"Thuddy" background noise during pick attack

[jmaf]

I fiddled with it between 50% and 70%, no change. I didn't go outside that range.

It's enough to rule that idea out. T'is a 3 month puzzle for M.E.F, they don't usually last that long.... If I get any ideas I'll bother you again

[wizard333]

Yeah its a stumper. Going to test some of the ideas from this thread this weekend and report back. Whatever it ends up being, if it can be solved, it sure will be interesting, and a good learning experience if anything like this ever pops up again.

[pdf64]

I suggest to rig up a hardwire bypass of all the IC based circuits, in order to eliminate it as being a cause / contributing to the issue.

[Randall]

Have we ruled out the speaker?

[wizard333]

Yes, and the cabinet, and oh btw everything else mentioned thus far.

[wizard333]

So yeah what this ended up being was a layout issue. Haven't seen one as bad as this but there were several revisions to this board and I'm sure some manufacturing variation to. I isolated the issue down on channel 2 to happening between C8 and the point marked "5" where a lead comes off the main board and goes to the control board. All that happens in an area the size of your thumb nail. Thought it might be the relay but I socketed that and switched in a few with no variation. Channel 3 didn't have the issue anywhere near as bad and they share C8 so dropping that value wouldn't really solve anything.

Ended up solving it by unsoldering the flying lead from point 5 on the control board and inserting a .0033uf cap at the control board. That effectively removed all the sub harmonic poop.

Once that was done, the standard mod I do to these includes changing C13 to .0022uf so that eliminated the small amount of sub harmonic poop on channel 3.

After modding, I have to say, the amp sounded better than the previous mod amps. I think the issue is there on all of them, but previously the ones I'd worked on didn't have it so bad that it irritated the hell out of me like this one did so I didn't address it. I'll be adding that .0033uf cap in that spot from now on. Removing that low end noise allowed the amp to breath much better, sing much better, get notes to feedback naturally much more easily than usually happens on these amps.