Princeton build - fuzzy decay at low volume

[BluegrassDan]

Hey guys. I just built a Mojotone Princeton Reverb and it has this very subtle fuzzy distortion at low volumes when the notes are decaying. It's strange because I used metal film resistors and all of the voltages in the whole amp are within less than 10% of the schematic. (Or maybe it's "okay" and I'm just being overly picky.)

Again, this is only at VERY low volume. Oscilliscope indicates a very slight "wobble" or ripple that is modulating the signal and causing distortion. Could this be just general power supply noise causing the decaying notes to sound fuzzy? As the amp is turned up it sounds beautiful.

The only mods to the stock design are:

Added grid stoppers and screen resistors to the 6V6s
Added switchable negative feeback using the ground switch. Cable is shielded.
470k resistor on the PI grid.

Here's what I've tried so far and nothing has fixed it:

I have pulled the reverb driver tube.
Adjusted the output tube bias hotter and cooler.
Swapped all tubes.
Tried the Paul C mod.
Swapped speakers.
Plugged in the amp in a different building.
Swapped out .1uF for .02uF coupling caps before the 6V6s

Any thoughts?

[BluegrassDan]

Here is a video of the scope with a low level 80Hz signal applied. Notice the bouncing around?

[sluckey]

Trigger the scope on the signal directly from the sig gen.

[aabbs20]

Great to have another bluegrasser on board!

I'd guess a bad solder joint somewhere but it could be a number of things. Do you have pics or audio you could share?  A voltage chart would also be very helpful. I know you've measured but getting another set of eye on them never hurts.

[DummyLoad]

output tubes may be biased too cold. easy test if you built adjustable bias PS. 


--pete