marshall 1978 jmp super lead 100w squeal.

[googy007]

hi guys I need some help with my 1978 marshall jmp 100w super lead, when im using inputs 1 bright or normal the amp makes a high pitch squeal noise if volume 1 is set at 4 or higher the sound changes to a type of oscillating noise when you turn the treble, mid or presence up the bass adjustment has no effect, the tubes are new jj el34s and have been biased correctly the preamp tubes are also new tungsols and mallard I have tried different preamp and power tubes but the squeal is still present. any help would be much appreciated as this amp is my fave !!!

[neikeel]

Suggest this gets moved to Marshall or Repairs section.

Presume it is a recent problem? Before or after the retube?

Anyone else been fiddling around under the hood before this happened?

Most likely issue is failed filter cap (preamp can next to choke).

[HTH]

I would try a 100pF cap between the plate and cathode of the first gain stage before anything else - these amps are notorious for squealing without it. In my first 2203, the squealing would come and go. This fixed it and is a 'mod' Marshall put in as standard in later years (JCM800 era).

[neikeel]

I would try a 100pF cap between the plate and cathode of the first gain stage before anything else - these amps are notorious for squealing without it. In my first 2203, the squealing would come and go. This fixed it and is a 'mod' Marshall put in as standard in later years (JCM800 era).

Question is why now?
Pretty standard cap (or over that stage plate resistor)on a tweaked 2203/4 amp but the question is why it has cropped up now?

[stef]

That is IMO normal for a stock SL, especially metal panes specs with low NFB, VOlume I 5nf bypass cap etc....badly potted pickups or true SC pickups.. add a boost (OD, EQ etc) pedal in front of it and it gets even worse.
When I stand in front of and close to my amp, facing the controls and speakers it always squeals
Try moving away from the amp and speakers and always keep your left hand across the strings thus muting it or turn off the volume control on the guitar when not playing
It takes some time and effort to learn how to control these beasts, good gear comes in handy too (guitar wiring and setup, pickups, leads, tubes etc)

I'd check the filter cans per Neil's suggestion, because he's always right

[neikeel]

I'd check the filter cans per Neil's suggestion, because he's always right

Sadly not , but I have had this happen to me and I did all the usual tricks (as per HTH) until I found the duff cap