Grid stoppers?

[EETStudent]

Ive typically seen grid stoppers after a volume control or before the power tubes, etc. However, in what is pretty much a Fender clone, there is a 470k resistor after the first stage coupling cap before the volume control. Is this effectively a grid stopper too?

[Enzo]

It is a voltage divider - the resistor and volume pot are in series.

[Joey Voltage]

It does indeed form a potential divider, but it also creates a high frequency roll off due to the miller effect. It also behaves as a grid stopper, as I believe it will limit grid current as well if that stage hits saturation. So yes, it is a grid stopper.

[R.G.]

The purpose of grid stoppers is to stop parasitic oscillation by causing a high frequency rolloff with the grid-cathode and grid-plate capacitance.

Note that I said grid-plate instead of Miller capacitance, as the stopper is effective in source-follower setups where there is no voltage gain to enhance grid-plate capacitance.

A grid stopper must be mounted as close to the grid's socket pin as the body and leads will allow, or it's just wishful thinking. The self-inductance of the wire lead will remove any benefits of the stopper and may make oscillation worse if the lead is not as short as is physically possible.

There are other uses for series grid resistors, including low pass filtering and limiting grid current during positive grid drive, but these may or may not also act as a grid stopper depending on how they are mounted.

[Joey Voltage]

Or dynamic input capacitance in other words. RG is right in that you should mount it on or as close to the socket as possible, but this is most important for the input stage, because the main purpose of this resistor is to roll off RF frequencies, and the resistor is useless if you nave a wire after the resistor.

In other stages it is used most commonly to roll off high end, but there are less noisy ways to do this.

[tubeswell]

Grid stoppers are especially useful at the grid pin of the 1st gain stage for preventing unwanted oscillation. Also useful if you are using shielded signal cable to your grid pins.

[EETStudent]

In my case, does the series 470kR increase the gain a bit? The volume pot is 1M, but it seems this resistor would make the signal further away from ground

[tubeswell]

Any resistor in the signal path will attentuate the signal in some way. With a voltage divdier you use Kirchoff's Law to figure out what the attentuation level is on the node between the 470K and the Vol Pot. (it will vary depending on how the pot is set). That is the signal going into the grid of the next stage.

[Enzo]

The original question was what is the resistor after the plate coupling cap going down to the volume control. It doesn't get wired to a grid at all, unless we consider the volume all the way up as wired to the grid. I don't know how we would mount this resistor "as close to the socket as possible." How is this a grid stopper?

[R.G.]

It also behaves as a grid stopper, as I believe it will limit grid current as well if that stage hits saturation. So yes, it is a grid stopper.

I'm sorry, I think this is not yet clear. These two sentences seem to say that
(a) you believe it will limit grid current if the stage hits saturation so therefore (b) is is a grid stopper, having met condition (a).
That is incorrect. Whether it is or is not effective as a grid stopper depends on where it is mounted physically, not whether it will limit grid current. A grid stopper might limit grid current, but that is an aside and accidental; properly designed, it will stop oscillations whether or not there is any grid current to limit.

As an aside, "saturation" as most semiconductor people understand it is not what you limit grid current for. Saturation is when the plate can no longer respond to the grid voltage increasing with an increase in plate current. While there is a saturation in tubes, it does not occur when the grid starts drawing current, limited or not. In both triodes and pentodes, you can cause higher and higher plate currents to flow by continuing to raise the grid above the cathode. This is the basis of Class AB2 operation in push-pull amplifiers.

I believe you meant to say "when the grid is raised above the cathode potential and starts conducting current" instead of saturation. Depending on the value of any grid stopper, the grid current might be limited by the stopper, but may also be limited by biasing resistors or the driving impedance of the previous stage; or a real current limiting resistor, put there only for that.

Or dynamic input capacitance in other words.

Can you explain that? It seems like a non sequiteur.

RG is right in that you should mount it on or as close to the socket as possible, but this is most important for the input stage, because the main purpose of this resistor is to roll off RF frequencies, and the resistor is useless if you nave a wire after the resistor.

I believe that "and" is the correct conjunction, not "but", or simply a full stop. Usefulness in some stages or importance to immunity from RF intereference is not a negation or an exception to the primary use of the stopper, or its positioning.

In other stages it is used most commonly to roll off high end, but there are less noisy ways to do this.

A resistor in series with a grid may do any or all of:
1) preventing the entry of RF into the grid
2) limiting grid current in extreme operation
3) rolling off high frequencies
However, since these are different effects, the correct value of resistor for one use may not be the right one for another use. For instance, it is common to use high value resistors for audio treble rolloff. This may make the stage MORE sensitive to RF interference by making the impedance at the grid higher. If the resistor is also fairly remote (i.e. an inch or so) from the socket lug, this may even tune in radio sources. Limiting grid current is itself a complex subject; the right value to limit grid current is not necessarily the right value for either grid stopping or treble rolloff.

[tubeswell]

The original question was what is the resistor after the plate coupling cap going down to the volume control. It doesn't get wired to a grid at all, unless we consider the volume all the way up as wired to the grid. I don't know how we would mount this resistor "as close to the socket as possible." How is this a grid stopper?

That's right - my understanding of a grid stopper is a (smallish ~up to about 68k) resistor in series with the grid pin (preferably right at the grin pin - for the reasons RG describes), whereas if it is connected to another resistor to ground before the grid pin, it is not a grid stopper but rather is part of a voltage divider (but even with the volume all the way 'up', to use your words in your example Enzo - it is still a voltage divider is it not?, because the pot is still there albeit that it becomes a larger resistor connected to the ground when it is turned 'up').

[Enzo]

It was always a voltage divider, regardless of whatever else it might or might not be doing. Many parts are doing more than one job, it all depends on what you are discussing. I understand grid stoppers, I am still not convinced that this is an example of one.

[Joey Voltage]

Maybe I was speaking in absolutes, and for that I stand corrected, but If you are calling a resistor that is in series with a tubes input grid a grid stopper only if it successfully performs the task of RF suppression, then most of the 68K resistors in series with the input tube grids of countless fenders, and marshalls are not grid stoppers, purely because the long (mostly unshielded) wire runs from this resistor to the grid, prevents this resistor from doing this task correctly. It then, under Mr. Keen's definition, just becomes a resistor in series with the input tube grid, because it fails to perform this task correctly. That is if we define this resistor purely by successful function. I do not know, I think many of us still label those 68K resistors on older marshalls grid stoppers, whether they perform or not. I see it more as an concise identifier, rather than a product of successful function. Same goes for the the series resistors often seen at the input grid of PA tubes. I Won't, excuse me, will not (sorry RG), call them grid stoppers anymore, because they don not perform this task in a lot of amps either, especially if PCB's are involved.

[jrfrond]

A grid stopper doesn't HAVE to be a resistor. A ferrite bead right at the tube socket will accomplish the same thing (i.e. RF suppression) without adding any series resistance at all.

[R.G.]

Maybe I was speaking in absolutes, and for that I stand corrected,

'S'OK. we all start somewhere.

but If you are calling a resistor that is in series with a tubes input grid a grid stopper only if it successfully performs the task of RF suppression, then most of the 68K resistors in series with the input tube grids of countless fenders, and marshalls are not grid stoppers, purely because the long (mostly unshielded) wire runs from this resistor to the grid, prevents this resistor from doing this task correctly. It then, under Mr. Keen's definition, just becomes a resistor in series with the input tube grid, because it fails to perform this task correctly. That is if we define this resistor purely by successful function.

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. A 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?

And 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. Well, OK, we do have the example of our dearly departed president, William Jefferson Clinton, who told us that "... it depends on what your definition of "is" is."

Calling it an elephant doesn't give it a trunk, but you might like it better.

I do not know,

OK. Sometimes I'm confused too.

I think many of us still label those 68K resistors on older marshalls grid stoppers, whether they perform or not. I see it more as an concise identifier, rather than a product of successful function. Same goes for the the series resistors often seen at the input grid of PA tubes.

Using 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", which we see as a more concise identifier than "plate resistor". We call the resistors from grid to ground "Mersienne Primes". Concise, easy to remember.

I Won't, excuse me, will not (sorry RG), call them grid stoppers anymore, because they don not perform this task in a lot of amps either, especially if PCB's are involved.

Actually, 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.