Divided by 13 flatulent distortion (like a broken speaker)

[Ortodoxo Tono]

Hi there!
I wonder if some of you guys could help me with this issue. I have a Divided by 13 CJ11 in my workbench right now with a very awful flatulent distortion. It seems you are playing through a broken speaker though the speaker is not the problem. I tried with a new one and it sounded just as awful.

Well, in fact it starts to sound that way wen the signal level reaches certain point. I scoped the signal through the hole preamp section and I saw a clean waveform everywhere except the concertina phase splitter. When the signal reaches that level the waveform cut in a very sharp fashion. I tried this with the output tubes removed from the amplifier so I am sure the issue is related to the concertina circuit being overdriven itself.
I just need to understand what the hell is going on here.

This is the oscillogram of the two outputs of the split load (concertina), with a voltage swing of 31Vp on the positive side and 28Vp on the negative side, the largest unclipped signal level:



And this is what it looks like when the concertina starts to be overdriven:



And this with the PI heavily overdriven:



Sorry for the low trace intensity. The camera doesn't see as well a I do Despite of that you could see these sharp edges on both outputs. The flat area corresponds to the negative voltage swing of the input signal.

Well, the fact is that I scoped while I heard and I can relate that sharp, flatulent and with lots and lots of Intermodulation Distortion sound, with the signal waveform hitting this flat area on the split load circuit.

These are the voltages I founded in the circuit:



Well, this is a good amp that used to work fine so I don't think there is the need of modifying it. There must be something wrong somewhere, but I don't know where! By the way, the amp sound very good until it reaches this point.

Do you think this waveforms are normal in this circuits in this circuit? Can this sharp clip cause the flatulent distortion I hear on the speaker?

This is the CJ11's schematic:



Thank you for your suggestions!

[J M Fahey]

Not sure what you find unusual.
**Every and all** gain stages clip whn reaching power rails or ground, since they can´t go beyond.
That is a *hard* limiting point by definition so if you keep rising signal level, you´ll get flat top and bottom, exactly what your scope signal shows.
There is nothing "broken" there that we see.
In fact, that amp has a PPIMV , which expects the concertina PI to clip so you can add distortion to that amp at lower volume.

I´m curious about how you are now annoyed by this, while you were happy earlier.

I can just think that you always used this amp at low volume (bedroom/garage?) and now find it low powered (loud drummer/Club or larger space use?)
Measure amp power out, you should be around 18/20W or so.

See that plate sits at 206V on a 248V rail so maximum theoretical peak value is 248-206=42V and cathode sits at 41.1V from ground, same thing, so the actual peak voltages you measure, lower than theory ones are in fact quite correct.

That you find it farty is something else, and usually symptom of way too uch bass in the signal: did you switch from single coils to humbuckers?
Are you using active pickups?
Are you using some bass heavy pedal?

As a side note, find it funny that Divided by 13, like hundreds of "Boutique" makers out there, isn´t actually adding anything new to general knowledge, simply rehashing old Fender circuits, adding **minimal** tweaks, many of them almost inaudible ... yet some musicians swear by a certain brand over the others .... which are basically the same under the hood.
Oh well, the power of marketing and repackaging.

[Dave H]

I think it's as Juan said. It's looks to be working normally. The PI is perhaps biased a little on the hot side causing the asymmetrical clipping. The way it's designed with a mid scooped tone stack, large coupling capacitors and no output stage negative feedback is asking for flatulence in overdrive. What's with the 1u coupling capacitors to the 6V6 grids?
Edit: Now I've zoomed in I see they are 0.1u (still too big)

[Ortodoxo Tono]

Thank you for your reply, J M Fahey.
I understand that hitting the rails means getting clipping (cutoff or grid limiting). But I expected a soft clip instead of that sharp, flat clipping. I turned the volume up slowly util and I found hard clipping just from the begging, I mean just when it starts to clip —not so hard pushed. Maybe it's because of my inexperience: I have never scoped an overdriven concertina and I haven't found similarities with the waveforms described by Merlin on his (very very nice) book: no spikes, no doubling effect...

Well, in fact what I am trying to fix is the farting out sound I get when the PI is overdriven. I thought the culprit was the PI itself but it could be just a blocking distortion issue located in the coupling capacitor in the output of the third gain stage. And I assumed that because I heard the "farting out" while I saw that flat clipping on the PI. So you suggest I should look elsewhere? Maybe it has an excess of gain from the preceding stages. I will try to find something in that way.

I have recorded an audio clip of that awful distortion sound

Well, In fact this amp is on a recording studio and it is used pretty loudly without any farting issues. In fact my band recorded in that studio and the other guitarist used it in a song or two. The amp had an issue with a shorted rectifier tube that I fixed. I also upgraded the filter capacitors from the Illinois Capacitors to the Sprague Atoms. I don't think it has something to do with the farting out issue.

I played with a single coil guitar plugged directly into the amp's input: no humbuckers or active pickups and no pedals. This farting sound is present despite of the bass control position is low...

I agree with you J M Fahey. This amp is nothing different from the Tweed Deluxes I have built but those **minimal** tweaks you mention (bass and treble controls, booster switch...). But I assume that this is a good amplifier and that it worked fine so I am not very prone to make modifications on it. I don't think my client will accept it anyway.

Thank you for your input. I appreciate the help!

[Ortodoxo Tono]

Yes, Dave, Juan convinced me that the PI is working normally. And I agree with you that it is biased pretty hot. What my client says is that the amp never did that before so there must be something different now, I mean not related to the design itself. I haven't found that flatulence in any of the Tweed Deluxes I have built despite they use those 0,1uF coupling caps. Maybe because they don't have to handle such a large signal.
I have just posted a link to a sample of the farting sound I get from it. Now I am inclined to say it is blocking distortion on the coupling capacitor in the output of the third stage. Do you think it is the sound you could expect from that circuit?
Thank you for your input, Dave!

[Richard]

After listening to your sound clip I don't believe it's supposed to make that sound.

[Justin Thomas]

That doesn't sound like any kind of "clipping" or even "farting" that I've ever heard. That the one note can be so clear and the other, not too far from the first in pitch, so off, makes me think it's something else. While it may benefit from some circuit tweaks, I think there is an actual "broken" something-or-other in the amp that needs to be fixed before we can start tweaking. It sounds like some kind of oscillation artifact to me...

Just my ears...

Justin

[nickb]

After listening to your sound clip I don't believe it's supposed to make that sound.

Is that intermodulation from 50Hz (maybe 100Hz) hum I can hear in that clip? Do you think the hum is greater than normal Ortodoxo?

[Steve A.]

Have you tried replacing the preamp tubes, specifically with lower gain varieties? The 3.3k cathode resistor for the 1st stage is larger than the traditional Fender and Marshall values. The .022uF cap going into the PI could be replaced with something smaller... just a thought.

Perhaps the guitar has super-loud pickups?

Steve Ahola

[frus]

That the one note can be so clear and the other, not too far from the first in pitch, so off

Correct me if I'm wrong, but he's playing the second note while letting first one ring. They are a second interval apart and beat frequency can clearly be heard. But I can hear the same thing on an acoustic. Since the two strings are louder than one, the amp clips, and as we know from a recent thread, asymmetric clipping accents the sum and difference frequencies, hence it's a bit exaggerated, but more or less something I would expect to hear if I played that particular sequence

How about a clip of some ordinary power chords and soloing in full tilt?

[tbonuss]

Does moving the wiper on the bass and / or treble rheostats change the resonant frequency of the oscillations?

[Ortodoxo Tono]

No, I haven't seen that behavior... But I haven't looked for it specifically. I will try to check it as soon as I get back to my shop in two days.
Thank you, tbonuss!