Cracklin' Tweed

[JimiRayBoneKing]

Ran across an odd problem last night with my Mission 5E3 that I'd like to share with other builders.

When the amp was fully hot and playing lower notes at substantial volume it would crackle as the note decayed. Oh crap, now what?
I noticed that it only seemed to do it when the channels are bridged (of course more bass..well, more everything really).
Popped open the back and looked at the input section to V1 and thought maybe the little ceramic cap across the plate load resistor was messing up (part of Bruce's tone mod). Disconeected it and fired it up. No cracklin'. Great!
So maybe it was a bad joint.
Reflowed the solder and fired it up again. Still no noise! Sweet!!
Replaced the back panel and the Crackling is back!!
Evidently the shielding that I put on the panel was making intermittent contact with the chassis and the vibration was pushing it away and it would snap back. These boxes shake like a son of a gun when at full tilt. (In all honestly I don't know why anyone needs a fan to cool the tubes on a combo when considering the amount of air being moved back there !)

I loosened the chassis and moved it back so that the cover would be in constant contact with the chassis. This cured the problem.
Don't know if anyone has run into this but it is a place to look when a stange noise occurs.

[Leif H]

I'm going to give this a try, all was good with my 5E3 until I had the back panel on.

I hadn't had a chance to play it until after the back panel was put on. all was fine on the weekend after I cleared up a hum issue.

I had glued tinfoil to the back panel, and after reading this, I guess this may be an issue as well... I'll give it a try.

[tubeswell]

I had glued tinfoil to the back panel, and after reading this, I guess this may be an issue as well... I'll give it a try.

Yes that can make (a) ground loop(s) esp if it touches/brushes against more than one part of the chassis.

A better way to perhaps shield the back (if you want to shield it, is to make the foil slightly undersized and run a single copper wire from the chassis to the foil - mount it firmly with a solder tag screwed hard up against the foil.

But I can't guarantee this will work adequately either.

Probably the best thing is to leave if off altogether. I don't bother putting that stuff inside back panels on my amps.

[Leif H]

Yes that can make (a) ground loop(s) esp if it touches/brushes against more than one part of the chassis.

A better way to perhaps shield the back (if you want to shield it, is to make the foil slightly undersized and run a single copper wire from the chassis to the foil - mount it firmly with a solder tag screwed hard up against the foil.

But I can't guarantee this will work adequately either.

Probably the best thing is to leave if off altogether. I don't bother putting that stuff inside back panels on my amps.

Foil off, I didn't notice any real hum... problem solved!