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.
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.
My oscilloscope decided to die, so I can't track it down. Does anyone have any ideas as to what it is and what might cause it? Or is this normal and I'm just obsessing?
I'll add that the amp sounds very nice the louder it gets. It's just the very tail end of decays at low volume that bug me. Gets crunchy.
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.
My oscilloscope decided to die, so I can't track it down. Does anyone have any ideas as to what it is and what might cause it? Or is this normal and I'm just obsessing?
I'll add that the amp sounds very nice the louder it gets. It's just the very tail end of decays at low volume that bug me. Gets crunchy.
The symptom makes me think crossover distortion. First thing I would do after answering PRR's questions would be to measure part values and voltages around the output tubes to get a picture of what's happening there.
Also, you mentioned you made the negative feedback switchable — does switching it on reduce the problem? That would offer clues about which stage it comes from.
Also, you mentioned you made the negative feedback switchable — does switching it on reduce the problem? That would offer clues about which stage it comes from.
Here is a video of scoping the output. A low level 80Hz signal is applied.
Princeton output scope - YouTube
Princeton output scope - YouTube
Here is a video/audio of the distortion in progress: Princeton Distortion Samples - YouTube
What does this sound like to you?
What does this sound like to you?
This has a resonant sound that seems to get louder on some notes than others, which makes me think something is physically rattling. My first guess would be a microphonic tube — I've had them sound exactly like this — but you say you've swapped them out. Still, get a chopstick and lightly tap each of the tubes while the amp is on to see if one of them makes noise. You might also try tapping the components in the circuit with the chopstick (one hand behind your back, carefully) while you're at it, though I'd expect bad connections probably would sound different. If none of them are to blame, I'd start to wonder whether the sound is coming from somewhere other than the speaker, maybe metal parts somewhere that aren't quite secure. One way to check that would be to try using a speaker mounted in a separate cabinet, if you have one handy.
Nice playing, by the way!
Nice playing, by the way!
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Hello, Bumping this old post. Did you ever resolve this?
I have something similar (actually, exactly as you describe) happening with a Princeton Reverb and it's driving me nuts.
Here's an audio example -
Princeton Distortion
And some 'clean' audio, for reference. Exactly the same guitar, lead, speaker, cabinet, etc. The only thing that is different is the chassis/circuit -
Clean reference
Thanks!
I have something similar (actually, exactly as you describe) happening with a Princeton Reverb and it's driving me nuts.
Here's an audio example -
Princeton Distortion
And some 'clean' audio, for reference. Exactly the same guitar, lead, speaker, cabinet, etc. The only thing that is different is the chassis/circuit -
Clean reference
Thanks!