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  • #46
    It occurs to me that (again) there is an error in the reporting. Because you say that at this time the volume control doesn't affect the noise. You also report that grounding the grid of V2A silences the noise. But the volume control (ideally) grounds the grid of V2A. I think you need to slow down and take more time and consideration with the processes.
    "Take two placebos, works twice as well." Enzo

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "If you're not interested in opinions and the experience of others, why even start a thread?
    You can't just expect consent." Helmholtz

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    • #47
      I totally get that! And you’re right, it should work theoretically. I’ve had a lot of success in the past with the grounding scheme that Doug suggests for his amps. I built a 6G6 Bassman a few months ago that couldn’t have been quieter, but I was also working out of a Bassman chassis, and copied the grounding scheme exactly. Overall I’d say it was a much easier amp to build.

      The reason I’m suggesting it’s power supply ground is because I managed to follow this hum from the 100k Ra for the first gain stage, back to the positive end of the filter cap, and found what I *think* is this hum, in a much quieter form on the PS ground. It seems like by design this wouldn’t be extremely quiet, but I’m wondering if it’s not quiet enough from a design standpoint. Doug suggests grounding the PS with the mains ground. is it worth separating them and seeing if there’s a result? A lot of amp designs suggest not doing this.

      Just trying to make sense of this: is it possible that the point of ground for the power supply entirely is noisy enough that it’s being amplified only by the most sensitive stages? V1 and V2? I did get a different result pulling tubes today because of this: I’ve been searching and testing for where this hum is originating with the input jack switched to ground. So i unswitched the input jack, and there is undeniably a 60hz hum with a guitar plugged in. After pulling V1 that way, some of the hum was eliminated but not all of it. Same result when I grounded the input grid. Pulling V2 eliminated the rest, just like grounding the grid of V2 eliminated the entirety of the hum. I would imagine that poor grounding in the PS would cause more hum issues than this, but maybe the first few stages are the only ones sensitive enough to amplify it. Does that line up? The reason I’ve been looking to V2 specifically is with the input jack grounded, the CF was still amplifying this hum but it didn’t make a significant impact since its originating from the same source and it’s the last stage sensitive enough to amplify it, and just didn’t come through a grounded jack. I’m sorry I didn’t try this earlier!

      This is kind of where I’m coming from when suggesting that if another ground point for the PS doesn’t work, it’s possible that grounding the first 2 filters to the preamp bus, which is extremely quiet right now could potentially have a good result. Maybe I’m off the mark here though

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      • #48
        Originally posted by Chuck H View Post
        It occurs to me that (again) there is an error in the reporting. Because you say that at this time the volume control doesn't affect the noise. You also report that grounding the grid of V2A silences the noise. But the volume control (ideally) grounds the grid of V2A. I think you need to slow down and take more time and consideration with the processes.
        I may have said something unintentionally to suggest that the hum bleeds through the volume control. It doesn't as far as I can hear! I didn't see the hum at V1 initially, but I didn't have my scope set properly to see the low frequency. Still kind of getting the hang of scoping specific frequencies.

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