Ok, so I started chasing that last remnants of mains hum. 100 Hz hum in the speaker when all volumes turned down. Also present with preamp tubes removed, I concluded that the problem is not in the preamp. I measured AC ripple with my scopemeter before and after the choke.
Before choke
Before choke.bmp
After choke
After choke.bmp
The ripple measures around 2,4 Vac before the choke, and around 0,2 V after the choke. I think that is good? So I concluded that the B+ line is probably noise-free.
So if the B+ is noise-free and the noise is not from the preamp, I started looking at grounding as a possible cause. I found one grounding I found somewhat suspicious: The speaker jacks (also from the OT secondary) are grounded at the same point as the bias adjustment pot. I thought this might cause grounding hum. The speaker jacks are isolated from the chassis, so that point is the only grounding for the speaker jacks. See the original layout where the jack is grounded together with the bias pot, and the picture where I have removed this grounding:
Speaker jack grounded with bias pot.jpg
Speaker jack grounding unsoldered.jpg
So I temporarily moved that ground to the front of the chassis (not soldered, only mechanical connection):
Speaker jack grounding moved.jpg
This removed that 100 Hz hum!! Look at the spectrogram from my phones recording in front of the speakers before and after moving that ground connection:
Spectrogram new grounding.jpg
And after moving that ground:
Spectrogram original gounding.jpg
Those spectrograms do not convincingly show the real audible difference in hum, but still, there is no noise in this amp now when all volumes are turned down using this new grounding.
So my questions are:
That original wiring of the speaker jack, grounded with the bias pot, is that correct, or is it a design flaw from the manufacturer?? If so, this makes me question the rest of the grounding scheme in this amp, so I'll then have to take a deeper look at the rest.
Could you point me to the correct grounding schemes that are verified noiseless? Where should those speaker jacks be grounded?
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