Marshall JCM800 clone - hum and oscillation

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Hi, i build the clone of Jcm800, Output transformer: Hammond 1750N.
If i set all pots values to max, its humming and whistling. I used ceriatone layout. I dont know whats wrong. In attachment is photo of amp. Thanks for your help.
 

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I'd start with those wire-clippers in the picture. Snip all your neat wire ties. Space wires, especially tone-pot wires, 1/4" 3mm apart, and try it.

I had a Champ with reasonably neat tone-wires but not exactly Fender's wire routing. It squealed at full-up. I put a pencil in the wires. By pushing one area I could make the squeal worse, so I knew the hunch was good. With more pushing around I got the squeal to stop.

Another squeal-maker is the field from the output transformer plate wires. This is a 300 Volt signal! It sneaks-back to sub-Volt stages and amplifies-around again... squeal. I actually had my plate leads run on the other side of the chassis (not a safe idea) with only a wee bit coming right to the socket lug. I can't see your plate leads but if they are bunched-up under small-tube stages, move them.

Good luck!
 
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