Grid stoppers?

[Enzo]

And speaking empirically, I have learned that calling an amp a "son of a bitch" doesn't seem to make it work any better either.

[Joey Voltage]

I think you're missing the point again, if more slightly. It's not successful performance of the function (although that would be very nice if we go to the trouble to put it in) that makes something a grid stopper; rather is is the possibility that it can do that function.

I understand your point

resistor a long way away from the grid it's "stopping" is simply going to be ineffective for that function. Is it a grid stopper? It can't possibly be in fact, so why muddy the waters by labeling it as such?

This reasoning seems very retrospective. You don't know that this resistor is going to be ineffective just by looking at it in a schematic. It is the assembly that dictates this, not necessarily the original intention behind the resistor.

let's get off this "Mr. Keen's definition" stuff. If you put a part in an amp which cannot possibly do the function you ascribe to it, but insist on calling it by a label it cannot possibly live up to, then you can't very well be surprised when someone else thinks it deserves a different label.

Lets go back to my Fender, and Marshall examples. When looking at the schemes of any of these amplifiers, most have little squiggly lines in series with the input tube grid with 68K written above them, or somewhere near them, correct? Most of us refer to these as grid stoppers. Why? They certainly have the potential to be, sure. It's not because I like the way "grid stopper" sounds any better. conventional/traditional labeling, sure (concise identifier was maybe a poor choice of words). It is the presumed intention of why it's there. We simply don't know whether or not this resistor is going to meet the proximity criteria to function properly as a grid stopper in any given design until the manufacturer has produced it. In my marshall/fender example, the physical layout of the component and long wire leads prevent it from suppressing RF, Therefore in the "Working Product" it is NOT a grid stopper, but the intent was there, and we couldn't possibly have known about inadequate layout just by glancing at a scheme, so why mess with the traditional labeling? Most people here know what you are talking about when you call these particular resistors in these particular amps, grid stoppers, whether or not the layout makes it impossible for them to function as such.

that line of reasoning, I and a number of my friends have decided that those 100K resistors on the plates of preamp tubes are "dingleberries", we see as a more concise identifier than "plate resistor".

Thats good stuff, you should update your website with this information. What do you call 220K plate resistors? I think you are going a bit to extremes. Calling a 68K resistor in series with the grid of an input tube, a "grid stopper," because it was most likely intended to be (but didn't quite translate in the layout), and calling plate resistors "dingleberries", are not even in the same ball park.

, you can - and will, if I know human nature - call them whatever you like. But be clear that calling a resistor a "voltage-controlled hyperlink" doesn't make the circuit operate differently.

Speaking of conjunctions, you can leave "but" out of the second sentence

is is the possibility that it can do that function.

I didn't know that two verbs occur successively like that in the english language. You my friend are in desperate need of a subject. Hey! you started it.

As for "Dynamic input capacitance", I have seen that term used in more than one publication. I can see if I can dig it up one of the sources if you are really curious in where I got that term.

[Enzo]

little squiggly lines in series with the input tube grid with 68K written above them, or somewhere near them, correct? Most of us refer to these as grid stoppers. Why?

Because we are continuing the common mis-use of the term. Some person refers to them that way mistakenly, other person hears that and assumes it is proper so he then uses the term that way.

It is the presumed intention of why it's there.

I have to honestly state that I disagree. When I see the two 68k resistors at the input, I see them mainly as part of a pad. The two resistors plus the grounding contact on the jack makes them a series resistance on one jack and a 2 to 1 voltage divider on the second jack - the classic Fender 6db pad. This view is bolstered by the knowledge that they are usually mounted on the jacks, not at the tube socket, or in some cases on the eyelet board instead of at the tube socket.

Most people here know what you are talking about when you call these particular resistors in these particular amps, grid stoppers

Most people here know what you are talking about when any number of misnomers are used. That is no defense of a bad habit.

Calling a 68K resistor in series with the grid of an input tube, a "grid stopper," because it was most likely intended to be

And there you go, it has come back around. I don't accept your premise. I don't accept that was in fact the intended purpose when the amp was designed.

I don't buy that all the engineers at Marshall and Fender and a whole list of other places are so inept that they have heard of grid stoppers and tried to stick them in their amps, but "didn't quite translate in the layout." I would give them the benefit of the doubt and assume that if they felt the need of a grid stopper, they are most likely knowledgable enough to implement it correctly. Note that on the Fender amps, the grid stopper for the power tubes is in fact mounted right at the socket. So inadequate layout? Phooey. They know what they are doing. The chosen layout makes the resistor ineffective as a grid stopper because that was never its intent.

When faced with your wiggly lines on the schematic, you can either choose to think that amp design engineers tried and tried and tried in model after model after model over years and years and years to implement grid stoppers in a totally ineffective manner, OR you could realize you are misusing the term.

[R.G.]

This reasoning seems very retrospective. You don't know that this resistor is going to be ineffective just by looking at it in a schematic. It is the assembly that dictates this, not necessarily the original intention behind the resistor.

It's not really retrospective reasoning. It is simply a recognition that schematics are an abstraction, and do not necessarily contain all of the information needed to make an electronic device work. Well drawn schematics intended for manufacturing reference will be annotated with physical details, including things like grid- and gate-stoppers being close to the device. Circuit reference diagrams often omit that data, since a repair tech is expected to replace bad parts, not redesign the amp.

Lets go back to my Fender, and Marshall examples. When looking at the schemes of any of these amplifiers, most have little squiggly lines in series with the input tube grid with 68K written above them, or somewhere near them, correct? Most of us refer to these as grid stoppers. Why?

Because most of us use it incorrectly, not understanding that there is a physical component to how grid stoppers work, and simply labeling any series resistor as one. Sure it sounds cool, like the person really understands tube amps, but it betrays a lack of in-depth information.

The Sorcerer's Apprentice strikes a chord in all of us because we all would like to appear to have all the knowledge, whether we do or not. But as the SA found out, a little knowledge is a dangerous thing.

They certainly have the potential to be, sure. It's not because I like the way "grid stopper" sounds any better. conventional/traditional labeling, sure (concise identifier was maybe a poor choice of words). It is the presumed intention of why it's there. We simply don't know whether or not this resistor is going to meet the proximity criteria to function properly as a grid stopper in any given design until the manufacturer has produced it. In my marshall/fender example, the physical layout of the component and long wire leads prevent it from suppressing RF, Therefore in the "Working Product" it is NOT a grid stopper, but the intent was there, and we couldn't possibly have known about inadequate layout just by glancing at a scheme, so why mess with the traditional labeling? Most people here know what you are talking about when you call these particular resistors in these particular amps, grid stoppers, whether or not the layout makes it impossible for them to function as such.

I'm not sure why you're pushing this issue. Presumed intention and traditional labeling have nothing whatsoever to do with what the parts do. As we'll get to in the next ply of this exchange, you can call it anything you like. But if the label does not reflect what it actually does, you may as well call it anything.

Thats good stuff, you should update your website with this information. What do you call 220K plate resistors? I think you are going a bit to extremes.

Of course I'm going to extremes. I'm using hyperbole to try to illustrate the silliness of insisting on calling something by a misleading name. I'm hoping that humor reaches you if reason can't.

Calling a 68K resistor in series with the grid of an input tube, a "grid stopper," because it was most likely intended to be (but didn't quite translate in the layout), and calling plate resistors "dingleberries", are not even in the same ball park.

You've inadvertently picked out the worst possible example to support your case. The 68k resistors at the front end of an amp do have a clearly defined role, that of padding the inputs to be high/low gain.

What you're missing here is that Enzo and I are not just being difficult. We are trying to help you learn something that no one has taken the time to tell you yet. And you're vigorously defending your right not to learn.

As for "Dynamic input capacitance", I have seen that term used in more than one publication. I can see if I can dig it up one of the sources if you are really curious in where I got that term.

I'd appreciate it. The term is no mystery, as I suspect that it's another term for Miller capacitance, but I would like to go learn what was really meant by it instead of using it as a red herring.

But just out of casual interest, what did *you* mean by it?

[Joey Voltage]

I have to honestly state that I disagree. When I see the two 68k resistors at the input, I see them mainly as part of a pad. The two resistors plus the grounding contact on the jack makes them a series resistance on one jack and a 2 to 1 voltage divider on the second jack - the classic Fender 6db pad. This view is bolstered by the knowledge that they are usually mounted on the jacks, not at the tube socket, or in some cases on the eyelet board instead of at the tube socket.And there you go, it has come back around. I don't accept your premise. I don't accept that was in fact the intended purpose when the amp was designed.I don't buy that all the engineers at Marshall and Fender and a whole list of other places are so inept that they have heard of grid stoppers and tried to stick them in their amps, but "didn't quite translate in the layout." I would give them the benefit of the doubt and assume that if they felt the need of a grid stopper, they are most likely knowledgable enough to implement it correctly. Note that on the Fender amps, the grid stopper for the power tubes is in fact mounted right at the socket. So inadequate layout? Phooey. They know what they are doing. The chosen layout makes the resistor ineffective as a grid stopper because that was never its intent.When faced with your wiggly lines on the schematic, you can either choose to think that amp design engineers tried and tried and tried in model after model after model over years and years and years to implement grid stoppers in a totally ineffective manner, OR you could realize you are misusing the term.

Enzo,

Here's another example for you. Please, if you can, expalin it to me. Most marshall 2203/4 amplifiers have high/low sensitivity jacks. In these amps, the 6db pad using the classic 68K method was ditched in favor of bypassing an entire gainstage by means of the low input jack. The high sensitivity input now has a single 68K resistor in series with the grid of the input stage. In practice (having owned one), marshall mounted this 68K resistor on the PCB between copper traces from input jack, and long unshielded wire to the grid of the input stage. It cant possibly be a grid stopper, because it doesn't have the potential to perform this function, but it cant be part of a 2:1 voltage divider either because there is no other resistor to perform this function with. This was 1986, so the engineers at marshall, as you said are not inept, and knew what they were doing right?. So why is it there? What is the intention in this case? What function does it serve? simply just roll off at a frequency most people can't hear? If it doesn't serve a useful perpose, why is it there? otherwise, to me at least, it is just one more part in the BOM that doesn't need to be there. I'm not trying to be a pain, I really just want to know.

[Steve Conner]

What is this, Guitar Amp Philosophy 101?

The 68k resistor can be part of an input pad.

It can be a RF input filter. 68k of series resistance, in conjunction with the ~100pF Miller capacitance of a 12AX7 grid, will form a ~20kHz low-pass filter that keeps LW and MW radio out of the amp. I made an amp with no 68k input resistor, and it picked up Radio Moscow after dark, and adding the resistor stopped that.

But is it a "grid stopper" in the traditional ham radio sense, which implies that it stops parasitic oscillations in the tube stage whose grid it's attached to?

On power tubes, maybe, and that's why you find it right at the tube socket, because it wouldn't work if it were placed further away. But preamp tubes are probably stable without it: the tendency to parasitics, all other things being equal, is a function of internal lead length and transconductance.

The resistor will still suppress external RF no matter where it is in the input wiring, as long as it's inside a screened chassis, because the screening of the chassis implies that any RF present must have got in through the input jack.

And by that token, the same 68k resistor(s) can be both a RF input filter and a 6dB pad.

[Joey Voltage]

I'd appreciate it. The term is no mystery, as I suspect that it's another term for Miller capacitance, but I would like to go learn what was really meant by it instead of using it as a red herring.

But just out of casual interest, what did *you* mean by it?

R.G.,

I haven't been able to track down my resources for that term yet but IIRC the term was used to describe the capacitance that results when tubes internal grid-to-plate, and grid-to-cathode capacitances are conjoined with the miller capacitance (effect). I suppose, and this pure speculation, the term "dynamic" is used to describe the variance in the grid-to-plate capacitance due to variance in stage gain. It makes the most sense to me at least.

[MattT]

Great info here. I do know that in several cases I've just willy-nilly mounted/moved a resistor that, in the schematic, appeared to be a grid stopper (like the 68k at the input) and didn't even think to test it to see if it adversely affected tone...I just assumed I had made it 'better' by putting it 'where it was supposed to be in the first place'. Woopsie.

It's no wonder my homebrews start out so pretty but end up so ugly inside...I keep going back in.

[Dave H]

Keep up the good work chaps. This is more fun than a Saturday night punch up at Wetherspoons.

I'm not sure what to call the squiggly dangleberries at the input of my amp now. When I first built the amp I mistakenly put the input tube right next to the input jacks and wired the 68ks directly to the grids hoping for some RF attenuation and stopping action. I hadn't seen inside a Fender or Marshall at that time so I didn't know how it was supposed to be done.

Dave H.

[Enzo]

I see nothing wrong with mounting the tube near the jacks and running the resistors directly. When I build little Champ-like amps, that is how I do it. Fender wouldn't do it that way because it would interfere with the eyelet board where the parts live and the tube would be in the way near the jacks.

[Joey Voltage]

Thats kinda how I do it too, except I use a piece of co-ax from the input jack to the resistor/socket. I usually will have the preamp tubes near the front of the amp anyway.

[booj]

Ok, so I should put grid resistors close to the output tube socket. That's very helpful!
Now what about those darn screen resistors? Oops! That's a different subjct!