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Thread: Ac30 build Hummmmm

  1. #36
    Supporting Member The Dude's Avatar
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    Quote Originally Posted by Chuck H View Post
    ......I'll wager two beer PNW that's power supply noise. Even though your scope seems to show about 51kHz!?! Maybe something about the settings is misunderstood?......
    The 51kHz could be the hiss or harmonic/byproduct of the hiss, which is not what is being chased. You'd need to turn the timebase down on the scope to see 60-120Hz.

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    Quote Originally Posted by The Dude View Post
    The 51kHz could be the hiss or harmonic/byproduct of the hiss, which is not what is being chased. You'd need to turn the timebase down on the scope to see 60-120Hz.
    byproduct of the hiss amplified by the phase inverter.. is it possible even with the CF not driving the tone stack?

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  3. #38
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    I will report back about grounding the grid, Chuck. I left the shop for the night. but a few things layout wise that occurred to me be contributing to this relating to the PS:

    I noticed there was 4.2vAC coming through the rectifier tube (using a 5U4) and it can be seen on the first filter cap. The rest appear to be clean DC with 0vAC on them. Seems a bit high, right? Maybe thereís some kind of failure going on in the rectifier.

    Another thing is after reading up a lot on Merlinís grounding chapter, it seems like a common grounding scheme is to ground the preamp filter caps to the preamp ground bus. Right now all of my
    filter caps are grounded to the main ground point. Theyíre also in close proximity to each other. The Hoffman layout recommended ground all of the filter caps together, but is it worth trying to reroute the ground of the filters for the input stage, CF and PI?

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  4. #39
    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Steelwitch View Post
    byproduct of the hiss amplified by the phase inverter.. is it possible even with the CF not driving the tone stack?
    A little bit, yes. Wherever there is amplification there is Johnson noise. Thermal noise. Series resistance is the most responsible culprit. And there is always series resistance. Even when there are no resistor components in the circuit.

    You did imply that the noise level of the amp was not objectionable with V2 pulled. But you posted the shot of a very low level high frequency read detected in the noise floor at full volume... Offering this observance for your consideration

    But as I inferred, perfection is not possible as long as amplifiers have built in resistances that are unavoidable from a physics and design standpoint. And there's not enough advantage to chasing it for the effort to be worth while. Trading hours for a couple or a few percent improvement in most cases. But you don't get anything if you don't ask. So you should always ask. If there had been a way to shave off the hiss noise you would know about it now. But as it happens, there's not a lot to be done. Keeping series resistances in the signal chain to a practical minimum (not always as easy as you might think) is the best you can do.

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    Quote Originally Posted by Chuck H View Post
    You need to test the ground you used. Grounding that grid should only shunt signal to ground. And just for the hell of it you should measure for DC voltage on that grid. There shouldn't be any.
    Yes itís super quiet with the grid grounded! No DC voltage on it either

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  6. #41
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    Any recommendations on the next move?

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  7. #42
    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Chuck H View Post
    Now what happens when you ground the input grid of the triode feeding the cathode follower?
    Your initial response to this was that grounding the grid as suggested resulted in a burst of oscillation squeal. It seems you were able to edit that post without the reference to it's having been edited. Which seems like some sort of forum super power to me. It's possible that you edited it within a time and lack of viewership that the edit didn't register I suppose. That happens for me sometimes. But the change does put following posts out of context. Because to me you have reported that grounding the suggested grid both induced oscillation AND is "super quiet". So I am inclined to question the care in the tests. That said...

    If, with V2 in and it's input grid grounded the amp is quiet. And if the volume control no longer has an affect on the buzz/hum level then the noise is being introduced between the volume control and the grid of the following stage (according to the schematic and assuming V2A is the triode feeding the cathode follower!?!). This seems unlikely since there is nothing between the two but a wire.

    Because of the lack of consistency in the circumstances, testing and changes to the thread record, I'm sorry, but I don't have a bead on this or what to suggest anymore. If you pull the tubes it might make a good wheel chock (for parking on hills). Or maybe even a big game fishing weight?

    No offence intended. But I don't take well to being hung out in the wind by post editing when I was only responding to your mistakes.

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  8. #43
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    Quote Originally Posted by Chuck H View Post
    Your initial response to this was that grounding the grid as suggested resulted in a burst of oscillation squeal. It seems you were able to edit that post without the reference to it's having been edited. Which seems like some sort of forum super power to me. It's possible that you edited it within a time and lack of viewership that the edit didn't register I suppose. That happens for me sometimes. But the change does put following posts out of context. Because to me you have reported that grounding the suggested grid both induced oscillation AND is "super quiet". So I am inclined to question the care in the tests. That said...

    If, with V2 in and it's input grid grounded the amp is quiet. And if the volume control no longer has an affect on the buzz/hum level then the noise is being introduced between the volume control and the grid of the following stage (according to the schematic and assuming V2A is the triode feeding the cathode follower!?!). This seems unlikely since there is nothing between the two but a wire.

    Because of the lack of consistency in the circumstances, testing and changes to the thread record, I'm sorry, but I don't have a bead on this or what to suggest anymore. If you pull the tubes it might make a good wheel chock (for parking on hills). Or maybe even a big game fishing weight?

    No offence intended. But I don't take well to being hung out in the wind by post editing when I was only responding to your mistakes.
    Ah Iím really sorry chuck! I realized a few minutes after I posted that I was taking a DC voltage reading and not what you had initially suggested.. I changed the comment a minute or two after I posted, I didnít think anybody saw. But youíre totally right, I should have either made a separate edit and had the continuity of the thread in mind for later readers, and for you and others helping me out here. I appreciate and rely on the knowledge yíall have here from time to time. Maybe itís because it was a direct reply to your comment, who knows. Anyway, my apologies and just wanted to express my gratitude for suggesting your process for tracking this hum down.

    At this point Iím going to reflow the solder joints on V2 and see it maybe itís just a bad connection or possibly a bad volume pot. Maybe itís the connection between the plate of V2a and grid of V2b, or a bad connection coming from the 100k plate resistor since I didnít see any hum on pin 7. I will report back.

    Iím sorry that some of my results werenít clear! I should be more careful before posting my results here for the sake of everyone helping me out and people looking for answers in the future. After that Iím kind of at a loss to know what might be happening as well, so maybe I will rethink this stage a bit.

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  9. #44
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    Chuck, I think you were right on the money when you mentioned it was a power supply issue. The power supply ground is really noisy from what I see on the scope! And Iím pretty sure V1 is amplifying that noisy ground. Iím going to revamp some of the filter cap grounding. I have all of them in close proximity which may not have been the best choice in terms of layout.. so I may take the last 2 filter caps and move them over to their respective stages. I just did a lot of reading on Merlinís galactic grounding chapter, and maybe this will help the cause. First Iím going to try to locate a quieter ground point for everything in the PS

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  10. #45
    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Steelwitch View Post
    Chuck, I think you were right on the money when you mentioned it was a power supply issue. The power supply ground is really noisy from what I see on the scope! And I’m pretty sure V1 is amplifying that noisy ground. I’m going to revamp some of the filter cap grounding. I have all of them in close proximity which may not have been the best choice in terms of layout.. so I may take the last 2 filter caps and move them over to their respective stages. I just did a lot of reading on Merlin’s galactic grounding chapter, and maybe this will help the cause. First I’m going to try to locate a quieter ground point for everything in the PS
    Ok, here's my take...

    A great many amps, especially those of vintage design, have all the filters grounded together. When I started modding and building I did it this way for my first several projects. It shouldn't cause a problem such as you are having. Maybe a bit too much noise in a design with cascade stages. (<period) So chasing design ideals right now isn't going to fix your problem because you actually have A PROBLEM. And having all the filters together, in your amp designs case, isn't a problem, it's just not the most ideal way to do it. So by my logic you should fix the problem first. Otherwise any potential errors that occur with changes may complicate the matter further, yet again.

    The problem isn't the 8uf filter (yes it could be larger, but it never has been for that design) and yes it's technically better to ground filters near their circuits grounds (but it's never caused objectionable noise levels with vintage type amps). I hope you get my point. Complicating the matter with changes that don't specifically address the real problem is a mistake that has shelved more projects here than anything else. You would be smart not to repeat this mistake.

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  11. #46
    Bent Member Chuck H's Avatar
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    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.

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    "Never bet your life on somebody else doing their job." SoulFetish's good friend

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

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

    "Back to the amp. It makes horrible sounds when I play my guitar thru it... because I suck at playing guitar." Mike6158

  12. #47
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    I totally get that! And youíre right, it should work theoretically. Iíve 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 couldnít have been quieter, but I was also working out of a Bassman chassis, and copied the grounding scheme exactly. Overall Iíd say it was a much easier amp to build.

    The reason Iím suggesting itís 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 wouldnít be extremely quiet, but Iím wondering if itís not quiet enough from a design standpoint. Doug suggests grounding the PS with the mains ground. is it worth separating them and seeing if thereís 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 itís being amplified only by the most sensitive stages? V1 and V2? I did get a different result pulling tubes today because of this: Iíve 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 Iíve been looking to V2 specifically is with the input jack grounded, the CF was still amplifying this hum but it didnít make a significant impact since its originating from the same source and itís the last stage sensitive enough to amplify it, and just didnít come through a grounded jack. Iím sorry I didnít try this earlier!

    This is kind of where Iím coming from when suggesting that if another ground point for the PS doesnít work, itís possible that grounding the first 2 filters to the preamp bus, which is extremely quiet right now could potentially have a good result. Maybe Iím off the mark here though

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  13. #48
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    Quote 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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