jcm800 clone with noise

[12afael]

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.

here is a sample of the noise.
http://www.soundclick.com/fireland

I also have some doubts about the wiring of the presence pot. in this layout there is DC through the pot , not like in the schematic.
http://www.dreamtone.org/pdf/NEW2204...mDreamTone.pdf
also in this one
http://www.sonicdeli.com/ThunderTwea...fuellayout.htm

I have checked voltages and it seems to be ok, unfortunately my oscilloscope only can measure AC.

someone had any similar problem or have any idea of what more I must check?

cheers
Rafael

[Twist]

Try reversing your plate wires on the output tubes. It's the wires that go to pin 3 of each output tube.

[12afael]

I already did that , I had some power amp oscilations due the positive feedback.

Rafael

[dai h.]

it seems like it should not be a particularly difficult build (since it's not too complicated). What did you do differently from the original? Maybe there might be a clue there.

[12afael]

HI Dai, well it is almost stock , I put two 220uF 250V (with two 220k R in parallel) in serie for the first filter, I have done my own choke, also there is 0.1uF between PI and power tubes, 1ohm between cathode and ground for bias and 5.6k in the grids, I`m using a bridge rectifier and I have 430V at b+.

cheers
Rafael

[12afael]

DONE! it was oscillating. I put a more big cap in c18 and problem solved.

thanks for the help

Rafael

[dai h.]

cool. Is C18 the 47pF in the phase inverter?