Star grounding problem

[trobbins]

Alan0345, your schematic of the rectifier and output stage should really show the output stage B+ current loop going through the power supply filter capacitor, not through the bridge rectifier portion. The bridge rectifier loop will only be conducting for a small percentage of time. Ciao, Tim

[Alan0354]

Alan0345, your schematic of the rectifier and output stage should really show the output stage B+ current loop going through the power supply filter capacitor, not through the bridge rectifier portion. The bridge rectifier loop will only be conducting for a small percentage of time. Ciao, Tim

Agree, my bad. I have to change that part. Actually I am thinking about how to accommodate the screen current so it does not affect the rest of the circuit.

[Alan0354]

Well, as you acknowledge here, you didn't just ask a question, you said all kinds of stuff.

On point one above, you disagree, fine, but I'm not sure that you adequately justified WHY you disagree with that strategy so vehemently. In my amps, the one-chassis-connection modified star ground system works for me first time, every time - I think a big selling point for hobbyists is its predictability. On the other hand, with my admittedly limited understanding of these issues, it seems like the scheme you advocate might theoretically have some benefit at RF, but it's entirely unclear to me why it should have any superiority, generally speaking, at audio frequencies. You make a big deal about the low impedance (as opposed to resistance) of the ground plane, etc., which leads me to suspect that you're still really thinking in high frequency terms.

And I guess I have to disagree with you about your subjective assessment of the noise performance of Fender amps. My own admittedly subjective opinion, frankly, is that it's generally just OK at best (even with brand new filter caps, etc.), and often just not very good, really. At any rate it's easy to build an amp with equivalent or greater gain that is much quieter than most blackface or silverface Fenders, although I acknowledge that there is much more in play here than just the grounding scheme.

As to what has emerged as your simple question -- Theoretically, I don't know, and I look forward to others chiming in. What I do know (and with RGs comments firmly in mind) is that in practice, with several new amp designs I've built with an otherwise rationally motivated ground scheme, it has never made a scintilla of difference that I could detect if the power tube cathodes returned to the -Ve of the screen or to the plate supply. And as Steve has pointed out these are bound to be both electrically and physically close.

No, not only high frequency, current choosing path of least resistance applies to very low frequency. If what I said is not true, there will be no issue using star ground however which way. The problem you can get into problem when you put ground onto a wrong star.

I use Fender platform to design my amps, I never run into issue. You crank up the cascade gain, it will be very obvious whether you have hum and I never with two Fender that I have been working on. Ground plane is well accepted already. I would say it is safer to have ground plane. I saw problem with star ground but never see problem with a well layout ground plane.

[Alan0354]

OK, I have been looking at this in light of taking into account of the screen current when in case of driving into clipping where grid voltage of the power tube can be +ve and screen current conduct. I half way appologize. Only half way!!! From Fig 3.11 where it is understand that each note of the ground is important and potentially by a star ground for that note. The drawing is still not correct. Below is my updated drawing. If Merlin draw it the way as I drew on the left drawing, I'll be ok with that as it has to be VERY SPECIFIC that the low side of the screen filtering cap has to go to junction C and become a star junction.

My updated drawing is on the right side that correct the mistake that Trobins caught me cold. Also, I modify and incorporated the filter cap for the screen to the star junction C. Now all the current should be accounted for in the island.



Yes I am very nit picking as I feel this is important when comes to grounding.

As for ground plane, I stand by my words, if people don't like it, they have their opinion. I presented my case and ground plane is well accepted in low and high frequency circuits.

What fool me is the screen normal don't conduct much current until you have big swing and it can produce noise.

[Tom Phillips]

...My updated drawing is on the right side ...
[ATTACH=CONFIG]20317[/ATTACH]

...

I don't understand what you are trying to show in that updated drawing is on the right side. It appears that there is a filter cap before the rectifier and that the input side of the choke is shorted to ground.

[Alan0354]

I don't understand what you are trying to show in that updated drawing is on the right side. It appears that there is a filter cap before the rectifier and that the input side of the choke is shorted to ground.

Yes, I correct the drawing already. Check out the drawing now.

[trobbins]

I think most of us would connect an output stage tube screen bypass capacitor back to the tube cathode's star point (eg. along with the OT B+ filter cap negative leg; bias voltage decoupling cap positive leg if used), as that localises the screen current loop - it doesn't matter per se whether the screen is supplied via dropper resistor or choke or even half-capacitor supply as per a doubler power supply.

Perhaps you may want to clarify if you use the chassis for multiple grounds connections, or just that you prefer to make the chassis ground point at the power supply 'end' of the ground circuit, rather than a more common connection at the signal input end. I've used both ends of the 0V chain, and with good layout, and some standard amp uses it doesn't matter - but like all things, there can be advantages to bonding at the signal 'input' end.

[dai h.]

doesn't that (connecting cct. ground to chassis ground at input end) make more sense to help keep high currrents off the chassis (so it's just a shield)? I think that helps with interconnectivity (i.e. ground loop) issues. Seems like the original style (everything on chassis) can cause problems if a 3-prong AC is connected and another AC powered device is connected (lots of stories about lifting safety ground using an old Fender Reverb into an amp to solve a hum problem--this can solve the audio problem but disconnecting the safety ground--or actually, relying on the shield for the connection to-- isn't a good idea).

[Alan0354]

I think most of us would connect an output stage tube screen bypass capacitor back to the tube cathode's star point (eg. along with the OT B+ filter cap negative leg; bias voltage decoupling cap positive leg if used), as that localises the screen current loop - it doesn't matter per se whether the screen is supplied via dropper resistor or choke or even half-capacitor supply as per a doubler power supply.

Perhaps you may want to clarify if you use the chassis for multiple grounds connections, or just that you prefer to make the chassis ground point at the power supply 'end' of the ground circuit, rather than a more common connection at the signal input end. I've used both ends of the 0V chain, and with good layout, and some standard amp uses it doesn't matter - but like all things, there can be advantages to bonding at the signal 'input' end.

It all started at post #30 when Jpfamps said connect cathode to screen ground. I would have no issue if he said the screen ground also connect to the point of the cathode/rectifier/main filter cap star ground point.

[Alan0354]

Well, as you acknowledge here, you didn't just ask a question, you said all kinds of stuff.

On point one above, you disagree, fine, but I'm not sure that you adequately justified WHY you disagree with that strategy so vehemently. In my amps, the one-chassis-connection modified star ground system works for me first time, every time - I think a big selling point for hobbyists is its predictability. On the other hand, with my admittedly limited understanding of these issues, it seems like the scheme you advocate might theoretically have some benefit at RF, but it's entirely unclear to me why it should have any superiority, generally speaking, at audio frequencies. You make a big deal about the low impedance (as opposed to resistance) of the ground plane, etc., which leads me to suspect that you're still really thinking in high frequency terms.

And I guess I have to disagree with you about your subjective assessment of the noise performance of Fender amps. My own admittedly subjective opinion, frankly, is that it's generally just OK at best (even with brand new filter caps, etc.), and often just not very good, really. At any rate it's easy to build an amp with equivalent or greater gain that is much quieter than most blackface or silverface Fenders, although I acknowledge that there is much more in play here than just the grounding scheme.

As to what has emerged as your simple question -- Theoretically, I don't know, and I look forward to others chiming in. What I do know (and with RGs comments firmly in mind) is that in practice, with several new amp designs I've built with an otherwise rationally motivated ground scheme, it has never made a scintilla of difference that I could detect if the power tube cathodes returned to the -Ve of the screen or to the plate supply. And as Steve has pointed out these are bound to be both electrically and physically close.

If it works for higher frequency, it works for low frequency, not the other way around. I worked a lot with Fender SF amps, modified to high gain, never once have problem with hum. Be careful about "noise", hum and ground noise is very distinct from other noises. I say nothing about noise that is relate to thermal, current and other. This subject is just grounding noise. My experience with SF are they are dead quiet for good reason.

If you ground a one point and only one point, you better triple check everytime you modify the amp, one accidental contact to the chassis from the tone circuit, or any other part will immediate cause problem. All the volume pots has to have the low side solder back to the board and not to the casing. If you have traditional two channel, you two input at different part of the chassis will create two grounding point.

I don't think anyone look into what I said about current choose the path of least resistance at low frequency and path of least impedance( inductance) at moderate and higher frequency. This is how circuit works because signal DO NOT TRAVEL as current and voltage. Signal travel as EM wave through guided structure. It is the guided structure that force the current path in the ground plane. Don't believe me, look up the speed of electron movement. They are so slow, you can put a voltage across a 3' wire, you inject an electron at one end, go get a cup of coffee and come back to wait for that electron to come out the other end. Signal travel at close to light speed ONLY because it is the EM wave that travel, not the electrons. The current and voltage you measure is ONLY THE CONSEQUENCE of the boundary condition of the EM field interaction with the conductor. This applies to frequency down to Hz.

If you look at signal this way, it will become much clearer how the ground works. BUT this is way too long to get into. If people overlook this, then ground plane become very confusing. I am not going to get into this, look into what I said when you have time instead of laughing at it. Look at the two link in Post #40 where I talked a little more in theory if you interested and look up EM books. This is where EMC theory starts.

[jpfamps]

It all started at post #30 when Jpfamps said connect cathode to screen ground. I would have no issue if he said the screen ground also connect to the point of the cathode/rectifier/main filter cap star ground point.

I quote from my original grounding scheme (post No.28):

"The heaviest currents in the amp are the charging currents to the first filter cap. I always connect the bottom of the main filter caps to either the bottom of the bridge rectifier, or the HT centre tap for amps with full wave rectifiers. This confines to the heavy ripple currents, and prevents them encroaching in to the audio. Call this star 1

The power valve cathodes, and bias supply filters are connected to the bottom of the screen supply filter cap. Call this star 2. I run a wire from star 1 and star 2, and then a wire from star 2 to the star ground point."

The phrase "I run a wire from star 1 (rectifier/main filter cap star) to star 2 (screen cathode)" would suggest I do recommend connecting these together..........

Regardless of this minor topological issue (Steve points out that we are arguing over an 1" of wire), my post No 28 was designed to be helpful to the original poster (rather than an exercise in demonstrating my knowledge) and was based on my experience of building amps, and eliminating earth loops, as the OP was having trouble with earthing.

The scheme I proposed was one based on what, in my understanding from reading around the subject (you aren't the only one who does this), is generally regarded as best practice in analogue audio.

Other posters in this thread also use a similar scheme and have no trouble with earth loops, which would suggest that this grounding scheme is robust.

Much of the information you provide is either irrelevant, or unnecessarily complicated (or both); and without wanting to be seen to be making an ad hominem point, looking at some of your contributions on Physics Forum (which you linked to), you do seem to be a serial offender in this regard.

For example, I can't see how discussion (or indeed knowledge) of "EM waves" and "guided structures" is of practical use in building a properly grounded valve guitar amplifier; I have seen no discussion of this any book I've read on analogue audio, and I expect Leo didn't know much about them either.

[Alan0354]

Accepting the signal traveling as EM wave open up a lot of technique on making use of the ground plane and simplify the layout. It is every bit relevant to electronics even in audio.

Speed of electricity - Wikipedia, the free encyclopedia

With this point of view, signal don't travel in all direction on the ground plane, the direction is predictable and you can localize the signal. That's why Fender have no ground noise issue and is easy to implement.

Now the noise refer here is not the hissing noise from the high gain amp, those are totally different. Ground loop cause 60, 120, 240 Hz type of hum. The hissing noise are from current and thermal noise in the gain stage. I mean Fender is dead quiet only refer to ground humming noise.

Yes, this sound overly deep for audio or even in normal electrical engineer. Very few people really get into this unless you specialize in EMC. I did not even know about signal travel as EM wave until 4 years ago. I just know about the image current tracking the signal. The EM wave nature of all electrical signal explains everything about the behavior of signals and simplify the design. You can actually guide how the current float inside the ground plane to avoid each other.

Again, it is pointless to go much further on this, I don't think you guys believe a word I said anyway. As for Physics Forum, you don't know me, I have been helping young engineer and student for about two years and I was nominated by the forum for the contributor of the year in EE. It is very critical over there, if anyone give wrong info or irrelevant info there, the senior adviser will immediately point out. I lost the vote mainly because I don't have the time with the forum yet, there are adviser that has been there for years and they got more votes.

[Wombaticus]

Notice the modifier superiority in my comments? I didn't say the ground plane approach couldn't be made to work, I just said that I don't see why it should be intrinsically better at audio frequencies. And from my point of view, it actually seems like it would be a lot easier to screw up a ground plane strategy than it is when running discrete wires over which you have total control. You don't have to sweat about inductive properties of areas bounded by runs and attachment points, etc. etc (or at least, you can re-route, whatever). I'm very well aware also that there are multiple sources of noise & hum, and what their properties are, which is why I also qualified my comments. But even still I am not at all convinced that the Fender scheme performs very well.

Don't forget that most of the comments and suggestions here are predicated on the assumption that most of the readers are hobbyists in need of simple-to-implement strategies that will have a high probability of yielding good results.

Finally, keep in mind that music gear tends to get connected to other music gear, so to my mind putting currents on the chassis is just asking for trouble when it's time to daisy chain a few pieces of gear. Also, just because a scheme seems to work well in the environment of your workshop, doesn't necessarily mean it perform adequately in a hostile electromagnetic environment like a club, where you're likely to have dirty power (maybe even a DC offset), cruddy EM from every imaginable source, interconnected devices, and so on.

By the way, it's not that we disbelieve everything you say... it's just that you seem, frankly, like a bit of a pedantic dick. I don't think this group is as thick as you seem to think we are.

If it works for higher frequency, it works for low frequency, not the other way around. I worked a lot with Fender SF amps, modified to high gain, never once have problem with hum. Be careful about "noise", hum and ground noise is very distinct from other noises. I say nothing about noise that is relate to thermal, current and other. This subject is just grounding noise. My experience with SF are they are dead quiet for good reason.

If you ground a one point and only one point, you better triple check everytime you modify the amp, one accidental contact to the chassis from the tone circuit, or any other part will immediate cause problem. All the volume pots has to have the low side solder back to the board and not to the casing. If you have traditional two channel, you two input at different part of the chassis will create two grounding point.

I don't think anyone look into what I said about current choose the path of least resistance at low frequency and path of least impedance( inductance) at moderate and higher frequency. This is how circuit works because signal DO NOT TRAVEL as current and voltage. Signal travel as EM wave through guided structure. It is the guided structure that force the current path in the ground plane. Don't believe me, look up the speed of electron movement. They are so slow, you can put a voltage across a 3' wire, you inject an electron at one end, go get a cup of coffee and come back to wait for that electron to come out the other end. Signal travel at close to light speed ONLY because it is the EM wave that travel, not the electrons. The current and voltage you measure is ONLY THE CONSEQUENCE of the boundary condition of the EM field interaction with the conductor. This applies to frequency down to Hz.

If you look at signal this way, it will become much clearer how the ground works. BUT this is way too long to get into. If people overlook this, then ground plane become very confusing. I am not going to get into this, look into what I said when you have time instead of laughing at it. Look at the two link in Post #40 where I talked a little more in theory if you interested and look up EM books. This is where EMC theory starts.

[jpfamps]

Accepting the signal traveling as EM wave open up a lot of technique on making use of the ground plane and simplify the layout. It is every bit relevant to electronics even in audio.

Speed of electricity - Wikipedia, the free encyclopedia

However, current best practice in analogue audio is to make only one connection to the chassis, so you don't have a ground plane.

Regardless, even if the original poster was using a ground plane (which he isn't), can you provide any practical advice regarding reducing the noise in his amp based on the knowledge that the signal travels as an EM wave?

With this point of view, signal don't travel in all direction on the ground plane, the direction is predictable and you can localize the signal.

You can localize the signal even more if you use a wire.....

That's why Fender have no ground noise issue and is easy to implement.

My guess is the Fender's grounding scheme evolved more by trial and error than theory.

Presumably however we can look forward to a mathematical proof that Fender's grounding scheme is the best. If you can't provide this, then your advice is I'm afraid pointless.

Now the noise refer here is not the hissing noise from the high gain amp, those are totally different.

I don't know why you feel you need to tell me this, as I have not made any statement in any other posts to suggest I don't know the difference.

Ground loop cause 60, 120, 240 Hz type of hum. The hissing noise are from current and thermal noise in the gain stage.

Yes I know this.

I mean Fender is dead quiet only refer to ground humming noise.

Hmm. Possibly not the prevailing opinion.

Furthermore from reading your posts, in your experimental data set n=2, which is probably not statistically significant.

I have worked on a few of these amps.............

Yes, this sound overly deep for audio or even in normal electrical engineer. Very few people really get into this unless you specialize in EMC. I did not even know about signal travel as EM wave until 4 years ago. I just know about the image current tracking the signal. The EM wave nature of all electrical signal explains everything about the behavior of signals and simplify the design. You can actually guide how the current float inside the ground plane to avoid each other.

I have no reason to doubt that you are correct; but how does this information help me (or indeed anyone else) reduce ground hum in their amp? Where does it instruct me to put the wires?

Again, it is pointless to go much further on this, I don't think you guys believe a word I said anyway.

It's not that I don't believe anything you say, it's more I find your posts devoid of any relevant practical information.

Stating that you should study EMC to work out how to eliminate a ground loop in your valve guitar amp is hardly helpful to anybody, even if it is technically sound advice.

[Steve Conner]

I'm the closest thing our company has to a "signal integrity engineer", and I've got all of our products to pass EMC. The second last one I worked on had about 10 switched mode regulators, two processors running at 300 and 500MHz, and 13 cables hanging out of it.

And, I say Alan is over-analysing it. The circuit impedances in a tube amp are so high that shared ground impedances hardly matter, except for a few cases, Loudthud's "stack of lugs" being one of them.

Alan, you might like to try reading Douglas Self's books to see where we're coming from.

Someone above suggested star-shaped solder tags with 6 solder bits and one bolt hole in the middle. They actually exist, I've seen them in the Morgan Jones book. No idea where to get them though. For important connections I make my own out of sheet copper.