Hi I have built a JCM800 clone with an original OT, four EL34, 100W and I have an issue with the presence control, if I put the control on a low value it make some loud noises. It`s worse if I turn up the master volume control. it don`t happen if I cut the Feedback wire. so I suspect is a weird problem of the power amp and the NFB, maybe too much NFB.
here is a sample of the noise.
www.soundclick.com/fireland
I`m using this layout
http://www.dreamtone.org/pdf/NEW2204LayoutSchemBomDreamTone.pdf
at this point it`s clear it is oscillanting, I reduced the high frequency responce of the PI and it work a little better. I also reduced the feedback resistor to 50k but it didn`t help. I`m using 0.1uf caps between the pi and the power tubes could this noise be blocking distorcion?
someone had any similar problem or have any idea of what more I must check? I just have an old rca 75KHz one channel oscilloscope that just can read AC so I can`t see all what is happend in this beast.
I try to keep guitar items elsewhere but this case is killing me.
cheers
Rafael
here is a sample of the noise.
www.soundclick.com/fireland
I`m using this layout
http://www.dreamtone.org/pdf/NEW2204LayoutSchemBomDreamTone.pdf
at this point it`s clear it is oscillanting, I reduced the high frequency responce of the PI and it work a little better. I also reduced the feedback resistor to 50k but it didn`t help. I`m using 0.1uf caps between the pi and the power tubes could this noise be blocking distorcion?
someone had any similar problem or have any idea of what more I must check? I just have an old rca 75KHz one channel oscilloscope that just can read AC so I can`t see all what is happend in this beast.
I try to keep guitar items elsewhere but this case is killing me.
cheers
Rafael