Regarding grounding schemes of stereo input

[pjd3]

Hi there and thanks,

Can someone elaborate a bit on what may be the implications of grounding the preamps either to the main B+ filter cap cathode vs the 1st stage input filter cap cathode? Thats something I struggle to resolve in my head.

thank you,

Best,
Phil D.

[R.G.]

Yes, the chassis ground connection should be only one place, not at the main ground but rather at the input stage instead.

In my mind, that works when you use non-isolated input jacks.

My quibble with this method is that any RF or noise intercepted by the chassis in shielding the circuits runs along two paths, one to the inputs, and one to the AC safety ground, which is where non-eddy-currents goes. There is a voltage difference between the two places, preamp ground and AC mains safety ground in the amount of preamp "sewer" return current times the resistance of the preamp ground wire.

Yes, the voltage difference is tiny, and it's almost certain to be good enough, as proven by the many amps that are wired this way. Isolating the jacks from chassis and running a reference ground from the power supply first filter cap negative removes the preamp return current from the wire between the chassis and the first filter cap negative.

In my mind you could do either. My preference is for isolated jacks and star grounding, including referencing the chassis to the power supply star point, not at the preamp.

[pjd3]

OK, again, thank you everybody for this great coverage of grounding schemes for the type of amp I am building. I'm at the point now where this recommended grounding scheme(s) is taking place.

I did install fully isolated jacks for inputs and speaker outputs and did run 100pf ceramic caps from input jack grounds to solder rings on the first volume pot of each associated channel for RF (about an 1.5" away from input jacks).

If I'm getting this correct, I will not need (yet) to use preamp-to-chassis ground lugs since each input jack is isolated, and each of the input jack grounds will be connected to its corresponding preamp ground right at the first filter cap ground (where that 22uF cap negative meets its ground bus wire).

Am I good so far?

From there I will simply follow the grounding scheme of the first diagram (version 1) at least regarding the electrical signals and electrical ground.

I will however, still need to address the earth grounds. This amp has both separate power transformers AND, separate AC cords each with its own green earth ground wire. I see three ways to approach this, initially:

1. Each earth ground wire connected to a chassis lug near its associated 120vac and power transformer.

2. Bring each of the separate earth ground wires to a common lug nearing the middle rear of the amp.

3. Connect one of the earth ground wires to a lug at its chassis corner, and bring the other earth ground wire all the way over to meet with that one.

One of the engineers stared at my hand drawn amp diagram for a couple of minutes and suggested trying number 3. I hadn't thought of that configuration but, I know this guy deals with some pretty tricky grounding schemes in out defibrullators. I trust his opinion more that mine (I'm just the dumb electrical R&D tech).

So if having your blessings, I will complete the wiring of this stereo amp in this way, and see what we have.

Thank you for very valuable and insightful contributions to my amp. I'm really hoping to make good of this. I wan't the best stereo guitar sound in the land!
Best,
Phil D (pjd3)

[Phil_S]

You might search out the hifi community for a stereo tube amp. Unless I misunderstand, it is basically the same thing. This page has a fairly good gut shot of the guts of one but probably not good enough to answer your questions:
https://www.stevehuffphoto.com/2020/12/ ... ainer-buy/
Maybe you get lucky and find a schematic or better yet a layout diagram?

[R.G.]

Here's how to think about it. There's a section on wiring input jacks in it. (Guitar Amp Wiring Notes Draft 2.15)
https://www.dropbox.com/scl/fi/8fscwmvn ... sl1j8&dl=0

There's a link to just the pictures in a thread here under Guitar Amp Wiring Cartoons.

I personally would use isolated jacks with RF shunt caps, and return the input jack ground wire to the local ground of the first amplifier stage that the jack connects to; two different places in this case. This ensures that there are no parasitic currents in the input ground wires.

[pjd3]

Thank you, I will certainly reference that image.

I did install isolated inputs and speaker jacks with 100pf caps from each input jack ground to nearby chassis (solder ring on first volume pot).

I already have wires soldered to the inp jack grounds ready to solder to each jacks associated preamp ground. What I am curious about is it optimum to send each inp jack ground right to where the first tube stage filter cap negative joins the preamp ground, or, is that insignificant and anywhere on the preamp ground buss will do? I'm just checking in to see if those things matter, such as what specific part of the preamp ground it is connecting to.

Thank you1
Best

Phil D

[pjd3]

Hi RG,

I've begun to read your article on grounding and already I have garnered some new insights.

Thanks for sending this link along.

Best,

Phil D

[pjd3]

As I"m studying these articles it certainly is looking to be that the location of where a ground is sent, as in isolated input jacks is important.

I'm just starting to think about how and where current is flowing and where it is coming back from.

Thank you,

Best,
Phil D

[pjd3]

Alright.

So there was alot here for me to digest and use to implement wiring in my "two amps in a chassis" stereo amp.

I pretty much adhered to what is proposed in the "Version 1' diagram, with the caveat that I must be ready to change things should there be noise/buzz/hum or any other artifact that is not acceptable.

I believe I only have one more connection (well, two actually) to make before doing a check over for any shorts and finally power one side at a time up, and look to see that proper voltages are found in the right places.

My final question to the "last wire" is, is there a optimal place on the preamp ground buss to bring to the main electrical ground reference where the HT centertap meets with the first filtercap? Is there a difference between say, taking it from the input end of the preamp ground buss wire, or at the end that is furthest upstream on the preamp ground buss? Or somewhere in between?

Just to re-iterate - Both the input and output jacks are isolated and so far, I did not bring the preamp ground to chassis. The isolated input jack grounds have been brought to the negative end of the first 22uF filter cap that is on the turret board for V1 and connected to the preamp ground buss.

So I think that is it. I can begin testing both of the amps once the preamp ground buss has been connected to its final reference place. Ha. I'm scared. I've been fairly confident with the handful of single "normal/classic" amps I have built but, this one has some notable compromises in placement of a couple of things in order to make room in one chassis for everything. Not too crazy, as I'm aware of the critical guidelines that drive most tube amp layouts but there were a couple of awkward wiring schemes used near the very end. We will soon see how that all plays out.

Thanks so much everyone. I do know how fortunate I am to have the kind of guidance I get from Ampgarage.

Best,
Phil D. (pjd3)

[martin manning]

Is there a difference between say, taking it from the input end of the preamp ground buss wire, or at the end that is furthest upstream on the preamp ground buss? Or somewhere in between?

I will guess the end closest to the inputs, but why not find out for sure with an experiment?

[pjd3]

Thanks Martin,

I can surely start there, and it there is a hum or buzz, I have someone to blame it on other than myself...... Kiddin.

Sure, I'll begin there, and adjust as is called for.

Thank you,
Best

Phil D

[martin manning]

Sure, I'll begin there, and adjust as is called for.

You can actually test this with a clip lead.

[lonote]

I know that Merlin's grounding chapter has been already mentioned on page 1 of this thread, but your quandary reminded me of the hum-block circuit that he included towards the end of that chapter, basically to allow safety grounding to chassis, while keeping things isolated enough to discourage ground-loops (2 separate devices in his example).

I have successfully used that circuit in a 6G15 build, which is somewhat notorious for creating grounding issues when connected to an amp.

Seems like your two sections could each use one of those circuits to make the chassis ground connection & minimize any interaction between the two.

Apologies if this has already been mentioned, didn't see it it an a quick skim to catch back up on the thread.

https://valvewizard.co.uk/Grounding.pdf section 15.10 Ground Lift

[pjd3]

Thank you lownote, every bit helps and I'll be sure to check this out.

Talking about safety ground, I decided to have a separate AC cord for each PT at the far rear corners of the amp. Its funny how alot of diffent people, all very experienced had different ideas about how safetly ground for this amp should go. Some thought that each ground should meet together in the middle of the chassis, another thought that a safety ground from one side of the amp should go meet the other on just one side of the amp, and another thought it best for each safelty ground to remain in its own rear corner of the amp nearby all the associated power components for that side of the amp.

And now that I think of it, since these preamps don't carry much current, I probably can get away with using clip leads.

thanks guys! Good night.

Best,
Phil D