Marshall Plexi circut noise..minor sizzle on top of notes

[Ampzone]

Hey guys I'm getting a a bit of sizzle on a 100w Plexi circuit, when the notes trail off there is a little sizzle, buzzy distortion that is mild but percievable. It's intermitent as it goes in and out as you let the note ring out and decay. Swapped tubes no change, checked bias, all is well there. Resistors are all carbon film not carbon comp. Voltages look to be to spec. Power supply filtering is all new, cathode caps new, bias caps new.

Speakers check out ok as well.

Thanks for any ideas on where to keep hunting

[Jazz P Bass]

'The speakers check out o/k'
Have you tried a different cabinet?

[Ampzone]

'The speakers check out o/k'
Have you tried a different cabinet?

Hey Jazz thanks for chiming in Ok so speakers good, tried another head of the same style and it behaves noramlly. Could this be blocking distortion I wonder? I need to try the amp at lower volumes and see if it still fizzes on the decay. The two grid stoppers are 1.5k and are spaced between the output sockets. I may need to raise them to 5.6k and put one on each control grid (4 total). Just brainstorming.

[Bruce / Mission Amps]

Sounds more like it could be a little oscillation caused by the presence control and NFB circuit.
Usually those are wire dress issues.
I know it's hard to track down without an O' scope but first thing I would do is remove the negative feedback wire from the output jack and see if it goes away or changes radically. Yeah the tone is going to change but the oscillations will be better if that is the culprit.

[Ampzone]

Sounds more like it could be a little oscillation caused by the presence control and NFB circuit.
Usually those are wire dress issues.
I know it's hard to track down without an O' scope but first thing I would do is remove the negative feedback wire from the output jack and see if it goes away or changes radically. Yeah the tone is going to change but the oscillations will be better if that is the culprit.

Bruce - thats an excellent suggestion. I will try this directly. Fingers crossed.

[Ampzone]

Sounds more like it could be a little oscillation caused by the presence control and NFB circuit.
Usually those are wire dress issues.
I know it's hard to track down without an O' scope but first thing I would do is remove the negative feedback wire from the output jack and see if it goes away or changes radically. Yeah the tone is going to change but the oscillations will be better if that is the culprit.

Well I tried it (NFB removal) with no change, a little duller sound overall but it still wants to sizzle on the notes decay a bit on the mid to lower notes.. I upped the grid stoppers to 8.2 from 1.5k with no difference. Cord, pre-amp tube swaps do nothing. What about blocking distortion? There is some talk about the "Ruby Mod" to remedy this issue by adding Zener diodes on the 220k bias resistors. The description of the mosquito sizzle sounds like what I have but I am not 100% sure. Lead dress looks clean, no cold solder joints to be found so far. No leaky DC on the caps voltages look right to spec. Hmmm

[Ampzone]

Still searching for this noise, I could replace the output tubes with another set but not sure how this could be the cause. Very puzzling.

[Ampzone]

Solved A part was loose on a lug. Thanks for everyones help on this.

[Bruce / Mission Amps]

Solved A part was loose on a lug. Thanks for everyones help on this.

DOH!!!