47 Ohms Above Ground

[g1]

Sort of see what you mean Daz, but I think the resistor helps avoid ground loops which may occur from parallel ground connections. With the resistor there is no ground current through it when things are ok.

[daz]

Yes, but thats what i was just saying i said in an above post and enzo said no.

[g1]

Sorry to confuse things Daz, there are a couple separate issues here.
1) there are no areas that are 47 ohms above ground. If you measured, you would find that both ends of the resistor are at ground potential. The resistors are not there to prevent ground loops, they are there to provide a ground path in event of a fault condition. They are like a "back-up" ground.
2) You asked why not use a straight jumper rather than a resistor. This is where you would create ground loops. You would be duplicating grounds when there is no fault condition. We only want to have a back-up for the ground in case of a fault (lost ground). A resistor will allow ground current when there is a fault, but will NOT act as a duplicate ground when there is no fault.

[silverfox]

Pop Goes The Jack

The point, daz, is that no section of those Peaveys is 47 ohms above ground, all points are grounded to chassis ultimately, the 47 ohms only enters the picture if the chassis connection is removed from one of the jacks.

Isn't that what happens momentarily when the cord is removed from the input jack? As I now recall there never was any of that nasty snap crackle pop when the cord was plugged in. Just a click.

Thanks for the schooling all,

Silverfox

[daz]

Sorry to confuse things Daz, there are a couple separate issues here.
1) there are no areas that are 47 ohms above ground. If you measured, you would find that both ends of the resistor are at ground potential. The resistors are not there to prevent ground loops, they are there to provide a ground path in event of a fault condition. They are like a "back-up" ground.
2) You asked why not use a straight jumper rather than a resistor. This is where you would create ground loops. You would be duplicating grounds when there is no fault condition. We only want to have a back-up for the ground in case of a fault (lost ground). A resistor will allow ground current when there is a fault, but will NOT act as a duplicate ground when there is no fault.

Yes, yes !!! Thats exactly what I suggested about 3-4 posts ago !!! I saw it as a way to keep then grounds seperate from the jack but not create ground loops. And thats the one i said enzo shot down, or at least it seemed so.

Heres my post i'm talking about:

I hope this isn't a stupid question enzo, but i'm pretty sure it will be. But hey, look who you're talking to. Anyways, are you saying that this allows the sections that are 47R above ground to NOT cause a ground loop with the input jack thereby allowing it to a be a non isolated jack?

[JoeM]

Yes, yes !!! Thats exactly what I suggested about 3-4 posts ago !!! I saw it as a way to keep then grounds seperate from the jack but not create ground loops. And thats the one i said enzo disputed.

Enzo disputed? Where?

As i stated, it insures a complete ground path even in the event the jacks lose contact with the chassis.

[Chuck H]

I guess I read through too quickly and opened a big can of "S" on this one... Apologies.

[Enzo]

Isn't that what happens momentarily when the cord is removed from the input jack?

No, the jack is grounded via its bushing, which remains firmly attached to chassis regardless of plugs. The input is grounded or not depending upon plug, but that is a different issue.


I'll do some research, if I am mis-stating the resistor purpose, I will want to clarify it.

[Raybob]

That 47 ohm resistor makes no sense to me at all. If it's there for some type of 'safety' for a problem, why not attack the problem, not just 'hide' it with this resistor?

I've been building many tube amps, learning about grounding early on, separating power section grounds from preamp grounds. For long time, I would have buss bar for jacks, pots / preamp grounds; and separate power grounds to a star. Used to always ground the buss bar to chassis, near input jack, but later realized that bar doesn't need to tie to the chassis at all. The preamp ground buss is floating, essentially, no ties to chassis, with VERY quiet idle hum.

[Merlinb]

That 47 ohm resistor makes no sense to me at all.

As stated, the resistor is totally shorted out under normal circumstances. But if the jack socket is not connected to the chassis then the 47R does create a partial ground lift for the preamp. I suspect it was added to give the designers the option of having the ground lift or not, depending on whether they found ground loops to be a problem when the prototype was built. After all, it's a lot easier to include redundant components on your PCB and then not use them / short them out, than it is to add them after the fact!

You often see redundancies and apparently pecuilar component choices in modern designs, because it is easier to prototype this way. Then when the final version is made, some or all of these things are left in place, much to the puzzlement of DIYers. For example, you might find of a lot of zero-ohm or low-value resistors in a power supply. They do nothing for the power supply, they just allow the different sections to be isolated and tested individually and progressively, to make sure each bit actually works as planned, rather than powering the whole thing up and hoping you got everything right first time!

[Enzo]

One thing I thought of but decided I didn;t think it likely was Spice. I have zero experience with Spice, but in reading my engineering magazines I see stories about getting a spice simulation to resolve itself. Converge I think it the term. And I have seen examples where a circuit sim would not converge until they added a resistor or other component not connected to anything else, and grounded at both ends.

[R.G.]

Ground issues are about current flow paths and the voltages which ground currents create when they flow through the resistances of the ground path.

At low frequencies, ground current follows the lowest resistance path. At high frequencies - RF - ground currents follow a transmission line path, clustering under the conductor trace that has the signal flow, due to field effects.

By inserting a low-value resistor in a ground line, you force the ground currents to choose an easier, lower resistance path. It's a smaller, less intensive way of choosing where and through which conductors ground currents flow than by using star grounding. It's useful when you really need star grounding for lower noise, but simply can't practically avoid having two paths through ground to the same circuit section. This is especially true when there is a large, pervasive "ground" like a metal enclosure and you can't practically NOT use the chassis for ground at two points - or if you're really, really trying hard to be cheap.

It works well - if a bit quirkily. I had an 11 ohm ground isolator between the signal star ground and the "ground" of the 9Vdc output on the amp. This cut the residual hum quite a bit as it moved the signal ground reference from the chassis out to the pedals being powered, as the ground current would preferentially follow the pedal signal ground out at the pedalboard, not the chassis ground where the jack was. But when some budding guitar genius would manage to force more current through the 9V output than it could do, the 11 ohm resistor could be overheated and burned out, and then the hum got much worse. The solution was to put in a jumper there, and revert the sound to dramatically quieter than the no-ground situation, if a bit humm-ier than with the 11 ohm resistor.

As noted, there are many reasons why someone MIGHT put a 47 ohm resistor in a ground path, including simple imitation of a neato, high tech thing they saw in a schematic one time. But control of where the current flows is probably the best reason to do it.

[eschertron]

Look at the Classic 20, and at the input jack note the triangle ground for the preamp joins the chassis ground there. Now look down to the external speaker jack, the circled triangle there is joined to the chassis. So when the amp is buttoned up, the two triangle grounds, circled and not circled, are joined together through the chassis. And that shorts across the 47 ohm resistor.
So the resistor does no ground lifting. What it does is provide a path to the rest of the system ground if the jacks lose contact with chassis.

:spank: I've been referring to the Classic 20 drawing for several months as I've worked on a DIY design and NEVER noticed the preamp grounds joining the chassis at the input jack. D'oh! I'm still confused why the 47R resistor would be specified on the drawing as flameproof, though.

[Enzo]

OK< I contacted my friend at Peavey, Roger Crimm, and asked him for the scoop. He put it to engineering,,,

Hey Enzo,
Here is what I got from engineering:
These used to be routine for two reasons:
1.) To allow for very specific grounding of various circuits and;
2.) To allow servicing of PCBs outside the chassis....the resistors break up any potential ground loops.

This isn't common practice now because we are moving to a unified ground on nearly everything, but we still do it, as needed, for Service and to break loops.
So there you have it!

Regards,
Roger C

Note the second item, service outside the chassis, as in when the jacks are not grounded to chassis.

[tubeswell]

Good ole Roger Crimm. So he's still kicking? - that's nice.

FWIW Fender also uses a couple of FP 15R for signal-ground elevation in their RI '63 reverb 'tank' (one to the pot brackets, and the other one to lift the power rail ground - bypassed with a couple of 6A nose-to-tail diodes). Done in the name of regulatory compliance AFAICT.