce enough neg bias voltage to properly bias the tubes. IOW, the bias current is too high at the highest bias voltage setting.
The secondary tap is only about -40vac which by the time you rectify it & filter it & such, is about -35V or so. Lifting the divider resistor to ground & or jumping the series resistor in the bias supply section only just barely get the tubes to bias at 35ma. Then the supply is no longer 'stiff' & too unstable to be safe that way.
In lieu of building a doubler circuit in there I got lucky & found that Lord Valve (Fat Willie @ NBS Electronics in town here in Denver) had a set of 6L6 JJ's that had a naturally low idle bias reading that he couldn't get rid of (8ma on his tester). Got them cheap & all was fine. Of course the next time will be an issue.
Just wondering if anyone has built a doubler for this amp and what that looked like. Thanx, glen
I am wondering if anyone has had issues with the bias adjustment on this 1x12 Ampeg combo.
It appears it is like the HiWatts where they were originally designed for tubes that naturally have lower idle bias current, but with the newer tubes no longer produce enough neg bias voltage to properly bias the tubes. IOW, the bias current is too high at the highest bias voltage setting.
The secondary tap is only about -40vac which by the time you rectify it & filter it & such, is about -35V or so. Lifting the divider resistor to ground & or jumping the series resistor in the bias supply section only just barely get the tubes to bias at 35ma. Then the supply is no longer 'stiff' & too unstable to be safe that way.
In lieu of building a doubler circuit in there I got lucky & found that Lord Valve (Fat Willie @ NBS Electronics in town here in Denver) had a set of 6L6 JJ's that had a naturally low idle bias reading that he couldn't get rid of (8ma on his tester). Got them cheap & all was fine. Of course the next time will be an issue.
Just wondering if anyone has built a doubler for this amp and what that looked like. Thanx, glen
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