Help Analyzing Scope Images (Images attached)

[jordan86]

Humbly calling on all the scope readers and wave experts!!!!

I've been chasing down a distortion in my Princeton build and finally got some scope shots. Had a previous thread in which many of you helpful gents thought it may be blocking distortion. At first, putting some .022 coupling caps off the phase inverter *seemed* to help, but it's still there. I have a friend who just built a Hoffman AC30 who was kind enough to come over and scope it with me. He's an electrical engineer and great with the scope, but we are both puzzled by the waveforms.

Wondering if any keen eyes would be willing to share any ideas of what they think this may be. The distortion seems to happen most on the low notes. Open E, G (fret 3), A (fret 5), etc.  It's sort of a scratchy buzz/fizz that almost feels like its under or behind the note.

Here's what the scope waves are:
(1) YELLOW: Speaker output jack
(2) LIGHT BLUE: Post phase inverter, pre 6v6
(3) PINK: Pre Phase Inverter, after grid stopper
(4) Dark Blue: Input jack

NOTE *I BELIEVE* we flipped the polarity on the pink and light blue waveforms in the scope trying to get the waves to match up to the output jack wave. That may make a difference?!?!

If you flip the waves on scope channel 3, it does seem similar to one of Merlin's pictures of frequency doubling from high grid current on a cathodyne PI. I tried a 820K grid stopper last night and it seems to have made little difference. Wondering if this is something I can fix, or if this is just the nature of the little Princeton and Cathodyne PI that I need to live with.

Any ideas if this is a circuit issue that could be fixed? Or maybe operator error, component failure, bad solder joint? The amp sound really great between 3-5 on the volume, but after that is where this starts to happen. Also, attached a schematic with my current mods. PT is also a Hammond 290ax, which supplies 100ma instead of 70ma.

[jordan86]

More images

FWIW...We tried this a few times at 100, 200, 400 and 800hz. I don't recall which of these screen grabs are from.

(1) YELLOW: Speaker output jack
(2) LIGHT BLUE: Post phase inverter, pre 6v6
(3) PINK: Pre Phase Inverter, after grid stopper
(4) Dark Blue: Input jack

[PRR]

The "Dark Blue" is clearly showing supersonic oscillation on the rising(?) wave.

This suggests leakage from output to input.

I suspect if you poke in there with a dry 'lead' pencil you can make it worse (even better). Layout problem. Did you follow Fender's layout like the gig depended on it?

[jordan86]

Layout problem. Did you follow Fender's layout like the gig depended on it?

The guts are a mojotone kit, so the layout is pretty tried and true I think. The only layout changes I made were adding a grid stopper to pin 7 of the phase inverter and moving the upgraded OT from Allen over about a half inch. The larger bell wouldn't clear the eyelet board mounting screw. You can see the original mounting holes for the OT in the photo. This was my first pic upon building. Nothing has changed in the layout. I did solder in the bypass caps after this pic:)

Would the oscillation create these strange waves down the line?

[tubeswell]

Hard to tell from the resolution of that last pic, but some of the joints (especially the 'wrap around' resistors leads) don't look like they're soldered

[sluckey]

I agree

[jordan86]

Hard to tell from the resolution of that last pic, but some of the joints (especially the 'wrap around' resistors leads) don't look like they're soldered

Yeah this was right after the build. All the cathode resistors are now soldered to the bypass cap leads on the preamp stages. In hindsight, I would do it differently...but it was my first build and that's how the mojotone kit instructions said to do it 

[thetragichero]

The "Dark Blue" is clearly showing supersonic oscillation on the rising(?) wave.

This suggests leakage from output to input.

I suspect if you poke in there with a dry 'lead' pencil you can make it worse (even better). Layout problem. Did you follow Fender's layout like the gig depended on it?

not directly related to the op but this explains some readings I've gotten on some builds that had both audible issues and issues on the scope (i figured the "furry" part of the waveform was bout a good thing)

thank you once again for helping me without even knowing it ;)

[jordan86]

This suggests leakage from output to input.

PRR, what does that mean exactly? Leakage from output to input. Is that a NFB thing? That blue probe is on the first grid of the first tube. Right after the grid resistors on the input jack. Does that mean that first grid is having oscillations? Would a shielded cable fix that?

[acheld]

So, even the pink (pre-phase inverter) waveform is heavily clipped.   And then every waveform following is even more distorted.

Is there too much gain in the pre-amp section? 

There are three triodes before the PI -- I'd be interested in seeing what your waveforms look like at each stage -- with volume at "3" (where it sounds good), and the volume set where it sounds baa-ad.

[jordan86]

Yes, my HUNCH was that the cathodyne PI was the culprit which is why I was curious to compare the amp's input and output directly with the pre-and post PI signals. You can see from the scope images on the right hand column that the two waveforms at the PI (pink and light blue) are inverted polarity. Not sure if that matters. But yes, they are heavily clipped which is what I am most curious about. And the double frequency thing on the pink line.

I will check on the input jack 1M grid leak. I have gotten an occasional weird pop and squeal when plugging in. Maybe 5% of the time. I presume that may clear up the dark blue line oscillation, but not sure what that would change down the line.