Interesting. It would be revealing to see a second trace with the filament voltage, to be able to spot the relative positions of the peaks and valleys of the noise and the AC mains waveform. It seems like the noise magnitude envelopes are similar to full wave rectified sine(ish), but inverted, with the round ends nearest the zero volts line. This is kind of, maybe, consistent with the noise from a switching-type power factor corrector - I think. Just guessing, could be confirmation bias (
) but it might be. If you have a single biggish transformer supplying AC mains voltage to several units, somebody on the same feed line could do this.
I quite curious to know the cause myself, will be investigating further once the grounding of the outlets is fixed.
Now i see you are too young and green.. it is ashame that you dont know this
: 220uF is very easy to make, from 470uF + 470uF regular electrolytic, + poles jointed together and you have 235uF bipolar...
No I'm actually aware of that trick. Its just not one I necessarily trust to be reliable. However, you reminded me that there are in fact non-polar electrolytics. I just forgot about them cause their use doesn't come up much in the work I normally do. But its important to be mindful of the ripple current. There's a 22uF from Mouser with a ripple current of 90mA. At 3.15VAC that gives a ripple current of 26mA so that would put the 10-20mH choke back on the table. They also aren't particularly long life and I don't know how they fail so I can't say I recommend anyone do this but I think I'll experiment with it.
40k sinusoide is not following the line AC signal and noise, it is clear sinusoide showing that you have oscillations from doing not correct wiring layout and lead dress, so you are on the wrong trace.
Sorry it absolutely isn't. I'm just gonna straight up say it.
I spent idk how long screwing around thinking it was before realizing it was noise coming in through the filaments.
To me thread looks like uber-complicating on just basic things, i allready help a lot in previous topic - 14VAC 0.3A small transformer, form CRC filter and then to 7812. Cannot be cleaner heater supply. For cheap. And i highlihgted that is important only V1 heater.
The thread title has impractical in it for a reason. I never meant this to be a solution that people should necessarily try. I started this thread, and I should have made this more explicit in the beginning so apologies for that, to highlight a root cause of heater induced noise that probably a lot of people who build guitar amps casually might not be aware of. Other methods of solving the problem don't make the elimination of the CM noise explicitly obvious. So I just thought this would be a nice way to demonstrate that. Again I should've been more clear from the beginning.
Okay, I went back to your first post and found where you mentioned a bridge rectifier. I had misread that as "the bridge rectifier in this amp being tested" and not "some other bridge rectifier in a different piece of equipment".
So on one hand it might be nice to over-engineer the amp to the point where it can handle whatever grody power happens to be coming out of the wall at whatever dive bar a person can get a gig at. That's a goal. Have you tested this amp on clean power? If it's also noisy with clean power going into it then you should address that first, imo, and then see what additional steps are needed for a bad power source.
On the other hand, are you located near any large industrial/commercial sites where they might be causing power line issues for you, or do you own any other pieces of equipment that you can unplug and see if the line voltage issues are even more local than that?
Its becoming apparent clarity is not my strong suite.
I should probably try it at home where the power is much nicer, I've just haven't felt like lugging it back, maybe I will do that soon.
Thats the odd thing, its a residential neighborhood so I wouldn't expect this sorta thing here. As stated above investigation of the source of the line noise to follow after the outlet grounds are fixed.
I can see in your pics something that does look like it's probably amplitude modulation, but according to your pictures this AM is happening in the audio signal path and not in the power supply or on the line voltage as you originally indicated. So we're making progress helping you correctly identify the actual issue that needs to be solved.
But with limited understanding (limited by having never seen the schematic or the layout) of what your circuit actually is, it will be difficult to backseat troubleshoot why the circuit you intended to be a DC power supply is behaving more like a ~40kHz oscillator (or whatever the problem ends up being).
Also, can you post a picture of the line voltage with this dubious amp unplugged?
What does your "B+" look like?
More accurate details are likely to afford more accurate troubleshooting.
I appreciate the help but trust me the root cause has been identified and the problem is basically solved. I'm just screwing around with impractical solutions because I find it interesting and documenting it in case someone might find it helpful. If you look at my last post you can see that the noise on the output of the second stage is around 600mV peak. After the caps are installed to ground on each of the filament lines the noise is reduced to roughly 100mV peak and is basically down to the noise floor of those two stages. You certainly can't hear anything other than tube hiss in the speaker with everything dimed.
I will indulge your curiosity however and say that the B+ is clean as that was one of the first things I checked. Since you and RG both requested more images I will try to get some tonight after work.
