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