ab763 bass problem

[stokes]

Try reversing the blue and brown leads.When these arent connected to the correct tube,you get oscillations from the feedback network.

[lky]

I haved just reverse brown/blue but when I switch on standby I hear from the speakers a luod sound how when you have jack in connected to input ad you touch the other side of positive jack.
Is this OK?

[stokes]

This is not right.Put them back and check the feedback circuit.I would disconnect the feedback circuit where the 820ohm connects to the junction of the 22k and the .1 cap are and try the amp that way.Also check the schem to be sure you have the feedback connected right to begin with.I think you have an oscillation somewhere in this circuit.

[lky]

I haved disconnect feedback circuits but the oscillation remain.
I haved ceck feedback circuits and it's all OK.
I don't understand where is the problem.
I replace OT with 50W OT of another tube amp.
Thanks

[stokes]

Does it have this problem with the Vibrato or reverb engaged?Is it the same if they are on or off?It sure sems like you are getting a low frequency oscillation somewhere.Its tough to pinpoint without having the amp in front of me,I would suggest the obvious and go over every connection starting at the input and work your way through to the output looking for bad solder joints.Check that the bypass caps are oriented correctly on the preamp tubes.You could also try removing each bypass cap one at a time and trying the amp each time to see if there is any change,this would help to isolate which stage might be causing the problem,removing the bypass cap will reduce the gain in that stage,maybe enough to stop the oscillation.Check all the voltages in the amp.They wont necessarily match the schem,but if something is way out it will give you a clue.Are you using a power tranny with the proper ratings?I still have a feeling this is related to that feedback circuit,check that the PI is wired right,the 82k and 100k plate resistors are on the right side,and you dont have that feesback connected to the triode with the 82k load resistor.