Grid stopper matching?

[Cartman]

In a push-pull power amp with grid stoppers, is it necessary or beneficial to match the resistor values? The reason I ask is I have a Marshall 18W clone with EL84s that has 5.1K grid stoppers, but they have drifted to about 6.4K and 7K. I'm trying to decide if I should change them or it's really nothing to worry about. Thanks.

[bepone]

no problem, up to 47k is ok,
but if they are cc and drifted so much this can be indication of humidity

[Cartman]

Great, thanks. Yeah, CC and about 15 years old so I may go back in and change them at some point.

[bepone]

CC is the best to use in that place because of non-inductance, good RF properties, so just replace for new cc's one day

[maxkracht]

CC is the best to use in that place because of non-inductance, good RF properties, so just replace for new cc's one day

Is this a theoretical or have you noticed instances where film caused problems?

[bepone]

it is practical, with metal films it is easy to have oscillations when paired to hi transconductance devices (mosfets , out.tubes)

[Roe]

CC is the best to use in that place because of non-inductance, good RF properties, so just replace for new cc's one day

how about resistive hiss? wouldn't CC grid resistors in the first preamp gain stages add unnecessary hiss?

[Raja_Kentut]

CC is the best to use in that place because of non-inductance, good RF properties, so just replace for new cc's one day

how about resistive hiss? wouldn't CC grid resistors in the first preamp gain stages add unnecessary hiss?

Yes. CC Resistors are worst regarding hiss (thermal noise, resistive noise). The higher the resistance value, the more. ( that applies to every resistor )
As you have written above, change them. If they have drifted already so far, it‘s a matter of time when they fail.

[bepone]

CC is the best to use in that place because of non-inductance, good RF properties, so just replace for new cc's one day

how about resistive hiss? wouldn't CC grid resistors in the first preamp gain stages add unnecessary hiss?

no on grids because there is no current passing, i put cc everywhere, but also in power supply,
usually grid stoppers are small value so they just protect , not adding,
hiss is produced in anode resistors usually

[Roe]

CC is the best to use in that place because of non-inductance, good RF properties, so just replace for new cc's one day

how about resistive hiss? wouldn't CC grid resistors in the first preamp gain stages add unnecessary hiss?

no on grids because there is no current passing, i put cc everywhere, but also in power supply,
usually grid stoppers are small value so they just protect , not adding,
hiss is produced in anode resistors usually

makes sense. I also use CCs as dropping resistors in the power supply if I can find 2w CC resistors (MO sounds horrible to me)

[Raja_Kentut]

there is often a misunderstanding. A „real world“ resistor can be seen as a ideal resistor in parallel with a current (edit: not voltage, see below) source. The noise of resistor a is not originated from current passing through it. (at first sight. Current- and other noise sources is an addition)
It is material and resistance. A resistor lying on a bench, completely unconnected, produces a noise voltage.
There is a lot of literature out there about this phenomenon.
Anode resistors could have more noise, because they have most often high values… now think what a 1Meg resistor does when connected grid to ground…
Our grandfathers used the components they could get at that time for a resonable price. Thats the only reason. Of course when talking about music, a lot of subjective impressions and emotions are involved. And when someone is really feeling better when choosing particular components, nothing is wrong with it.

[maxkracht]

Our grandfathers used the components they could get at that time for a resonable price. Thats the only reason.

There are other reasons, our grandparents were smart as well as frugal. As Bepone mentioned, induction is one reason. In the RF world you see stuff like a cc resistor bypassed by a solid core wire wrapped around it, because our grandparents also studied black magic…

I have used carbon or metal film as grid stoppers in more amps than I remember, without incident, but i am open to the idea of edge cases where cc would be a better choice. Maybe film and a ferrite bead would work in those edge cases as well?

[martin manning]

A „real world“ resistor can be seen as a ideal resistor in parallel with a voltage source.

A non-ideal resistor can be represented as an ideal resistor with either series voltage source or parallel current source. See: https://en.wikipedia.org/wiki/Johnson–Nyquist_noise

Large and/or noisy resistors are not the best choice for grid stoppers: "...the input grid stopper adds the most amount of Johnson noise (hiss) of any resistor in the amp. (A 68k stopper generates at least four times more hiss than a typical 12AX7!)." See http://www.valvewizard.co.uk/gridstopper.html

[Raja_Kentut]

Martin, you are right, I change that in my post !

The resistor wound around by a wire forms a lowpass. R parallel to L.
HF will not go through the wound wire as this forms a inductor.
e.g.
0Hz -> 0 Ohm (only the wound wire resistance)
xxx MHz -> almost only resistor‘s value is left because the inductor has a high resistance (called reactance in english?) for the HF.
So the HF is more dampened than the low frequency.

[bepone]

It is material and resistance. A resistor lying on a bench, completely unconnected, produces a noise voltage.

you are speaking now about RF characteristics, every component act like antenna and receive noise from the outer world (very dirty environment, from RF signals around, internet, routers, wall 230 garbage radiation) which is just normal, all resistors make that.
I started to test all resistors and measure them for the noise mVpp, on the table, nothing connected, there is difference..

But u will be surprised which resistors receive and produce most of the noise.

For me, i have no any problem with CC resistors..maybe only i can complain that they tend to make dark sounding amp.. and after 20 years in operation maybe also cracking and hiss..but where are we now and where we will be in 20 years!