Ampeg V4 clone parasitic oscillation help

[robotgraves]

hey, first post and it is of course for the all important advice.  I have an Ampeg V4 clone that I have been working on that is throwing me for a loop.  I love my V4 but it has seen way better days, so a clone of sorts was in order.  I already finished one clone, that I was mostly happy with but did have parasitic oscillation at very high settings.  Redid the entire layout and redesigned the power supply to be better.  I now have a motorboating issue that is also only happening at hotter settings, but much more readily than previous designs.

What have I tried?

• Redressed all the grid leads.  They now come from under the board, and avoid being parallel with the plate lines
• Redressed the Master volume.  It now is shielded, and it's grid resistor is direct off the lug
• Redressed the NFB.  It is now direct from the output 8 ohm lead, and the spare turrets from the MV resistor were reused to hold the filter parts
• Redressed the power tube grid resitors.  They are now direct off the lugs, and direct paths to their sends from the PI
• Pulled the pi filter power modification.  It was a 50 uF / 7 H / 50 uF filter, but the motorboating was the exact same frequency that this filter can resonate at.  it is now a 470 ohm resistor for a standard RC power filtering (also this had no results)
• pulled the flyback diodes
• redressed a plate to grid stage that, in layout, reached over way too far
• using mspaint, did the highlighter from my schematics to layout drawing, and layout to chassis.  Did manage to find a mistake, but have yet to find the oscillation

So, what data do I have on the oscillation?  I have access to an oscilliscope and other tools, so here is the known information I have

• In order to oscillate, I need some gain, bass, and master volume; all in healthy ammounts.
• the oscillation is about 8ish hZ
• The mid selector can also pitch the oscillation, but there is an 8-ish hZ oscillation / motorboating underlying whatever the mid makes it sound like
• The oscillation, when it does occur, is visible on the power supply, which has made isolating it very difficult.  Plate and cathode of stage 1 oscillate all the way down and back through the amp.
• The oscillation is also visible in the power tubes as blue plating; and in the indicator light, so it pulls down the heater circuit too (which could stem from the heater DC elevation circuit that also pulls off the main power)
• pulling all the power tubes DOES stop the oscillation.  I can monitor the audio from a preamp out, and no issues no oscillation.  that is testing from input all the way through PI
• Using power amp in, which disconnects the preamp before the last gain stage and PI, also DOES stop the oscillation (but the test signal was not nearly as hot as the dimed preamp)
• Bypassing 4 stages, and going directly from the bax tone stack into the master volume ALMOST entirely stops the oscillation.  the oscillation that is audible is everything dimed, it will hang with an almost "wind" type sound for a couple of seconds.

I did get the help of Dusky and Science amps, both of which had several things that helped get some of this data.  Science's best guess right now is that my layout is the primary cause.  I can't disagree with this, I have little to no experience making layouts and everything on this build is from scratch except for the schematics.

I have schematics, layout diagram, and photos to boot; and will take any and all requests for data or modifications seriously and immediately.  I'm on day 5 of fighting this, and the well of ideas is dry.

Schematic: https://drive.google.com/file/d/1s_Wj6aompyNGcySbpNXONfvsKQTpvURD/view?usp=sharing
Practical Build guide: https://drive.google.com/file/d/1Tou3TPn6pbcZW4HptYwwMtN1M8cOB5RT/view?usp=sharing

[sluckey]

Science's best guess right now is that my layout is the primary cause.

Not only the layout, but plenty of long wire runs that can cause the symptoms you describe. Also, grounding on those later Ampegs is quite picky. We just recently saw an SVT with slightly deviated filter cap grounding that was causing problems.

[robotgraves]

Science's best guess right now is that my layout is the primary cause.

Not only the layout, but plenty of long wire runs that can cause the symptoms you describe. Also, grounding on those later Ampegs is quite picky. We just recently saw an SVT with slightly deviated filter cap grounding that was causing problems.

Science mentioned ground in terms of that i shared the IEC ground with another connection, which is a no-no.  I can move that, it's the back side of the Bias transformer tap

[AL]

Is the second F&T filter cap from the right (in between the blue Ohmite) soldered? I can't tell from the picture but it looks like you may have missed that connection.

AL

[robotgraves]

Is the second F&T filter cap from the right (in between the blue Ohmite) soldered? I can't tell from the picture but it looks like you may have missed that connection.

AL

yes it is. I am checking all connections, and did a reflow on their ground connections on back for good measure

[robotgraves]

As a follow up on this:

• I've redone the output jack ground such that the OT secondary ground goes to the jack instead of direct to ground -> no effect on oscillation
• Redid the rectifier ground to go to the backside of the reservoir caps instead of direct to ground -> no effect
• Redid Heater CT DC lift ground to go to the backside of the reservoir caps as well -> no effect
• Rebuilt the stage one cathode circuit to be more optimal layout -> RESULTS.  still oscillates but needs more than before for sure.

Considering how to approach other parts of the circuit, I don't know what is or is not bad layout.  May move the Baxandall tone stack onto the pots to clear some space on the board for rearranging.  Also may rerun more ground wires based on some in depth articles that I was shown: https://www.valvewizard.co.uk/Grounding.pdf

[cfortner]

Which PI does the amp have? LTPI?

Try a gridstopper 470k-1M direct at the junction to the grid of the first PI-tube. If it gets too muddy, lower the gridstopper until it sounds ok and the oscillation is gone.