Any reason not to use smaller grid stopper at input?

[Gaz]

I only worked on a few amps, I never seen RF problem. All I've seen is in cascade stages, the tube start talking with the guitar and squeal. It is not from guitar to speaker, it is from the amp to the guitar direct.

I do do very careful layout. I am an RF and signal integrity engineer working with all sort of high speed circuits, I do pay attention to the layout to prevent EM pickup though.

If you look at some of the non high gain amp, I don't recall seeing any shunt capacitor as I never see any issue with noise pickup.

It's common in guitar amps to pick up RF at the input. The input tube has a Miller capacitance, which forms a RC filter with the grid resistor to get rid of the RF stuff.

You don't need an extra parallel capacitance at the input (with the Miller capaciatance), but it allows for a smaller resistance, which reduces resistor noise at the most critical stage of the amp.

And that's why I started the thread because I'm wondering how low you can make the resistor, and how high the capacitance without causing adverse tonal effects, etc.

For now I have a 7.5K resistor and 470pf resistor to the cathode of the input stage.

[Alan0354]

It's common in guitar amps to pick up RF at the input. The input tube has a Miller capacitance, which forms a RC filter with the grid resistor to get rid of the RF stuff.

You don't need an extra parallel capacitance at the input (with the Miller capaciatance), but it allows for a smaller resistance, which reduces resistor noise at the most critical stage of the amp.

And that's why I started the thread because I'm wondering how low you can make the resistor, and how high the capacitance without causing adverse tonal effects, etc.

For now I have a 7.5K resistor and 470pf resistor to the cathode of the input stage.

I never put any cap. I have one amp with 20K in series from the input jack to the grid of the first tube. But that resistor is just there and I am too lazy to take it out. I use a 100MHz scope, I would have seen any AM or FM signals if it is there.

[Merlinb]

And that's why I started the thread because I'm wondering how low you can make the resistor, and how high the capacitance without causing adverse tonal effects, etc.

I would place 10k as the minimum sensible input grid stop, for several reasons:

A lot of commercial audio gear is not designed to drive impedances of much less than 10k, so with a 10k grid stopper the input resistance cannot fall lower even when the grid is overdriven.

The equivalent input noise resistance of a 12AX7 / ECC83 (and indeed most valves) is around 5-6k, so there is little point making the grid stopper even smaller than that, since its noise will become irrelevant compared with tube noise. 10k is not much worse.

10k is much greater than most pickup impedances over a wide range, so the input capacitance can be as big as you like without greatly loading the pickup. Any pickup resonances are therefore controlled by cable capacitance plus any extra capacitance you deliberately add, over which we have complete and predictable control.

[J M Fahey]

Originally posted by Merlinb
10k is much greater than most pickup impedances over a wide range, so the input capacitance can be as big as you like without greatly loading the pickup.

Dear Merlin, I please ask you to reconsider your answer.
1) 10 K is usually higher than most pickups *resistance* but not *impedance*.
Quoting just one serious Pickup maker (although all agree on this, it's simple Physics):
From: Bill Lawrence

Inductance ( henry) is another valuable parameter for the sonic evaluation of a pickup but requires some basic understanding of electrodynamics. As a general rule, the higher the inductance, the lesser the highs. For example, a traditional strat pickup has an inductance around 2.3 henry while a Gibson PAF has an inductance around 4.4 henry and some of the so-called "distortion" pickups have an inductance above 8.0 henry.

OK, let's calculate impedance at, say, 4Khz, where lies most guitars "sparkle" (guitar speakers don't go much higher)
From: Capacitor and Inductor impedance calculator

>>For a Strat pickup : 2.3 Hy , 4KHz , Z=57805 ohm , rounding to 58K.
Closest standard value= 68K .... mmmmhhhh, ring a bell? *Maybe* that's how Leo Fender picked that classic value.
Any higher, too much cut; much lower, no real gains, since the pickup impedance is still there.

>>For a Humbucker: 4.4 Hy , 4KHz . Z=110584 , round to 110 or 120K. Scary.

Now we also see why Strats and such are happy with 250K volume pots, while Humbuckers prefer 500K.

Things do not happen by chance, there's some justificacion for choice, even if we don't notice them.

>> now for a high distortion monster:
8 Hy , 4KHz , Z=200K ... scarier.

So, using that real world pickup impedance, apply the values I set in my table and you'll see my conclusions about influence on sound stand up.
Not to mention that they have been checked in the real world, not simulated on some software

An even more important consideration: an amp input capacitance *resonates*with the pickup inductance and causes an audible peak in the frequency response.
How high? Depends on Q which in turn depends on series resistance (both pickup winding DCR and our own added series resistor).

At what frequency? Let's calculate it using a Humbucker as a middle of the road pickup.

A Strat type will have a higher frequency peak, hopefully beyond audible range, and a monster distortion will be so heavily filtered, clipped and processed as to make it unrecognizable, that's why I choose a Humbucker.

From: Resonant Frequency Calculator

With 100pF 4.4Hy Resonant peak= 7600Hz. Beyond speaker response. Did I call it "inaudible"?

With 220 pF still 4.4 Hy Resonant peak=5 KHz Just beyond the edge.

With 470pF 4.4 Hy Resonant peak=3500 Hz ... "Starts to be audible". Not a bad effect, sounds bright and chimey (although it's killing RF very well) No wonder it's what I use in my amps.

With 1000pF 4.4 Hy Resonant peak=2400 Hz. Ugh!! "Tinny" "nasty" guitar with no real sparkle. Most musicians start to dislike it or complain.

With impossibly high 4700pF Resonant peak=1100 Hz. Nasal as H*ll. You might as well use a Wah switched on, permanently, in your signal chain. Not kidding.

So as we see, the pickup DCR is really the least important parameter.

Inductance rules and, of course, affects heavily what capacitance is acceptable in our cable+amp chain.

Now we see that even tiny 100pF become important, if added to any reasonable cable capacitance.

[pdf64]

'I only worked on a few amps, I never seen RF problem'
Try removing any such mitigation at the amp's input (as an experiment) and you might run into an RF issue, but it depends on the RF environment that the amp is operating in.
If there's a strong RF source nearby, the input stage can get pushed into cutoff.
And passing cabs and CB radios can break through.
If there's plenty of gain (eg fuzzface) my experience is that it's common to get some russian radio station coming through when the guitar volume is turned down.
Pete.

[Enzo]

The discussion is drifting.

Two issues. One is whether you are experiencing any noise from RF. The other is the question why would there be a small cap across an amp input.

If you are not personally having any RF interference, that is great. Your RF environments may be such that no protection is needed, and you never have any interference. But that doesn;t alter the WHY amps are designed with RF suppression at the input. My shop is a half mile from an AM radio station transmitting tower. I can flip on my scope and watch their modulation envelope any time i want. And I can easily pick the signal up with a guitar amp. So can anyone else in the area. SOme guy goes down the street with a sloppily matched RF amp on his CB, (Yes, there are guys running illegal 1KW finals on their CBs) and that can come bursting from the amp speaker. It may not come up for many people, but that is why it is there.

[Merlinb]

Dear Merlin, I please ask you to reconsider your answer.
1) 10 K is usually higher than most pickups *resistance* but not *impedance*.

Indeed, but I said over *a wide range*.

OK, let's calculate impedance at, say, 4Khz, where lies most guitars "sparkle" (guitar speakers don't go much higher)

You can argue that that is where the 'sparkle' is, but it's not where most of the sound is. 4kHz is about three harmonics above even the highest note on the guitar! Most of the energy from the guitar is well below 1kHz.

>>For a Strat pickup : 2.3 Hy , 4KHz , Z=57805 ohm , rounding to 58K.
Closest standard value= 68K .... mmmmhhhh, ring a bell? *Maybe* that's how Leo Fender picked that classic value.

Unlikely. Firstly, the closest value is 56k, not 68k. Secondly, it only works because you happened to pick that particular frequency out of the hat! Thirdly, using that resistance would critically damp the pickup resonance and kill the sparkle, not enhance it. But even then, only if that resistance actually loaded the pickup, which it doesn't since it's in series with the grid! It's the grid *leak* that provides most of the additional damping after the guitar's own controls.

Pickup resonance is always dominated by cable capacitance, not the amp's capacitance. To make the amp remotely important you would have to use a tiny grid stopper *and* add a lot of capacitance to the grid. Pretty unlikely. That's why I like 10k as a minimum, because it basically keeps the amp's capacitance out of the equation, and fewer variables are easier to control.

(2.3H sounds kinda high for a standard Strat pickup, to me. I would expect more like 1.5 to 2H tops.)

[Wombaticus]

JM, I think you must be underestimating the baseline of cable capacitance and ignoring the interwinding capacitance in the pickup (which appears, effectively, as a capacitance in parallel to the impedance of the pickup). I don't know what numbers you are assuming, but I can't imagine these would be less than 500pF or so for cable, and maybe 200-500pF for interwinding depending on if it's a SC or HB. For any reasonable values of these, the resonant peak will most affirmatively already be within the audible range for either the single coil or a humbucker with a 68K resistor.

For instance - assuming 200pF of interwinding capacitance, 500pF of cable capacitance, 4.4Hy, 6K DCR, and a 68K grid stop, 1Meg grid leak, and the Miller capacitance typical of a fully bypassed 100K/1.5K 12AX7, I come up with a 7.5 dB peak at approximately 2.5KHz. And probably that estimate for the interwinding capacitance is low - that's more like a single coil figure. If you assume 500pF of interwinding capacitance, you're looking at about a 6dB peak at about 2.1 KHz (in practice these peaks will be damped to some degree by eddy currents in the pickup).

On the other hand, if you keep everything the same, and replace the 68K with a 10K plus, say, 220pF, you're only shifting this peak down to about 2KHz, a very small difference. Now, mind, pickups are complicated beasts, and there's a lot that is ignored in this kind of lumped R/L + C model -- but my main point is that I'm reasonably confident that (especially for humbuckers) the resonant peak is pretty much always in the audible treble range, even before one begins messing with additional grid-ground capacitance. I'd speculate that the resonant peak is actually very much part of the sound to which we are all accustomed.

[Steve Conner]

My favourite guitar cord measures 750pF.

[Tom Phillips]

Comment about the "Classic 68k" grid stopper value.

Let’s consider the classic two input circuit as used by Fender and many other amp manufacturers over the years. The resistors in the circuit are obviously 68kΩ. However, when you trace out the connections and draw the equivalent circuit you will find that the actual grid stopper resistance is 34kΩ when the #1 input is used. (The #1 input is by far the most commonly used)

There are many builders that don’t realize this. The evidence being their use of a single 68kΩ grid stopper value for a single input topology amp. I’m not saying that this variation destroys the sound of their amp. If it did we would see discussions popping up about why the tone changed when a single jack input layout was used. It’s just evidence of the many people that just copy classic circuits rather than doing any of their own analysis and design. I don’t have a problem with that either except for those that proclaim themselves a guru of all things electronic and publish statements such as “resistors hate high frequencies.” I read that once as an explanation of why grid stopper resistors suppress RF. (Or maybe it was “High frequencies hate resistance.”) Based on the writings of some people it appears they think they “invented” the electron.

Sorry. This is turning into a rant. I’ll stop now.

Regards,
Tom

[Alan0354]

The discussion is drifting.

Two issues. One is whether you are experiencing any noise from RF. The other is the question why would there be a small cap across an amp input.

If you are not personally having any RF interference, that is great. Your RF environments may be such that no protection is needed, and you never have any interference. But that doesn;t alter the WHY amps are designed with RF suppression at the input. My shop is a half mile from an AM radio station transmitting tower. I can flip on my scope and watch their modulation envelope any time i want. And I can easily pick the signal up with a guitar amp. So can anyone else in the area. SOme guy goes down the street with a sloppily matched RF amp on his CB, (Yes, there are guys running illegal 1KW finals on their CBs) and that can come bursting from the amp speaker. It may not come up for many people, but that is why it is there.

As I said, I only design a few amps only, but I did that in different houses, I just never seen that. I looked at a lot of classic amp schematics, I don't see any input filtering. Case in point, Fender vintage amp don't have shunt cap at the input nor to the cathode. This is the first time I heard about this.

Besides if it is RF from radio, the capacitance needed should be really small as the frequency is much higher than the audio frequency. It should not affect the sound as you are filtering out RF of at least close to 1MHz.

My biggest problem is with the cascade gain amp, when I crank the volume up, the guitar feed back to the tubes, I have not found a layout that improve this. A cap in the signal path is a must.

[J M Fahey]

Dear Alan, in tube amps, 12AX7 supply all the capacitance you need and then some.
Google "Miller Capacitance".
The cap is there, and is measurable, it's just not drawn in the schematic.
In my particular case, I design and sell mostly SS amps (lost count ovet 10000 a long time ago) which do not suffer from that problem.
My typical TL072 input stage has negligible input capacitance so, as stated above, I had to *add* a ceramic cap.
Best result was with a 470pF.

Dear Steve: nice to know. To complete the equation, what pickups do you use, or find best with that favorite cable?

[Enzo]

Older amps lacked any overt RF suppresion. They also lacked DC blocking caps at the input. Back then no one had computers and other digital stuff all over their homes potentially radiating noise, plus the rules today are much more strict about what an electronic device can RADIATE.

Look at the input of the Peavey Classic 30 as a typical example of current practice.

Look at the input of something like a Marshall AVT150 for some hardcore input filtering.

There are numerous amps with input jack not grounded to chassis, but a ferrite bead in the ground lead.

[Alan0354]

Dear Alan, in tube amps, 12AX7 supply all the capacitance you need and then some.
Google "Miller Capacitance".
The cap is there, and is measurable, it's just not drawn in the schematic.
In my particular case, I design and sell mostly SS amps (lost count ovet 10000 a long time ago) which do not suffer from that problem.
My typical TL072 input stage has negligible input capacitance so, as stated above, I had to *add* a ceramic cap.
Best result was with a 470pF.

Dear Steve: nice to know. To complete the equation, what pickups do you use, or find best with that favorite cable?

I know miller cap, it's the one from the plate back to the grid. The Miller cap equal to the gain times the feedback cap. We are talking about adding extra cap at the input which I never find it necessary. I am sure miller cap can do the job of filtering out the RF as the pickup can easily goes up to over 100K ohm at that frequency.

[Gaz]

I know miller cap, it's the one from the plate back to the grid. The Miller cap equal to the gain times the feedback cap. We are talking about adding extra cap at the input which I never find it necessary. I am sure miller cap can do the job of filtering out the RF as the pickup can easily goes up to over 100K ohm at that frequency.

Alan, I think you may be missing the point: Typically additional capacitance is not needed with a grid stop over 10K or so in order to block RF, BUT if you want to reduce resistor noise you can, to a certain point (which I discovered in this thread), reduce the resistance value and add a cap to ground to approximately recreate the same frequency break point.

Didn't I just say that?