Ac30 build Hummmmm

[Chuck H]

With volume at 10, you are going to get some hiss. I can't say if you have more hiss than normal, but some gain noise is to be expected.

Oh, and this^^^^^^^^^^^^^^^^^^^^^^^^^^

But I'm thinking you knew because you only seem worried about the buzz/hum.

[Steelwitch]

Alrighty! Now what happens when you ground the input grid of the triode feeding the cathode follower?

I will check and report back!

[Steelwitch]

the cap that powers the cathode follower is 8uf.. is it possible that it’s underfiltered?

[Chuck H]

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.

[Chuck H]

the cap that powers the cathode follower is 8uf.. is it possible that it’s underfiltered?

The value of that cap isn't the problem. Put that consideration on the shelf for now. Please don't temporarily swap it out or parallel another cap or ANYTHING.

[The Dude]

......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.

[Steelwitch]

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?

[Steelwitch]

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 there’s some kind of failure going on in the rectifier.

Another thing is after reading up a lot on Merlin’s 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. They’re 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?

[Chuck H]

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.

[Steelwitch]

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 it’s super quiet with the grid grounded! No DC voltage on it either

[Steelwitch]

Any recommendations on the next move?

[Chuck H]

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.

[Steelwitch]

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 I’m 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 didn’t think anybody saw. But you’re 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 y’all have here from time to time. Maybe it’s 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 I’m going to reflow the solder joints on V2 and see it maybe it’s just a bad connection or possibly a bad volume pot. Maybe it’s the connection between the plate of V2a and grid of V2b, or a bad connection coming from the 100k plate resistor since I didn’t see any hum on pin 7. I will report back.

I’m sorry that some of my results weren’t 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 I’m kind of at a loss to know what might be happening as well, so maybe I will rethink this stage a bit.

[Steelwitch]

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

[Chuck H]

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.