I tackled this build and have been trying to bring the background noise down. V4 & V5 have a persistent 60 hz signal that drops off when capacitance of the grid wire increases. If I touch the coupling cap from the PI to power tubes, the hum drops to an expected level. If another probe touches the grid pin, the noise drops even more.
I’ve double and triple-checked the cathode grounds and applied more flux to the joints to rule out the potential of a less than ideal ground connection. I tried to upload a video here of it, but the file is too large.
Persistent 60 hz hum in 124 build
Moderators: pompeiisneaks, Colossal
Re: Persistent 60 hz hum in 124 build
Upload photos and videos to free sites (eg imgbb YouTube) and provide a link.
My guess is oscillation.
My guess is oscillation.
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joesnuffey
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joesnuffey
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Re: Persistent 60 hz hum in 124 build
Got it straightened out
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sluckey
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Re: Persistent 60 hz hum in 124 build
That's not a fix. It's just a band-aid that is masking the real problem. The presence pot should not be grounded. Look at the attached pic. I suspect there is a missing ground or wrong value/bad 390Ω resistor.
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joesnuffey
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Re: Persistent 60 hz hum in 124 build
The ground. Thanks!